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General Discussions => The Lab => Topic started by: soapfoot on January 05, 2011, 10:04:41 AM

Title: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 05, 2011, 10:04:41 AM
Hello.  This is my first post here-- sorry to dive right in with questions, but I do promise to stick around.  Before my membership was approved, I lurked for a long time and I've read many wonderful threads here, using the search function, about DIY U47-alike microphones.

I found one blurry schematic for use with an EF12 tube, of which I have a very nice NOS Telefunken.

I'd like to make a mic using the TAB/Funkenwerk stuff, Thiersch Blue Line capsule, and this tube.

Does anyone have an experience or wisdom to share, a cleaner/readable copy of the EF12 schematic, validation that this schematic is correct and works, or anything else?

Thanks a lot.  Great forum you have here.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: Sredna on January 05, 2011, 10:09:28 AM
Check out Klaus Heynes forum, there were recently a thread about EF12 and DIY U47.  
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 05, 2011, 10:35:03 AM
Thanks.  I'm a member over there as well.  When I get a few more of my parts/thoughts together, I might ask more in-depth questions there.  Klaus runs a pretty tight ship, and is pretty passionate about having the discourse be 'meaningful.'  I don't want to step on any toes.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: Biasrocks on January 05, 2011, 11:18:34 AM
I found this thread which has some good info.

http://www.groupdiy.com/index.php?topic=22730.0

And

http://www.groupdiy.com/index.php?topic=41549.0

Here's a schematic for the EF14 tube.

Mark


Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: zebra50 on January 05, 2011, 12:55:27 PM
Here's mine... again

(http://www.xaudia.com/omnip/Mics/STU47/STU47_b.jpg)

http://www.groupdiy.com/index.php?topic=39541.0
 
:D
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 05, 2011, 01:07:14 PM
I found this thread which has some good info.

http://www.groupdiy.com/index.php?topic=22730.0

And

http://www.groupdiy.com/index.php?topic=41549.0

Here's a schematic for the EF14 tube.

Mark




Thanks!  I plan on using an EF12 in mine, instead of the EF14 or VF14 (or UF14).

Question-- did you find you had to sort through a bunch of EF14 to get a "good" one?  Klaus, Oliver, et al seem to say that you can plan on doing this.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 05, 2011, 01:07:28 PM
Here's mine... again

(http://www.xaudia.com/omnip/Mics/STU47/STU47_b.jpg)

http://www.groupdiy.com/index.php?topic=39541.0
 
:D

beautiful!
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: MagnetoSound on January 05, 2011, 01:44:56 PM
You even did the badge?? (wags finger ...)  ;D
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: zebra50 on January 05, 2011, 02:17:39 PM
You even did the badge?? (wags finger ...)  ;D

 ;)

Only for the photos and a giggle, badge was a purple one off a KM84, stuck on with sticky tape.

Actually when it was all finished I didn't want to put someone else's badge on - it'll get an my own badge when I get round to it.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: MagnetoSound on January 05, 2011, 02:51:16 PM

Good man! We should all have our own logos and badges, IMO.

Which tube is that again? Did you have a chance to compare it in a 'shoodoud ziduation'?

Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: e.oelberg on January 05, 2011, 03:31:52 PM
this could be an inspiration, this is what Telefunken USA is selling as VF14.
(http://studiobeige.de/DL/VF14.jpg)


Very likely it's also JAN 6028
like in Max's kit, and the resistor looks like 3.6k


nicholas
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: zebra50 on January 05, 2011, 04:29:27 PM
408A also looks like an interesting possibility, 20V heaters IIRC. I have a load but have never really tested them properly in a mic. On the 'to do in 2011' list.

Good man! We should all have our own logos and badges, IMO.
+1. I have some nice enamel Xaudia badges for front panels, but they don't bend well

Quote
Which tube is that again? Did you have a chance to compare it in a 'shoodoud ziduation'?
Not with a real U47. Some of kind forum members offered to help out, but geography gets in the way. One day...
It's won a couple of vocal shootouts in our little studio, but has lost a couple also. (The Josephson keeps winning.)
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 05, 2011, 04:32:14 PM
This is what I'm working with right now, so far.  It's the only part I have.  

(http://img.photobucket.com/albums/v283/jazz347/47-inspired%20mic%20build/DSC00399.jpg)

I got a pretty good buy on it, NOS, and I've been wanting to build a mic anyway.  I've done a fair amount of reading, and everyone has a (passionate) opinion about tubes to use in "U47 Inspired" mics.  I use the word "Inspired" because some people get extremely offended at the notion that a microphone without the VF-14 could even be called a "U47."  So I'll not do that.  I don't have enough personal experience to have a genuine opinion either way.

However, the EF12 is (or at least was at one time) the "2nd place" tube of choice for Wagner and Grosser, according to what I read on the internet.  So I figured I'd go that route, as those two make nice mics.  I certainly cannot afford a real VF14.

I've built a couple of mics before, but not from scratch.  I put together two of the "Royer mod" MXLs and they came out really really nice.  So I'm not a total "rookie" but I do view this project as a "probable learning experience."  Hopefully not a "huge waste of money," but sometimes that's how we learn.

I'll slowly amass parts and pieces as budget/funds permit.

Thanks again for the links above.  I'll be looking.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: zero on January 05, 2011, 06:09:04 PM
Hi there,

For what i know of the ef12 in a u47, it has a reduced sensitivity and diferent heaters voltage.
i have a Flea with one in it and it is a great sounding microphone(and looking) although i never tried it with a vf14M and probably never will.

cheers
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: deuce42 on January 05, 2011, 06:12:43 PM
For what its worth - in my U47 clone I tried both an EF12 and EF14. I preferred the EF12 slightly. It sounded more "hi-fi" - which is a bad way to describe a microphone I think, but it is the world that comes to mind. Pin outs of the EF12 and EF14/VF14 are different. Voltages are different its true.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: zero on January 05, 2011, 06:21:51 PM
Yes, the guys at Flea decribed the ef12 as being a litlle less agressive than the ef14 so more hi-fi is probably the best definition.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 05, 2011, 06:24:31 PM
From what I've read, the EF12 was sort of a precursor to the EF86.  Can anyone validate?

I'm just hoping to end up with a good-sounding microphone.  Sounding 'exactly' like a 'real' U47 is of secondary importance to me; however, I'd like to end up in that "zone."

Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: zebra50 on January 05, 2011, 06:30:18 PM
Quote
From what I've read, the EF12 was sort of a precursor to the EF86.  Can anyone validate?

That's what I understood too. . That's where I started out with my STU-47 - I used Max's EF86 circuit and then tweaked (paying attention to heater current and max voltages). Best thing is to look at the data sheets.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: e.oelberg on January 06, 2011, 07:27:14 AM
408A also looks like an interesting possibility, 20V heaters IIRC. I have a load but have never really tested them properly in a mic. On the 'to do in 2011' list.

I think jan 6028 is the military version of the 408a
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: Gus on January 06, 2011, 07:51:06 PM
What about a solid state circuit?
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 06, 2011, 11:52:29 PM
there are great solid-state mics.  I'm not interested in making one right now, however.  Maybe some other day.  Right now, I'm interested in making a tube mic inspired by a U47.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: e.oelberg on January 07, 2011, 09:28:34 AM
What about a solid state circuit?

that is what I was thinking about. Grosser is using fets in his VF14 substitute anyway.
did you experiment with solid state amplifiers to get a u47 inspired sound ?


nicholas
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 13, 2011, 07:24:41 PM
Thanks to the links here and Google I've been doing a lot of reading and learning.  I've also sent some e-mails to a few experienced people trying to dig up a legible, correct, legitimate schematic for a U47 EF12 conversion.  I've seen a few "inspired by" ideas, one illegible schematic, etc.  But nothing stands out head and shoulders above the rest, and nothing seems like a real "conversion" of the original circuit for the new tube-- the STU47 from this board looks very appealing, but it seems like a "tribute" in the form of a ground-up design.

I've come up empty-handed on the "conversion" schematics, which is what I'm really after, so maybe I'll take a different tack.  I'm experience- and training-wise under-equipped to do work on figuring out the "conversion" entirely myself, but I've been trying to do what I can.

Looking at the data sheets, I can already see that the filament voltage of the EF12 is 6.3V.  I've seen it recommended (per Oliver Archut) to underheat it slightly, to about 5V or so.  Seems that this would be most elegantly accomplished through a separate conductor in the tube mic cable, it seems.  This much makes sense, and while it's obvious, it's at least a start.

Does anyone know what else would need to be changed from the U47 schematic to make the EF12 work?  Even bits and pieces would be helpful as I try to piece this together.

Here's a Philips datasheet for an EF12:
http://tubedata.itchurch.org/sheets/030/e/EF12.pdf

This sideways thing is the best I could come up with for VF14 data:
http://tdsl.duncanamps.com/pdf/vtubes.pdf

Also in the course of my detective work, I found that there was one mic in Neumann's history for which both VF14 and EF12 schematics exist: the CMV-5B.  Apparently, both versions were at least drafted, though nobody has seen a VF14 version.  Nevertheless, the schematics exist.

EF12: http://www.elektroakustika.cz/images/cmv/cmv5/schema.gif
VF14: http://recforums.prosoundweb.com/index.php/fa/16142/21957/

I've mostly been a kit builder and have done some rudimentary design stuff, but mostly with guitar amps.  I do not have any formal electronics training and am totally self-taught.  While I can identify the purpose of a fair amount of the components on the schematics, I cannot parse exactly what was done to convert from VF14 to EF12 operation, nor if I would be able to apply those changes to the U47 circuit.

I'm sure Gunter Wagner's design is proprietary, though I haven't asked him.  Just seems like he wouldn't want to freely give out that info.  That said, I'd be real curious to see how he's running the EF12 in his mic, and if it required major departures from the original circuit.  Most agree that his mic sounds very good.

Thanks for any help anyone can lend.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: zebra50 on January 13, 2011, 07:51:07 PM
As you know, the BIG change is having 6V heaters instead of 20V. With the original U47 supply you just have 105V for your B+, and a big resistor to drop it for the 20V heater & cathode bias. To get this down to 5 or 6V at relatively high current compared with VF14, you need to scrub off a stupid amount of heat.

So any sensible (to my mind at least) mic design with EF12 has a separate 6V supply. And at that point you've taken a big step away from being just a 'conversion'.

Max's MK47 design uses the heater supply to bias the cathode, which is closer to the U47 philosophy. I haven't really studied his new design but I imagine there is much to be learned there too. My mic is a bit simpler, with straightforward heater and cathode resistor, and a bit less like a real U47 design.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: milesjackson on January 13, 2011, 10:23:49 PM
Quote
Looking at the data sheets, I can already see that the filament voltage of the EF12 is 6.3V.  I've seen it recommended (per Oliver Archut) to underheat it slightly, to about 5V or so.  Seems that this would be most elegantly accomplished through a separate conductor in the tube mic cable, it seems.  This much makes sense, and while it's obvious, it's at least a start.

Underheating a vf14 with a specified filament voltage of 55V down to 36V in the original U47 circuit is not identical to underheating a ef12 from 6.3V down to 5V.  I've been down this "I'm gonna make a u47 with the parts I can get" road, and I have to say I gave up on that.  Build a good microphone, and don't worry about whether or not it vaguely resembles the original U47 circuit.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: Skylar on January 13, 2011, 11:06:41 PM
Sorry to interject.

I'm beginning to realize that there's nothing special about the VF14 (or that whole family of tubes for that matter).
You can get great sounds from many, many tubes.

The secret is the capsule...


If you follow the history of what Gus has gone through in his experiments over the years and start doing the same thing, you find yourself coming to the same conclusions.
As much as we love tubes, solid state starts to make real sense after a while.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: gary o on January 14, 2011, 07:15:42 AM
Am I correct in saying that different ways to power the heaters makes no different to the sound of the mic (longs as we give heater the right voltage a certain percentage less than usual as I understand it)and as long as the cathode is biased correctly without the heater connected ......maybe a dummy load would be used

So if I understand correctly in a real U47 we could disconnect the heater, replace with dummy load, externally feed the VF14 with the 36V & the mic would still sound the same ?





Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: MagnetoSound on January 14, 2011, 07:56:10 AM
... as long as the cathode is biased correctly ......

... we could disconnect the heater, replace with dummy load, externally feed the VF14 with the 36V & the mic would still sound the same ???

Yes.

Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: pasarski on January 14, 2011, 08:52:56 AM
...But a noisy heater supply would make the mic noisy in U47 circuit.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: rodabod on January 14, 2011, 09:14:15 AM
... And you'd need to keep the cathode resistor the same value, thus meaning you'd need an external bias voltage source.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: gary o on January 14, 2011, 09:17:56 AM
Hi again so getting LT from HT supply is a good way to get less noise in yr LT then.....do other mics do this ?

So I hope Im still right to say that we can just adjust the cathode bias part of real U47 circuit to be correct and supply the different LT for a different tube Im talking about the E14 that im told is same as the V14 apart from heater & then mic should sound the same as with V14.... I no other DIYers & designers are doing this but I seem to read that it might not same sound the same I think it should be but not sure thats why I ask.....thanks for your patience

very interested to learn about this interesting type of circuit

cheers
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: gary o on January 14, 2011, 09:24:04 AM
... And you'd need to keep the cathode resistor the same value, thus meaning you'd need an external bias voltage source.

Hi Roda......please excuse my lack electronics basic knowledge......can we not adjust the mic circuit to bias tube from the 105VDC going into mic.....Im keen to understand this ......again thanks for patience.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 14, 2011, 09:30:15 AM
Im talking about the E14 that im told is same as the V14 apart from heater & then mic should sound the same as with V14.... I no other DIYers & designers are doing this but I seem to read that it might not same sound the same I think it should be but not sure thats why I ask.....thanks for your patience

I've been doing a lot of reading, and though it seems that way on paper, according to experts like Klaus Heyne and Oliver Archut, the EF14 is not the same as the VF14 (with different heater voltages).  The numbers are the same, but the cathode material is different, and the internal construction of the tube is different.  Supposedly, the sound is quite different.

Several different builders have said they actually prefer the sound of the EF12 over the EF14 in a modified 47 circuit.  And since I happen to already have one, I'd like to figure out how to implement that.  However, I haven't yet figured out how they're doing that, and nobody seems too keen to share (understandably, for those who make their living doing this).

As for the underheating to 5V, this was a recommendation of Oliver Archut over on the Neumann pinboard.  I believe it was not necessarily for the sound, but to keep the tube living longer.  

As for the solid-state thing, I understand that for many this does make a lot of sense.  However, I love tubes and am doing this primarily as a learning experience.  I'm very interested in learning how to make the tube work.  Not even getting into how it may or may not sound to use a solid-state device, or how much "sense" it makes, I'd rather use the tube at this time.

...hopefully I'm not the only one who's interested in figuring this out.  :)
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: zebra50 on January 14, 2011, 09:35:30 AM
... And you'd need to keep the cathode resistor the same value, thus meaning you'd need an external bias voltage source.
... which is sort of what Max does in the MK7...


Quote
and nobody seems too keen to share

Well, we're all sharing! Really all the info is here now - 6V heater (or five if you prefer), and cathode bias from heater supply. Everything else as it was in the 47, then tweak til it sounds good.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 14, 2011, 09:53:15 AM
Well, we're all sharing! Really all the info is here now - 6V heater (or five if you prefer), and cathode bias from heater supply. Everything else as it was in the 47, then tweak til it sounds good.

And I do thank you!

So is that all there is, really?  That sounds easy enough.

So that brings up a couple of questions--

You say "cathode bias from heater supply," and I think we're getting into something I don't understand.  To me, "cathode bias" has always meant "place a resistor between the cathode and ground, of a particular value that will cause the appropriate positive voltage to develop on the cathode, making the grid appear negative by comparison."

So what does it mean to get cathode bias "from the heater supply?"  Do you mean take a small positive voltage from a 6v DC heater by way of a resistor, and apply that directly to the cathode, as opposed to using a resistor to develop the bias?  And at that point does the cathode "float," with no ground connection?

Bias should be approximately how many volts negative?

And finally... "tweak til it sounds good..." what would be some places to start tweaking, in your experience?  In other words, can we start a discussion on what sorts of things, when changed, have what sorts of effects?

Thanks for your patience.  I've never delved this deeply into microphone design before.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: rodabod on January 14, 2011, 10:24:46 AM
Hi Roda......please excuse my lack electronics basic knowledge......can we not adjust the mic circuit to bias tube from the 105VDC going into mic.....Im keen to understand this ......again thanks for patience.


Yeah, you probably could do this. Cost you about (at a guess) 40mA if you form a potential divider with the 29R resistor (in order to drop ~1.1V)
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: MagnetoSound on January 14, 2011, 11:48:20 AM

So what does it mean to get cathode bias "from the heater supply?"  Do you mean take a small positive voltage from a 6v DC heater by way of a resistor, and apply that directly to the cathode, as opposed to using a resistor to develop the bias?  And at that point does the cathode "float," with no ground connection?


Yes. The U47 is a fixed bias amplifier, although the (positive) bias is applied at the cathode, rather than (negative) at the grid, as is more commonly the case.


Quote

Bias should be approximately how many volts negative?


Bias is given as 1.1v for the VF14. The sweet spot may well be elsewhere for other tubes (1.6 or so for EF86). Remember, grid is negative wrt cathode, and vice versa.

Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 14, 2011, 11:54:58 AM
That's very helpful, thank you.  Makes a lot of sense.

Something else I've observed from looking at schematics and data sheets--

In the VF14, each grid, cathode, and metal case have their own individual pin connections on the tube.

In the EF12, the third grid, the cathode, and the metal case are all tied together at one pin connection.

The U47 schematic shows that the second and third grids are tied to the plate (triode operation).

How would I connect the EF12 for triode operation?  I'd assume that the second grid would be connected to the plate?
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: zebra50 on January 14, 2011, 12:10:17 PM
http://recforums.prosoundweb.com/index.php/m/0/32588/16/3507/#msg_32588

(In some circles cross posting is considered bad form!)

For tweaking, plate resistor, output capacitor value & type, and polarisation voltage spring to mind.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 14, 2011, 12:29:48 PM
Thanks for your thoughts.  I'm aware of that forum's rules against cross-posting.  I figured my reply would be within the 'spirit' of the rule-- instead of creating a new thread or bumping an old thread, I replied with similar thoughts to a thread that was already on "page 1,"  to share information I had gathered in the course of doing this that I thought would be relevant there.

If you feel like that's bad form, I will delete my post over there.  It wasn't my intent to "spam," but to hopefully steer a thread which seemed to have drifted off-topic.

Edit: I've added a note to my post over there mentioning this conversation, and stating that I will happily remove my comments there if Klaus feels like this qualifies as "cross-posting."  I know he runs a tight ship, and I don't wish to run afoul of anyone.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: zebra50 on January 14, 2011, 12:35:35 PM
I don't think it's really a problem. Just making sure you know.

I know a lot of people here who visit both forums. And the PSW mic forum can be stricter than here!

Anyway, back to the topic...
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 14, 2011, 12:36:20 PM
Also, Stu--

I noticed that on your schematic for the "STU47," the EF12 shows three grids, two of which are connected together and to the plate.  

In light of the EF12's internal connections (grid 3 to cathode and metal case), how did you manage to do this?  Or is it a misprint on the schematic (or am I misunderstanding something)?

Thanks,
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: zebra50 on January 14, 2011, 07:00:16 PM
My bad - just a copy and paste error from another schematic. It would be grid 3 to cathode, grid 2 to plate.

I did make a mic around an EF40, and played with the grid connections. It was an experimental build, but I couldn't hear a difference between connecting grid 2 to plate or cathode. Noise was about the same in each case too.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: electroslut on January 14, 2011, 08:19:37 PM
It seems you have got way more help than I ever have had on this very informative forum! You have every thing you need here except
a couple caps and resistors! weather you power the heater from the B+ or its own supply will not effect the sound at all if its nice and clean. trying to lower the B+ to the heater voltage to make it more U47 is just silly. Now that you know the proper way to power and bias the tube
the last little tweaks I would do by ear and test gear. also see if you can borrow a U47 (if you don't have one) and get it as close as you can!
by the time you get all the hard work done (building the rest of the mic) you will have more info then you will need!  :D
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: scott_humphrey on January 14, 2011, 11:27:11 PM
Here is another option for you from Oliver Archut:

http://www.tab-funkenwerk.com/sitebuildercontent/sitebuilderfiles/AMIU47alttubeschematic.pdf

I recently built one using his U47 metalwork and transformer with a K47 capsule. I sounds great to me, but I don't have access to the real deal to give any kind of fair comparison. I'm sure that it won't sound like the original   ...but then again, and API pre does not sound like a Neve, but they are both effective.

I ended up using a Telefunken EF802 tube. You can still find these on the German eBay. One deviation from the schematic that I did was to use two 1N4148 diodes in series to bias the cathode (they are connected from the cathode directly to ground). I tried this after reading about it from Brian at Fox Audio Research:

http://www.foxaudioresearch.ca/The47.htm

Having a fixed bias for the tube seems to be an important part of the sound from what I've read, and in my experience I like the fixed bias sound better    ...but who knows?

I tried Oliver's schematic as is and also tried it with the diodes for fixed bias at the cathode. The end result sounded the same to me (as was the final voltage on the cathode), so I left the diodes in the circuit rather than the resistors since that was the last configuration that I tried. I'll post pics at some point.

Happy building!
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: gary o on January 15, 2011, 07:46:19 AM
I read that fox diode stuff..... would like to learn more about the diode bias thing.....I dont fully understand the different biasing schemes ....I am a little slower than others but I get there in the end...

The mic Fox was modding had IR red diodes.......is that LEDs ?....
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 15, 2011, 09:43:15 AM
Another question about the STU47:

why the .33uf film capacitor?  The original value is .5, and I've heard of some people going as high as 1uf.  All the stuff I've read about EF12 versus VF14 has indicated that the EF12 sounds similar except the low-end sounds a little "relaxed."  This is not first-hand info, just from reading on the net.

I'm curious what led you to the .33 value.

Thanks!
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: scott_humphrey on January 15, 2011, 09:53:51 AM
You can use either two 1N4148 diodes in series or a single infrared LED to achieve a cathode bias of 1.1V. Brian Fox has done formal testing and reports that the IR LED doesn't stay "open" at lower current draws, so maybe the 1N4148 diode is a better option here. I don't want to misquote Brian, so I'll send him an e-mail to see if he can provide a more accurate description of what's going on.

The basic idea, though, is to replace the cathode resistor/capacitor with an LED or diode. Just like with having a resistor running from cathode to ground there is a voltage drop that occurs across the LED or diode that provides the biasing voltage for the cathode. The difference, though, is that with the LED/diode the voltage stays at the same level (it is fixed) regardless of the current draw of the tube. When using a resistor, the voltage changes as the current draw of the tube changes (due to Ohm's law V=IR) so the voltage is not fixed with that configuration.

The key to having this work is to find an LED or diode that will provide the correct cathode bias voltage.

As to whether or not this is "better" it is really just a matter of taste, so try it out for yourself and see. I personally think that it sounds much better than the typical cathode bias with the R/C. Punchier and more phase coherent is how I would describe it, but maybe I'm kidding myself.

I also switched the circuit in my diy Telefunken-Ela-M251E-style mic so that it now uses an infrared LED (and an old Blue Kiwi capsule):

http://www.treblebooster.com/m251e/telefunken_ela_m251e_circuit.htm

Again, I like the sound better, but your mileage may vary....
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 15, 2011, 09:58:22 AM
An unbypassed cathode resistor is a form of negative feedback, correct?  So using the diodes for bias would have a similar effect to removing negative feedback from the circuit, right?
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: Gus on January 15, 2011, 10:26:21 AM
First Scott good to see you post.  I remember when you first posted about your microphone build


Years ago I gave away a LED bias tube circuit and the schematic was even posted here.  The correct way to use LED or diodes for biasing.  Have fun looking for it.  Just using a LED alone in the cathode leg can sound worse than resistor cathode bias.

I wonder if the circuit is like the circuit I posted years ago?

EH tubes?
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: gary o on January 15, 2011, 10:37:59 AM
Well Im just interested to learn & if I can maybe make something that sounds good clone or otherwise most likely tweaked

Ill have a search for led biasing in mics...

Scott lovely DIY mic thanks for info very interesting

An unbypassed cathode resistor is a form of negative feedback, correct?  So using the diodes for bias would have a similar effect to removing negative feedback from the circuit, right?
ing

very interested to learn about the above too

Thanks to all posting here for yr knowledge.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 15, 2011, 10:52:03 AM
A few words on biasing from Klaus Heyne that I found.  The context was that he was discussing a U47 on eBay that had been converted to cathode bias by way of a bypassed resistor, and fitted with a dedicated regulated heater supply for the VF14.

Quote
changes in tube biasing substantially alter the sound of this or any other condenser mic, as anyone who has ever played with the two types of biasing will confirm.

In response to the question: "How would it sound when the U47/VF14 circuit is changed to cathode biasing?"


Quote
In a word: anemic.

The heft of the midrange is gone, and replaced with more of a hifi sound that adds clarity and speed at the price of removing texture and musicality.

By the way, most VF14 to EF14 conversions, which usually are done with cathode bypass, suffer from the same thing.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: Gus on January 15, 2011, 11:15:16 AM
The best way to learn is to build circuits and try them in a microphone.

I always question what I read on the web.

Tube microphones are so yesterday.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: Skylar on January 15, 2011, 12:54:08 PM
I always question what I read on the web.

+1
especially the quotes from two posts above
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 15, 2011, 01:36:46 PM
Just passing on what Klaus said.  I have no first-hand experience either way-- probably should've pointed that out.  But Klaus's opinion on the U47 is pretty well-respected, so I figured I'd pass it along.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: gary o on January 15, 2011, 01:48:24 PM
I always question what I read on the web.

+1
especially the quotes from two posts above


Im 2 post above..........what dont you believe?
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: Skylar on January 15, 2011, 02:06:55 PM
Gary, you're 3 posts above my previous post.

I was talking about the Klaus quotes.
He's saying that changing your circuit from fixed bias to cathode bias will "substantially alter the sound of this or any other condenser mic"
Question that.

I've designed/built 6 different tube mics over the past year...some have fixed bias, some cathode-biased. Some are steel tubes, some glass, some sub-miniature. Each mic uses a different transformer. Despite all of those differences, as I move the same capsule between each of these mics, the sound follows the capsule.
Even changing the value (and type) of the output capacitor doesn't change the overall tone as much as changing the capsule.

BUT, please don't believe me (or Klaus for that matter); try it for yourself and see.


Quote
In a word: anemic.

The heft of the midrange is gone, and replaced with more of a hifi sound that adds clarity and speed at the price of removing texture and musicality.

Oh, give me a break. This is that typical audiofool BS. Is he talking about a microphone or a Monster cable?
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: milesjackson on January 15, 2011, 02:26:16 PM
Quote
Oh, give me a break. This is that typical audiofool BS. Is he talking about a microphone or a Monster cable?

C'mon Skylar, Klaus isn't going far enough.  To preserve the texture and musicality of a tube mic, we must use $600 power supply cables! 


Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 15, 2011, 02:26:54 PM
I think it might just be the case that what Klaus's considers to be "drastic" might not seem so to most of the rest of us.  However, he's probably heard, handled, worked on, and been inside more U47s in his lifetime than almost anyone in the last 40 years.  That counts for something, in my opinion.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: e.oelberg on January 15, 2011, 03:00:20 PM
I think the most interesting about Klaus Heyne is how he established his web reputation. If I read his posts I'm always fascinated by his delicate language, his humble attitude, his ears are the best you'll find on the web, I'm sure he hears the color of the Neumann Badge.

Nicholas

Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: Gus on January 15, 2011, 03:13:35 PM
I was trying to give some advice about the web.  I have built a few microphones and changed a few circuits around in microphones (some call this simple stuff modding)designed solid state and tube gain stages for microphones for myself so what do I know?  I have also repaired a number of microphones for friends Lomos, Neumanns, Oktavas, china ones etc.   I have built both solid state and tube circuits and mixes.

My point is not too believe anything you read about microphones until you hear one or test one or build/mod 50 or more circuits.
I have heard some "web respected" microphone that I did not like  some >$3,000.  I have also heard some "web respected" microphones I thought were very good.

FWIW one example I bought a GT44 years ago because a "web guru" posted about how good it was.  That taught me to not believe what a certain person posted ever again on the web.  Don't forget people are often trying to sell you something.

Biasing matters but what matters more?   I heard two of one companies I believe 47 like "web respected" microphones that sounded like they were cathode biased.  I did not look inside the microphones because they were not mine.  They both had the m7 /47 capsule grill sound but were a little different sounding than a fixed bias circuit.  

But as always the Web exaggerates the sound differences in microphones.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: scott_humphrey on January 15, 2011, 03:22:47 PM
Hi Gus,

It's good to be back after my 5 year hiatus. Did I miss much? Has anything new happened in the last 5 years in this forum?

I did have fun searching for your schematic. I believe that you are referring to this one:

(http://www.xaudia.com/omnip/Mics/Square/SqrMicSchemSml.gif)

You're using a resistor connected to the LED presumably to keep the current that flows through the LED from shutting off. This would make the LED operate at the flat portion of its curve (so the current through the LED doesn't shut off and screw up the cathode's bias voltage).

Do I understand this correctly?   ...and, are you connecting that resister to the heater voltage supply?

I don't think that you would need the smoothing capacitor that you've drawn in parallel with the LED, but let me know if you think otherwise.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: Gus on January 15, 2011, 03:35:27 PM
Try with and without the cap bypass.  Also you need to be mindful of the semiconductor material and the curves

http://en.wikipedia.org/wiki/Light-emitting_diode

Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: gary o on January 15, 2011, 10:28:57 PM
Sorry for confusion Skylar......interesting about capsule sound noticed similar in my few DIY mics.
I doubt I could here a neuman badge but I no what I like when I hear it so i'll experiment ask questons copy old circuits till I hear it hopefully i'll learn as I go

thanks for interesting info chaps.

Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: Gus on January 16, 2011, 10:01:11 AM
Soapfoot

Google, Bing, Ask etc
Ohms law
power in circuits
Thevenin
Thevenin and Norton
Triode

If you are the same soapfoot I noted you got wrong answers to some questions at Klaus's.

A u47 is not cathode biased(a very small amount if one wants to be 100% correct but not to any extent it really matters to the operation of the tube) it might look that way but think about it.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 16, 2011, 10:46:07 AM
Thanks.

The bias in a U47 comes off the heater supply, correct?
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: electroslut on January 16, 2011, 11:08:47 AM
yes.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: electroslut on January 16, 2011, 11:13:08 AM
since R3 is there, there is still 1.2 volts above ground.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 17, 2011, 03:07:49 AM
OK, here's a draft I made.  Any thoughts/corrections/advice etc.?

(http://img.photobucket.com/albums/v283/jazz347/47-inspired%20mic%20build/EF12_47_10.png)
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: rodabod on January 17, 2011, 03:19:43 AM
Are you aiming for 1.5V bias? You could swap that 60R resistor for a trim pot.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 17, 2011, 10:33:57 AM
I did some searching around looking for the "sweet spot" for EF12 bias.  I found one schematic for an EF12 tube mic, and the bias voltage was 1.67v.  This made sense to me, because the EF12 is an ancestor to the EF86, and I've read (earlier in this thread, actually) that 1.68v or thereabouts is the 'sweet spot' for an EF86.

So I just figured I'd start there, and experiment if I had to.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: mad.ax on January 17, 2011, 11:35:50 AM
>Soapfoot
The biasing of the U47 is not about the value of the 29 ohms resistor, but more about the current flowing through it...
My advice: Study how Max did it in his MK7...

Axel
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 17, 2011, 11:51:59 AM
>Soapfoot
The biasing of the U47 is not about the value of the 29 ohms resistor, but more about the current flowing through it...
My advice: Study how Max did it in his MK7...

Axel

Axel,

Thanks for your comments.  I'm not sure what you mean, however.  The voltage divider created by the 60R and 29R in my schematic above should give about 1.6-ish volts if my calculations are correct, which was my target.  What do you think is incorrect about this arrangement?
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: mad.ax on January 17, 2011, 12:30:51 PM
I didn't say it is incorrect!
I just want to point out that 29 ohms is not a 'magical' number, neither would be 1.1, 1.25, pr 1.6V bias...
This famous 'sweetspot' may be 1.6... Or more... Or less... (Like in the MK7 schematic...)
The best way to find it it to experiment with it, and what easier way to experiment than using a trim pot like Rodabod advised?

Axel
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: gary o on January 17, 2011, 01:06:08 PM
Im gonna be making a U47 ish mic soon was thinking of have a trim pot on the outside of the mic case and a DVM connected so I can sing n record the mic and alter the bias as I go......cant wait for my parts to arrive....very interesting stuff.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 17, 2011, 03:14:02 PM
I didn't say it is incorrect!
I just want to point out that 29 ohms is not a 'magical' number, neither would be 1.1, 1.25, pr 1.6V bias...
This famous 'sweetspot' may be 1.6... Or more... Or less... (Like in the MK7 schematic...)
The best way to find it it to experiment with it, and what easier way to experiment than using a trim pot like Rodabod advised?

Axel

I see what you're saying now.  Yeah, that seems like a good idea.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 17, 2011, 05:26:42 PM
OK so I've just started thinking about designing a regulated power supply for this thing.  I've been reading the power supply meta and it's a bit overwhelming.  A lot to digest there.  I've also been looking at other microphone power supplies for examples.

Seems that some people think regulation is a good idea to ensure consistent sound from day-to-day, and some think it's totally unnecessary.  At the very least it seems it would help filter out that last tiny bit of ripple.

I'm always down to learn something new. 

Stewart, designer of STU47: what did you use for a power supply?  Regulated or unregulated?  Do you have a schematic?
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: gary o on January 17, 2011, 05:39:55 PM
Well Im gonna knock this up in a hurry and gonna modify to give me 120VDC for another mics HT

oops forget to post the link

http://www.groupdiy.com/index.php?topic=41379.0
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: boogietube on January 17, 2011, 11:21:28 PM
I think this is a good place to start the psu:
http://www.twin-x.com/groupdiy/albums/userpics/MK7_psu.JPG
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: boogietube on January 17, 2011, 11:25:06 PM
There is a BOM, and the transformer can be bought here:
http://www.edcorusa.com/default.aspx

This was their reply to my e-mail inquiry:

That is a PWRC100-1 cost is $23.03 each.  You can call in at 800-854-0259 to place your order as this item is not on the website.

Phyllis Weston
EDCOR Electronics Corporation
7130 National Parks Hwy.
Carlsbad NM 88220
Phone (575) 887-6790
Fax (575) 887-6880
www.edcorusa.com
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: deuce42 on January 18, 2011, 06:56:23 AM
At the risk of adding my unhelpful/ unsolicited opinion to this thread -  Soapfoot, enjoy experimenting and learning about the different permutations of your EF12 circuit, but for the sake of stating the obvious, do not underestimate the impact of the capsule as some earlier posters have noted.

In my experiments*, the differences in capsules used on exactly the same circuit was by far more drastic than using the same capsule with different tubes or different circuits. At least to me anyway.

Experiment with the EF12 for your own learning curve and the enjoyment in doing so, but don't become too caught up in it at the risk of neglecting really thorough capsule research.



*(granted these experiments weren't undertaken by someone working on a daily basis with Neumans for 40 years and whom has achieved "web golden ear deity status" but I did learn a thing or two)


Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 18, 2011, 09:48:27 AM
There is a BOM, and the transformer can be bought here:
http://www.edcorusa.com/default.aspx

This was their reply to my e-mail inquiry:

That is a PWRC100-1 cost is $23.03 each.  You can call in at 800-854-0259 to place your order as this item is not on the website.

Phyllis Weston
EDCOR Electronics Corporation
7130 National Parks Hwy.
Carlsbad NM 88220
Phone (575) 887-6790
Fax (575) 887-6880
www.edcorusa.com


Thanks for this.  Do you happen to know what the specs are on this transformer?  Secondary voltage? Multiple secondaries?  Thanks!!

Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 18, 2011, 09:49:24 AM
for the sake of stating the obvious, do not underestimate the impact of the capsule as some earlier posters have noted.

Noted.  I was planning on either a Sneesby K7 or a Thiersch Blue Line, depending on budget.  Thanks.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: 0dbfs on January 18, 2011, 10:17:52 AM
The best way to find it it to experiment with it, and what easier way to experiment than using a trim pot like Rodabod advised?

Axel

Chameleon Labs makes a LDC tube mic with a pot in the PSU that adjusts the heater bias. It is labeled as COLD/WARM. We have a bunch of them here at our studios.

Cheers,
j
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: boogietube on January 18, 2011, 11:05:45 AM
Yes, the transformer specs are exactly like the schematic. In fact, they were specially made for this psu. Someone here absorbed the setup cost so now that they have it on file, it is a minimal cost to buy one.
Sean

120V 60HZ primary
Dual secondary
160VAC @10MA
12VAC @500MA
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: MagnetoSound on January 18, 2011, 11:35:01 AM
The best way to find it it to experiment with it, and what easier way to experiment than using a trim pot like Rodabod advised?

Axel

Chameleon Labs makes a LDC tube mic with a pot in the PSU that adjusts the heater bias. It is labeled as COLD/WARM. We have a bunch of them here at our studios.



Two different things. Roddy was talking about the cathode bias. This is arguably a matter of sonic preference.

Heater voltage (not bias) should ideally be set at the optimum point for lowest noise and tube longevity and is not so much an 'adjust to taste' parameter.

Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: 0dbfs on January 18, 2011, 11:36:53 AM
The best way to find it it to experiment with it, and what easier way to experiment than using a trim pot like Rodabod advised?

Axel

Chameleon Labs makes a LDC tube mic with a pot in the PSU that adjusts the heater bias. It is labeled as COLD/WARM. We have a bunch of them here at our studios.



Two different things. Roddy was talking about the cathode bias. This is arguably a matter of sonic preference.

Heater voltage (not bias) should ideally be set at the optimum point for lowest noise and tube longevity, although some might choose not to.  ::)



Oops! Helps to read and connect words-to-brain.

Anyway, I was thinking of how the u47 fixed bias (and ioaudio's mk7) is tapped off of the heater and somehow connected this information with the mic I mentioned (which is probably cathode biased). Although the heater in the u47 is tapped off B+ so there would not be a dedicated heater line from the PSU.

details, details...

Best,
jonathan
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: boogietube on January 18, 2011, 12:36:13 PM
So, does that mean that the MK7 PSU can't be used? I wouldn't think so, but I'm a fresh newbie.
Sean
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 18, 2011, 12:38:24 PM
well, for one thing the voltages are way too high.  I need to build a supply that has 105v B+ and about 5-6v for filament/bias.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: zebra50 on January 18, 2011, 12:53:00 PM
Quote
Stewart, designer of...
...nothing really - it's all copied from standard circuits.

So, does that mean that the MK7 PSU can't be used? I wouldn't think so, but I'm a fresh newbie.
Sean

Yes, it can.  ;)

Make sure you have a beefy enough transformer for the extra heater current, and use ohm's law for B+. That's it really.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: boogietube on January 18, 2011, 03:36:36 PM
I thought ohm's law could be applied here to drop some b+ voltage and the variable resistor on the heater can be used to adjust the voltage. I'm working on an EF14 mic and that's what I understood from the replies to my posts.

Sean
 
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: zebra50 on January 18, 2011, 03:58:51 PM
Looks like EF12 (http://www.scottbecker.net/tube/sheets/082/e/EF12.pdf) and EF86 have the same 200 mA current requirements, so no problems.

EF14 (http://www.scottbecker.net/tube/sheets/082/e/EF14.pdf) is 450 mA!
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 18, 2011, 04:19:48 PM
This is all interesting.

So, apologies for the "noob" questions, but:

1) do you see any disadvantage in having an unregulated supply for B+ and a regulated supply for the filament?

2) When working toward a target voltage, would this math be right?  Say I wanted 105 VDC output.  To find the desired VAC input, is the following equation right?

(V -1.4) * √2 = 105V.  Solve for V. 

In this case, V = about 75.6 VAC.

Is that totally wrong?  My reasoning is that 1.4v is dropped in the bridge rectifier, and after smoothing the DC voltage will be at or very near the peak AC voltage, hence the multiplication by the square root of 2.

Also, how do I account for the load resistance in the microphone itself... i.e. absent a regulator, how do I know the correct voltage without the microphone connected?  Thanks for your patience.  I have 2 music degrees but was never better than a "B" student in math.

Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: zebra50 on January 18, 2011, 04:32:36 PM
1) do you see any disadvantage in having an unregulated supply for B+ and a regulated supply for the filament?
Cross posting - See the answers to the same question on Klaus's (and my comment on this above - I'm now reading 5 threads about your design across two forums).

Quote
2)...Is that totally wrong?

Not wrong, but that would get you 105V at the output of the bridge rectifier.
But of course you need filtering.
See the MK7 or G7 PSU
You don't have the experience to design one of these from scratch, so pick a known design and adapt.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 18, 2011, 05:04:20 PM
So wait... I thought I had accounted for the filtering by multiplying (v - 1.4) times the square root of 2?  What am I missing here?

You don't have the experience to design one of these from scratch

You can say that again

Quote
, so pick a known design and adapt.

The problem is finding the right one to adapt, and knowing how to adapt.  Evidently I'm missing some things.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: zebra50 on January 18, 2011, 05:24:14 PM
So wait... I thought I had accounted for the filtering by multiplying (v - 1.4) times the square root of 2?  What am I missing here?


That's the voltage relationship due to the rectifier.

http://en.wikipedia.org/wiki/Rectifier#Full-wave_rectification
(http://upload.wikimedia.org/wikipedia/commons/thumb/c/cc/Gratz.rectifier.en.png/500px-Gratz.rectifier.en.png)

But then you need to get that bumpy supply as smooth as possible - so you need caps and resistors/ inductors to make it smooth, just like in a guitar amp.


Quote
The problem is finding the right one to adapt

It doesn't matter.
Pick something, preferably something simple that is known to work.
Build it.
Debug it by taking measurements and asking when you get stuck.
Then start again and make something even better.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 18, 2011, 05:30:44 PM
That's the voltage relationship due to the rectifier.

http://en.wikipedia.org/wiki/Rectifier#Full-wave_rectification
(http://upload.wikimedia.org/wikipedia/commons/thumb/c/cc/Gratz.rectifier.en.png/500px-Gratz.rectifier.en.png)

But then you need to get that bumpy supply as smooth as possible - so you need caps and resistors/ inductors to make it smooth, just like in a guitar amp.

Right, this much I understand.

Oh!  right.  The voltage drop across the resistors in the Pi filter sections.  I forgot to account for that.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: Gus on January 18, 2011, 05:32:44 PM
zebra50

You still have the microphone with the switchable heater voltage power supply?

For the new to building a condenser microphone

A hint

The power supply is a big part of the tube microphone.  The details count.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 18, 2011, 05:36:16 PM
Right.  This is actually not my first tube microphone, but I believe you.  My experience with audio circuits (mostly guitar and hi fi amps, a couple microphones, a couple mic preamps) has ALWAYS been that the power supply is anything but an afterthought.

In a tube mic PSU, what are some of the "critical details" in your opinion, Gus?
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: zebra50 on January 18, 2011, 05:37:51 PM
You still have the microphone with the switchable heater voltage power supply?

Hi Gus,

Yes, I have a home built supply with variable tube, capsule and B+ voltages. It gets used a lot for experiments & development.

Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 18, 2011, 05:51:56 PM
Can we look at an example?

(http://img.photobucket.com/albums/v611/maxwall/U47PS.jpg)

Looking at this U47 PSU schematic, what calculations would I need to do to determine the voltage on the secondary of the power transformer?
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: Gus on January 18, 2011, 06:02:50 PM
You need ohms law.  You can figure out what you need from that.  Voltage Current Resistance If you have two you can find the third.

The words I posted before to search for were not a joke, they are things you need to understand how a power supply works.

You should just know things looking at a simple circuit like the PS above 40ma(about the u47 current) 1k 40VDC drop per resistor 1Ks in series.   105VDC 40ma is marked   so you are dropping 160VDC in the supply

Simple fast    1V,1K,1ma   Do you understand the relationship?  A teacher told me >3 decades ago 1K 1V 1ma is a useful thing to know for DC circuits

Do you understand series and parallel what voltage and current do? T and N stuff.

EDIT you are not trying my patience.  Often I will not just give an answer.  I would not be posting if you were trying my patience.

When I look at a tube microphone schematic for the first time I often just use ohms law even if there is no voltages given.  I take a guess at the plate current or look at the voltage divider string and calculate my first guess.  After you work with microphones you can guess capsule voltage or plate current and check it by the calculated grid bias voltage if it is in a workable area of the tube curves.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 18, 2011, 06:06:50 PM
Right.  I have done searches and I do know Ohm's Law.  I get the feeling I'm trying your patience, and I'm sorry if that's the case.  I'm doing my best over here.  I did read the first several volumes of the NEETS manuals in their entirety a few years back, but as I said, I have no formal training.

My problem is that I'm not necessarily sure how to apply Ohm's law in this situation.  I do very well with examples and reasoning from that, but I don't do as well with abstractions.  Thanks for all your help so far.

Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: zebra50 on January 18, 2011, 06:13:25 PM
Which power supplies did you build for your existing microphone projects? I'm sure we can pick an example from those?
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 18, 2011, 06:18:52 PM
The two microphones I built so far were cases of "recipe following."  I built two of the Royer modifications of the MXL 2001, using the 5840 tube.

The PSU for that was a voltage tripler design running off a 24V transformer... would not exactly apply in this case.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: zebra50 on January 18, 2011, 06:22:40 PM
Well, it's a fine microphone circuit design - very well respected and many people here, including myself, have started out with that as a successful first tube microphone build.

If I recall right, it will give you around 105V for B+, and 5 to 6V for the heater. With a chunky enough transformer, you might make it work.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 18, 2011, 06:25:09 PM
A very good point, and duly noted.

However, for purposes of learning, I think I'd really like to learn to do it the more 'conventional' way.  For me most of the fun of doing it myself is the learning part.  Additionally, while I'm at it, I'm more keen to do it the way that would make for the very best sounding microphone I could possibly make.  I don't want to spend good money on components only to sabotage myself with a power supply that might hold me back.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: boogietube on January 18, 2011, 07:13:38 PM
I learned a lot from studying the Gyraf PSU in this thread, There's also some great math there:
http://www.groupdiy.com/index.php?topic=25597.0
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 18, 2011, 07:55:31 PM
A couple of questions:

1) How do I determine the approximate amount of current that a tube microphone will use?  Filament current would be 200mA, if I understand correctly.  How do I determine the approximate current draw of the HT supply before having built the microphone?  Or at least make an educated guess?

2) Once I determine the approximate current draw of the HT circuit, how do I, for a given schematic, work backward to determine the necessary PT secondary voltage?  Using the U47 schematic above as an example, would I take the current draw, multiply it by the resistance, and get the voltage drop across that resistor?  Then add that to the total and proceed down the line until I get to the output of the bridge rectifier?

3) I assume, from looking at other viable schematics, that I can tweak the final voltage with a voltage divider (perhaps even variable) at the output?

Would love to know if I'm on the right track here.  I don't have the good fortune to have someone I can ask in person, a tutor, or anything like that.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: tommypiper on January 19, 2011, 12:45:50 AM

Simple fast    1V,1K,1ma   Do you understand the relationship?  

No.  Care to explain or point us to more information?  It's otherwise too oblique to know what you have in mind.  I never had a teacher, so I never came across anything like that.  I have no clue what you're referring to.   ???


A teacher told me >3 decades ago 1K 1V 1ma is a useful thing to know for DC circuits


Sounds useful ?  :D If I knew what it meant or what it was referring to? 


 T and N stuff.



This has got to take the cake for obscure references!  I've been hanging out on this board and built some things and I'm bright and a quick learner, yet never have I heard anyone say, "dude, T and N stuff, you know?" 

A lot of us have no electronics training, OK?  We just do DIY to the best of our ability and try to learn as we go.  We're not dumb, we're just not used to obscure "hints" from people who have been at this for, what did you say, 3 decades, and had a teacher...?  Ten times longer than many of us here!!!!!

I really don't get the whole obscure, hint from a master approach mentality that means nothing without explanation or at least referencing and context or some guidance...   please.  If you insist on only hinting and not explaining or helping in a usual fashion, please guide one to further contextually applicable material that will reveal these hidden gems of wisdom.   ;)
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: MagnetoSound on January 19, 2011, 04:06:22 AM

Simple fast    1V,1K,1ma   Do you understand the relationship? 

No.  Care to explain or point us to more information?  It's otherwise too oblique to know what you have in mind.  I never had a teacher, so I never came across anything like that.  I have no clue what you're referring to.   ???



This is Ohm's Law!

One Volt into 1K gives 1mA, simple as that.  :)

(I have no idea what is meant by T and N stuff either.)

Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: zebra50 on January 19, 2011, 04:14:07 AM
I assumed that was Thevenin & norton equivalent circuits.

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/thevenin.html
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: Gus on January 19, 2011, 06:03:27 AM
I posted words to search for in a post in this thread.  If you typed them in google you would find things to read.
Tommypiper did you read my other posts in this thread?

IMO one needs to know at least Ohms law to understand simple things like passive power supplies.  Just google bing etc.

The 1V 1K 1ma stuff makes it very simple.  I knew how use Ohms law >30years ago.  The1V 1K ohm 1ma relationship lets you do math fast in your head for simple resistor circuits.  Example follows

Look at the PS schematic 40ma at 105 VDC is marked as what the tube microphone "needs".
Series circuit of 1Ks I "see" 40VDC across each of the the 1Ks.
1ma and a 1K is 1 volt
40ma across 1k = 40VDC, 4 in series 160VDC

Or use a calculator and do the simple math.  .040A X 4000 ohms = 160VDC

Then do the math for DC power to find the power rating needed for the resistors.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: 0dbfs on January 19, 2011, 06:43:19 AM
So, with 160V of drop across the 4 1k's...

Add that to the 105V that is left-over/indicated/specified...

Gives us 265V..

The mic is a divider = 2625 ohms (in order to provide 105 at the node).

Start plugging in some number here (and/or elsewhere):
http://www.bowdenshobbycircuits.info/r2.htm

Looks like the 4x 1k's dissipate a total of about 6.4W. P=IE, P=.04A*160V, 6.4W=.04A*160V

I see the PSU schematic specifies 2W. I guess watts add in series because if they didn't, it wouldn't work and they wouldn't have sold many mics.


Now, with 265V at the bridge what does the BV03 need to provide, and what about those 50uf caps?

I see an indication of 250V/75mA at the bridge. Looks like additional info that might be helpful.

Best,
jonathan
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: gary o on January 19, 2011, 08:26:19 AM
This is interesting thread.....Im afraid Im just a builder too & struggle greatly to understand stuff like ohms law or more how to apply it....I also find the criptic clue type approach doesnt help me tho I appreciate anyone trying to help me  :) I no Im a pain.....also the mathmatical way some of the wesites explains stuff also blows my mind.....I used to get ohms laws....about 15 years ago Saturn sounds Ashly Styles told me how to apply ohms law to C28A PSU I was building, I got it ,it worked he had a great way of explaining to me that I understood...
So, with 160V of drop across the 4 1k's...

Add that to the 105V that is left-over/indicated/specified...

Gives us 265V..

The mic is a divider = 2625 ohms (in order to provide 105 at the node).

Start plugging in some number here (and/or elsewhere):
http://www.bowdenshobbycircuits.info/r2.htm

Looks like the 4x 1k's dissipate a total of about 6.4W. P=IE, P=.04A*160V, 6.4W=.04A*160V

I see the PSU schematic specifies 2W. I guess watts add in series because if they didn't, it wouldn't work and they wouldn't have sold many mics.


Now, with 265V at the bridge what does the BV03 need to provide, and what about those 50uf caps?

I see an indication of 250V/75mA at the bridge. Looks like additional info that might be helpful.

Best,
jonathan


Is the 6.4W shared between the 4 1K resistors 1.6W each ?

As said great thread.....

I have a transformer puts out 240VAC, 250 ma I want to run a MK47 mic kit 105VDC 40ma and so gonna try to apply all this to make this off to the shop to get some resistors

Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: 0dbfs on January 19, 2011, 08:59:15 AM
I found this in the PSU META:
http://www.kpsec.freeuk.com/powersup.htm#trsr

And going through that gives me about 190V at the transformer secondary referring to the U47 PSU schematic listed above. Max's mk47 specifies a 200V secondary and is slightly different but mostly the same. So, we are in the ballpark here with the theory and a known good implementation anyway.

240V at the secondary would require more dropping across the CRC filter stage.

Rectified / CRC'd 240V = 340V.

Need to drop it about 235V for 105V @ 40mA.

235 across 5k8 @ 40mA = E=I*R = 232 = 5800R*0.04A

5k8 across four resistors in series is a little less than 1.5k per resistor.

I would start by looking at some of those values and plugging it into max's mk47 PSU which also has an inductor and a tweaker pot/rheostat for fine tuning.

Also need to figure your dissipation in watts for the dropping resistors and overrate them so they don't burn up.

Not sure yet exactly how the inductor fits in WRT resistance so I would need to look at a datasheet for the inductor max specifies and probably look up more equations to understand it a little better. But the inductor generally opposes AC and passes DC based on the mH, and apparently has wattage/resistance ratings that play into it.

Best,
jonathan
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 19, 2011, 09:44:13 AM
Thank you all so much.

The "missing link" was right under my nose the whole time-- the 40mA listed right on the U47 schematic.  I understand the math now.  As for the question of the bridge rectifier-- that circuit used selenium diodes, correct?  Selenium rectifiers had a higher voltage drop than silicon diodes.  I suppose this voltage drop would depend on current/load, but in any case it would be greater than with silicon diodes, and it would be multiplied by 2 since two diodes in a bridge rectifier are always conducting.

Now comes the hard part, for me.  I have to figure out how to parse how much of the U47's 40mA current draw is due to the inefficient manner in which the filament supply happens.  I don't know how to do this, but maybe I can reason it out.  

Looking at the one limited data sheet for the VF14 I can find, I see a specification called "Heizstrom."  Plugging this into my little translator here, that is German for "filament current."  The specification given is 0,05, which I assume means 0.05A?  But wait... that's 50 mA, and the whole mic supposedly draws 40mA.  And the heater supply in that mic is inefficient and generates a lot of heat across the 1k78 resistor in series with the heater, which drops 69 volts according to the schematic (which would seem to add another 38.76mA, doing the math according to Ohm's law) and we haven't even supplied B+ or capsule polarization yet.

what am I missing?  I really am trying hard, I promise.   :)
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: MagnetoSound on January 19, 2011, 10:10:51 AM

Current across resistors in series is constant at each point (the same through the heater as through the dropper resistor).

Plate voltage drops from 105v to 34v across two resistors, and polarisation is negligible.

Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 19, 2011, 10:32:40 AM
OK so if I understand you right--

Most of the current drawn by the mic is via the heater supply, correct?

Since it's under-heated, perhaps it draws less than the full 50mA... hence the 40mA number.

Do I have that right?

So in your opinion, for my project-- with a regulated 6V supply for the EF12 heater, I can assume a very small load for the 105V supply for the rest of the mic?
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: MagnetoSound on January 19, 2011, 11:13:39 AM

Yes, yes and yes.

Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 19, 2011, 11:26:18 AM
beautiful.  Thanks so much.  I'm working on a schematic right now.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: 0dbfs on January 19, 2011, 11:40:13 AM
Please post your schematic when you are ready!


So in your opinion, for my project-- with a regulated 6V supply for the EF12 heater, I can assume a very small load for the 105V supply for the rest of the mic?

Should be small compared to the heater current but will need to be close to compute the voltage drop across the CRC in the PSU.

I believe the plate-current is largely dependent on the cathode/grid operating points and if you know plate current and voltage-drop you can then figure out the load. This gets into analyzing the tube data sheets (which there may be none for an ef12 in triode mode).

How does one estimate the effective plate-to-cathode resistance (other than measuring a range of operating points / V&I)?

Will you be using fixed bias with the cathode seeing a fixed voltage between heater return and 0V or will you use an auto-bias scheme maybe with bypass cap?

If the cathode bias is coming from the heater line a regulator may introduce undesirable artifacts into the grid circuit.

These tube mic circuits sure do look pretty simple. I learned a little more today so thanks.

Best,
jonathan
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 19, 2011, 11:42:45 AM
Ah, I see.  This gets more complicated than I thought!
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 19, 2011, 11:47:10 AM
Please post your schematic when you are ready!


will do!

Quote
Should be small compared to the heater current but will need to be close to compute the voltage drop across the CRC in the PSU.

I believe the plate-current is largely dependent on the cathode/grid operating points and if you know plate current and voltage-drop you can then figure out the load. This gets into analyzing the tube data sheets (which there may be none for an ef12 in triode mode).

Still trying to understand what you're saying here.  I do have data sheets for EF12.  Don't know if they include any provisions for triode connection.

Quote
Will you be using fixed bias with the cathode seeing a fixed voltage between heater return and 0V or will you use an auto-bias scheme maybe with bypass cap?

Here's what I've designed--

(http://img.photobucket.com/albums/v283/jazz347/47-inspired%20mic%20build/EF12_47_11.png)

Quote
If the cathode bias is coming from the heater line a regulator may introduce undesirable artifacts into the grid circuit.

Would love to hear more about this.  What type of undesirable artifacts are you talking about?

Thanks again!
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 19, 2011, 11:50:36 AM
data sheets for EF12--

Telefunken
http://www.mif.pg.gda.pl/homepages/frank/sheets/076/e/EF12.pdf

Philips (English)
http://www.scottbecker.net/tube/sheets/030/e/EF12.pdf
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 19, 2011, 11:55:57 AM
been doing some searching about artifacts from linear regulators in audio circuits. Seems that they allow some ripple through and add some noise of their own.  Perhaps I should avoid this and do a more standard DC supply with a choke.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: 0dbfs on January 19, 2011, 11:57:17 AM
Ah, I see.  This gets more complicated than I thought!

I don't know that for sure (whether it get's more complicated or not) so I wouldn't take my word on it but this is what I have observed in several instances.

Looks like the curves are for triode connected so that makes it easier. I don't remember exactly right now how to decipher/determine your operating points from the curves but there are threads around here about that too.


Would love to hear more about this.  What type of undesirable artifacts are you talking about?
Hum. It's because the heater line has to have absolutely no ripple if the cathode is referenced to it or it interacts with the grid operating point WRT the cathode. And if the grid voltage moves... That turns into audio.




Best,
j
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 19, 2011, 12:06:01 PM
Cool.  What I should probably do then is forego the regulation and create a standard DC filament supply with an LC and some RC pi filters and a voltage divider/rheostat allowing me to fine-tune heater voltage within a range of, say, 5-6.3v

Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: rodabod on January 19, 2011, 01:09:39 PM
Just a quick point; your curves change as you underheat. Any bias voltage assumptions may at this point be thrown out the window if you so wish!
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 19, 2011, 01:13:27 PM
I'm revisiting lots of things right now.  Judging from this curve, if I'm reading it correctly:

(http://img.photobucket.com/albums/v283/jazz347/47-inspired%20mic%20build/Screenshot2011-01-19at11200PM.png)

If I had a B+ of, say, about 40-50 volts or less, and a bias of about 1.7 volts, then (assuming no underheating), current draw would be somewhere in the half a mA to 1mA range.

Right?

This is very little.  Does it seam unrealistically small?

Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: MagnetoSound on January 19, 2011, 01:37:55 PM

Yes, now compare to (underheated) VF14.  ;)

Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 19, 2011, 01:42:04 PM
Yes it's unrealistically small, or yes I'm reading the graph right?   :)
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: MagnetoSound on January 19, 2011, 01:43:12 PM

It's not unrealistic.

Do your Ohm's Law on the U47 schem.

Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 19, 2011, 01:46:16 PM
The U47 circuit has 105V input, 130k plate resistance, and 34V plate voltage.  This is a voltage drop of 69v.  Dividing this by the 130k resistance gives me .53mA.

Is my logic sound, and would this be correct?
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: MagnetoSound on January 19, 2011, 02:00:48 PM
Your logic is sound, but your answer is incorrect!

Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 19, 2011, 02:11:23 PM
that's because of a simple mathematical error, I think:

voltage drop is 71V, not 69 (doh).

71 / 130 = 0.55mA.

Correct now?
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 19, 2011, 02:11:52 PM
Actually, I know it's correct, because 0.55mA is printed right on the schematic!   :D
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: MagnetoSound on January 19, 2011, 02:14:26 PM


... and relax :)

Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 19, 2011, 02:38:28 PM
You've been super helpful.  I can't begin to thank you enough.  I'm learning a lot and that is always good.

Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 19, 2011, 05:12:16 PM
Please post your schematic when you are ready!

Here is my first draft.  Can anyone see any glaring problems?  

Most of what I did is copied from other schematics and adapted a bit.  You will see obvious nods to the mk7 power supply.  I added voltage dividers/rheostats at the end to allow fine-tuning.  I added fixed resistors before the rheostats so that the voltage could never fall below a certain threshold, so that the entire range of the pot would be 'close.'

Please let me know where (if anywhere) I went wrong.

Thanks!

(http://img.photobucket.com/albums/v283/jazz347/47-inspired%20mic%20build/Screenshot2011-01-19at51014PM.png)
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 20, 2011, 12:54:50 PM
After showing someone my first draft schematic (pictured above), they expressed concern with all the resistance on the B+ rail.

Do you smart folks think that's a problem?  Is there anything else about my idea that looks wrong?

Thanks again,

--brad
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: boogietube on January 20, 2011, 02:18:58 PM
Nothing looks wrong to me, but I'm not really qualified. The only problem might be sourcing 370 and 420 ohm 1 watt resistors....
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: moamps on January 20, 2011, 06:48:16 PM
Hi,
that 370 ohms in front of the 6V/200mA heater doesn't look correct to me.
Regards,
Milan
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: MagnetoSound on January 21, 2011, 06:30:34 AM
Doesn't look right to me.

B+, what DC voltage are you expecting to see after rectification/smoothing?

Once you have this you can work out the total resistance required for the drop to 105 @ 0.55mA, and then work out the required drop at each filter stage.

The trimmer if you want one, can be simply a rheostat towards the end of the string, you don't need to have a shunt there, except maybe a high value to bleed off in case of no load.

Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 21, 2011, 09:51:00 AM
I was looking for 105VDC.

I asked the advice of a guy who taught me some stuff.  I haven't talked to him in years and he's busy.  But he said he'll draw me up something when he has time.  I hope he does. 
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: gary o on January 21, 2011, 09:59:51 AM
Thankyou Odbfs I have been breadboarding without the board bit of a scary mess but helps me to understand whats going on I no Im a bit slow but am getting there

quick drawn schematic.....requirements little bit different to Soap foot looks like he is doing ok there good luck soap foot.....I have 240VAC 250ma transformer I want to use to power a MK47 mic 105VDC 40Ma Im getting close with this circuit dont know if Im doing anything wrong with the resistances here

http://good-times.webshots.com/photo/2160397500103387484DfTAMC

from learning from this thread I now suspect a mic PSU I made some time back is under powering one of my mics....

Question what will happen if a tube mic cannot draw enough current from the PSU for its B+ ?

Anyway I have extra voltage in my PSU so I want to add a 120VDC output for a C12 type tube mic Im still a little confused about current draw....Im not sure what the C12 B+ requires but Im sure it will be a lot less than 40ma....so do I need to limit the current or does the mic only take what it needs?

thankyou for your patience
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: 0dbfs on January 21, 2011, 10:08:43 AM
If you close that 2k3 switch (or just connect the 2k3 across the output of the supply) that approximates the load the mic presents and you should be able to tweak until you get your 105V.

The mic will only draw the current it needs. If your supply can provide more that is fine. If you try to draw more than is available, something will most likely burn up (resistor, transformer, etc).

Cheers,
j
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: MagnetoSound on January 21, 2011, 01:21:13 PM
I was looking for 105VDC.

Yes, you want 105V at the output but you need to know what the voltage will be after the rectifier, at the first smoothing cap. Then you can calculate the drop you'll need to get your 105V (use Ohms Law again, drop voltage/0.55mA, to find the total R).

Rectifier relationships are good to know: http://www.davidbridgen.com/rects.htm

With a bridge rectifier you are going to have a DC of approx 1.4 times the RMS voltage at the secondary, so ... ?

Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: gary o on January 21, 2011, 02:25:06 PM
Thanks J yep i did the 2K3 load resistor thing I can adjust my last resistor before the pattern section to provide the 105VDC....Im now getting greedy and can see the opportunity to get 120VDC for the plate of my C12 type mic....so as you say the C12 or any load draws only what it needs in power as long as enough is available ........

Soapfoot can you bread board your circuit....

cheers
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 21, 2011, 02:27:00 PM
MagnetoSound: That's useful info.  Thanks again, sincerely.

I'm going to digest this and try again.  I admit that I'm getting a little bit overwhelmed/frustrated so I might take a step back and come back tomorrow with a clear head and try again.  I wish I had more knowledge so I knew what I was doing.

Thanks again for all your help so far.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 21, 2011, 02:31:59 PM
Soapfoot can you bread board your circuit....

I intend to eventually, but not yet.  I want to make sure I at least have some clear idea and some better understanding so that I make sure to at least purchase the correct "expensive" parts (i.e. choke, transformer). 

Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 21, 2011, 02:41:50 PM
I was looking for 105VDC.

Yes, you want 105V at the output but you need to know what the voltage will be after the rectifier, at the first smoothing cap. Then you can calculate the drop you'll need to get your 105V (use Ohms Law again, drop voltage/0.55mA, to find the total R).

Rectifier relationships are good to know: http://www.davidbridgen.com/rects.htm

With a bridge rectifier you are going to have a DC of approx 1.4 times the RMS voltage at the secondary, so ... ?


OK, so here's what I have so far...

Using a choke input filter for the filament/bias supply, given a PT secondary of 12VAC and a filament current draw of 200mA, after the choke I should have 10.8VDC and the current draw should now be 188mA.

Using a capacitor input filter for the HT supply, given a PT secondary of 160VAC and a current draw of .55mA, after the first capacitor I should have 225.6VDC, and a current draw of 0.34mA.

Is this correct?

If so, what's the best way to drop 120 volts to get to the final 105V supply?  High resistor values in the pi filters?  A simple voltage divider or series resistor at the end?  It seems that 360K of total series resistance would drop that much, but I don't know if that's the right way to go.  Something tells me "probably not."

Thanks again.  I'm ready to attack it again.  My "step back" was short-lived.  I want to learn and get this thing.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: gary o on January 21, 2011, 03:43:10 PM
Im struggling with the ohms law maths myself so I might not be much help but Im not sure the current you want to draw when you say .55ma & .34ma so thats in the region of half of 1 milliamp?...... are you doing your ohms law sums in Amps.....Im my case I want 40ma which is I think    .045A

 so 1 mA = 0.001 A  ....so your .55 = 0.00055 Amps

Sorry if I got this completly wrong

Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: gary o on January 21, 2011, 03:53:00 PM
Just been fiddling with the numbers & im adding up the same as you Soapfoot In your B+ circuit.....takes so long to sink into my brain
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 21, 2011, 04:15:18 PM
OK, I tried again.

Does this look any better?

(http://img.photobucket.com/albums/v283/jazz347/47-inspired%20mic%20build/Screenshot2011-01-21at41539PM.png)
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 21, 2011, 04:20:06 PM
By the way, my friend who offered to help me was really talking up regulation using 3-pin regulators.

However, over on PSW in a discussion about power supplies, David Bock says that he doesn't care for that type of design.

So who knows.  I'm kinda lost, but at least I'm learning.  I don't know what I'm going to do in the end.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 21, 2011, 06:36:30 PM
So I was looking at the old Neumann NG schematic.  This PSU has a really good rep for being very good and quiet.  It appears to have some sort of twin-coil choke.

(http://www.groupdiy.com/index.php?action=dlattach;topic=31210.0;attach=149;image)


So I looked and found a Triad choke that's just a few bucks.  15H at 20mA.  I drew up a schematic with two of these on the B+ side.  I figured the 20mA rating would be plenty, since unlike the original U47, the B+ side of this circuit would only draw about half a mA of current.  

(http://img.photobucket.com/albums/v283/jazz347/47-inspired%20mic%20build/Screenshot2011-01-21at82622PM.png)

Aside from being a slight extra expense (about 25 bucks extra) and maybe a bit of overkill, anyone see anything wrong with this?
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 21, 2011, 08:18:49 PM
Updated the schematic in the post above--

I downloaded an emulator for my computer so that I could download a simulator for the circuit.   :)

Downloaded PSU Designer II from Duncan Amps. http://www.duncanamps.com/psud2/index.html

Had to download a program to allow me to emulate Windows 98 on my Mac, though, but I got it working.  Handy little program, that is.

This allowed me to correct a few voltages on the schematic, and to see that I wouldn't have been able to get the voltage up where I needed it running just an LC filter for the first stage of the filaments like I had originally planned.  So I'll sacrifice a little bit of regulation to gain a little more voltage, and run a Pi filter instead. 
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: zebra50 on January 22, 2011, 03:49:49 AM
Something funny going one... that looks like a lot of current through the 820R! Imagine a scenario where you have 105V and that pot in the middle. R = 820 + 50 R

V=IR, I = 105V / 870R = 0.12 A

Power = I-square-R = 0.12 * 0.12 * 870 = 12.7 watts!

Did you mean 100K and 820K for those positions?


Why not use the MK7 / MK47 design and tweak the resistors to get your 105V? It's very close in function to what you've drawn, and it is well tested.

(http://www.twin-x.com/groupdiy/albums/userpics/MK7_psu.JPG)
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 22, 2011, 04:02:58 AM
Good eye.  Yeah, I might do 100k and 820k instead.  I might also use/adapt the MK7 design.

Just curious about that one though... what is the purpose of the .1 cap and 3W 4.7R resistor before the first Pi filter?  I've never seen that configuration before and it looked strange to me.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 22, 2011, 04:18:26 AM
OK, how about this?

(http://img.photobucket.com/albums/v283/jazz347/47-inspired%20mic%20build/Screenshot2011-01-22at41739AM.png)

Now I might be off-base, but I thought it was good to keep the impedance down in the supply rail so that fluctuations in current didn't cause the power supply to sag, correct?

Would a high resistance in the voltage divider cause the supply to sag under load?  Of course we're dealing in fractions of mA here so it practically probably wouldn't be a big deal, right? 

Anyway, I went with 10K and 8k2, and used a 1K trimmer.  That's a compromise that puts power dissipation at about 1.5W for most settings, which with a 3W trimmer should be fine, right?
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: MagnetoSound on January 22, 2011, 04:41:18 AM
Using a capacitor input filter for the HT supply, given a PT secondary of 160VAC and a current draw of .55mA, after the first capacitor I should have 225.6VDC, and a current draw of 0.34mA.

Is this correct?

No - current doesn't change! That 0.34mA is wrong (I'm guessing you extrapolated that from the rectifier chart I linked to, but that is the AC current in the transformer secondary, not relevant to your DC calculations.)  The current we are concerned with is the known DC current drawn by the tube, 0.55mA).

225.6 is nearly right. (There is 1.4V double-diode drop through the bridge - each diode will drop 0.7V and two are conducting at any one time.) For a secondary voltage of 160VAC, you are going to have a rectified DC of 222V. (158.6 x 1.4)


Quote
what's the best way to drop 120 volts to get to the final 105V supply?  High resistor values in the pi filters?  A simple voltage divider or series resistor at the end?  It seems that 360K of total series resistance would drop that much, but I don't know if that's the right way to go.  Something tells me "probably not."

Yes, much higher resistance in the filters.

Your tube wants 0.55mA @ 105V. 

You need a voltage drop of 117V, and you have a current of 0.55mA to make it happen. Ohms Law for this is R=E/I, so 117/0.00055 which is 212727, or approx 213k.

Thus you are looking for a series of preferred values that make up close to 213k in total (maybe a little less to allow range for the trimmer, but you don't want to shunt too much current to ground).


The MK7 PSU is indeed a great helper. Can you spot the mistake in the marked voltages on the HT side? What should it say?  :)

Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 22, 2011, 10:34:19 AM
No - current doesn't change! That 0.34mA is wrong (I'm guessing you extrapolated that from the rectifier chart I linked to, but that is the AC current in the transformer secondary, not relevant to your DC calculations.)  The current we are concerned with is the known DC current drawn by the tube, 0.55mA).

225.6 is nearly right. (There is 1.4V double-diode drop through the bridge - each diode will drop 0.7V and two are conducting at any one time.) For a secondary voltage of 160VAC, you are going to have a rectified DC of 222V. (158.6 x 1.4)

You're right, that's exactly the mistake I made.  I'll make the necessary adjustments.  Thank you.

However, some of my voltages come from simulations I ran using PSU Designer II.  I downloaded a Windows emulator and this program and ran simulations on my design.  It was really instructive.


Quote
Yes, much higher resistance in the filters.

Your tube wants 0.55mA @ 105V.  

You need a voltage drop of 117V, and you have a current of 0.55mA to make it happen. Ohms Law for this is R=E/I, so 117/0.00055 which is 212727, or approx 213k.

Thus you are looking for a series of preferred values that make up close to 213k in total (maybe a little less to allow range for the trimmer, but you don't want to shunt too much current to ground).

OK.  So let me ask you this question-- actually a series of questions.  Just so that I can learn...

1) What is the disadvantage to "shunt[ing] too much current to ground" besides needing components with high power handling capability?  What are the problems and potential pitfalls to that design?

2) Doesn't the amount of resistance in the filters increase the voltage drop across those resistors as more current is drawn?

3) Wont the .55mA of current draw vary somewhat depending upon what is happening in the tube-- how much the tube is conducting?  This is the case in other tube circuits I've worked on.

4) If (2) and (3) are true, then won't that cause the voltages to "sag" dynamically under load... 'browning out' the voltage as the load draws more current?

5) If (4) is true, wouldn't that cause a type of distortion especially when the tube is conducting near its maximum, made worse as resistance in the B+ string is increased?


Just trying to get a handle on this.  Thanks.

--brad
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 22, 2011, 11:46:57 AM
What if I just tried to find a transformer that had an 80V secondary or 100V secondary?  Then I wouldn't have to drop so much voltage.  I've been looking all morning but a transformer with an 80V and 12V winding doesn't seem to be a "standard" item.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: MagnetoSound on January 22, 2011, 03:16:21 PM
Wont the .55mA of current draw vary somewhat depending upon what is happening in the tube-- how much the tube is conducting?  This is the case in other tube circuits I've worked on.

It will vary somewhat, but how much? The tube is biased to an operating point which sets plate current at a point somewhere between fully on and fully off. Because we are dealing with really tiny signals, it can be biased fairly close to cutoff. At the point we've chosen the tube draws 0.55mA. From there it is modulated by the AC signal on the grid. As the grid goes positive, the current increases and the voltage on the plate drops. As the grid goes negative, the current drops and the voltage increases. But the signal at the plate is in the range of a few tiny millivolts - not enough to make a real difference across a resistance designed to drop more than a hundred volts.

Doesn't the amount of resistance in the filters increase the voltage drop across those resistors as more current is drawn?

Yes, that's how Ohm's Law works. You need to drop 117V - that's quite a lot, so you need enough series resistance, otherwise you end up wasting heat in a high-wattage shunt for no practical gain. Granted, the supply will be stiffer - but this is not a power amp, it is a tiny mic amp with no significant loading on it.

What is the disadvantage to "shunt[ing] too much current to ground" besides needing components with high power handling capability?  What are the problems and potential pitfalls to that design?

You try to avoid wasting power. Your mic is asking for a tiny amount of power to make it work, not quite 60mW! Any extra power that you pull through shunt resistance is not contributing anything except extra heat inside the PSU. Not a problem and it might not bother you, but too much is not good practice.

Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: MagnetoSound on January 22, 2011, 03:20:17 PM
What if I just tried to find a transformer that had an 80V secondary or 100V secondary?  Then I wouldn't have to drop so much voltage.  I've been looking all morning but a transformer with an 80V and 12V winding doesn't seem to be a "standard" item.

You'd probably find one with a 120V sec without searching too hard. You can always use a separate transformer for the filament supply. Or just use what you have and adjust the resistors.

Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 22, 2011, 04:35:18 PM
Thanks for your answers and explanations.  It makes sense to me. 

I haven't purchased a transformer yet.  I thought about doing a separate xformer for the filament supply, as you suggested and may indeed go that route.  5 coils in one PSU!  One heavy beast.

I saw this on Edcor's site:

Quote
EDCOR charges a $40.00 design and setup fee for quantities less than 10 pieces. We have a large database of designs going back over 30 years and if we already have the transformer on file then the design and setup fee is waived.

I sent them a note asking if they had anything similar/close to what I needed.  We'll see what they say.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 22, 2011, 05:49:32 PM
No luck on the Edcor front.

Now I'm deciding... do I waste a bunch of extra power with the 160V PT, or do I make my PSU heavier and more expensive than it needs to be?
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 22, 2011, 06:21:03 PM
I found this transformer, Hammond model number 262B24.

It has a 120V secondary at 27mA (should be plenty for the .55mA B+ supply), and DUAL 12V secondaries at .2A apiece.  Am I correct that I can connect those dual 12V secondaries in parallel for double the current rating, having 12V @ .4A?

If so, this might be a good transformer for my needs.

OR I might just ante up and have Edcor make me a custom one, which if my guess is right would probably run around 65-70 bucks.  The Hammond would probably run around 27 bucks.

http://angela.com/hammondlowpowerbiaspowertransformer262b24.aspx

Hmm.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: MagnetoSound on January 22, 2011, 06:50:48 PM

That one looks fine.

I'm going to stop here. All you need is in this thread, zebra50's points about the merits of adapting a proven design are extremely worthwhile (that's how we all learned it), and you have what you need to do that. I strongly suggest you go back and re-read everything that has been posted here regarding Ohms Law and rectifier relationships at least once and thoroughly digest it before you begin.

Good luck!  :)

Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 22, 2011, 07:01:46 PM
I will do that.

Thanks for all your help.  I've gotten some help from some other sources, too, and it's all coming into focus.  I don't know that there's much left except for me to review and adjust my designs, order some parts and start breadboarding. 

Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 23, 2011, 02:17:49 AM
Antique Electronics Supply has a transformer with a 140V secondary and a 10.5 @ 2A secondary for 20 bucks.  Looks nice.  I ran some simulations in PSU Designer II, tweaked some values, and this is what I have right now:

(http://img.photobucket.com/albums/v283/jazz347/Screenshot2011-01-23at23314AM.png)
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: boogietube on January 23, 2011, 08:20:07 AM
Just so I'm following you, it's this transformer right?

P-T292

Catalog Page 36

Transformer, Power

    * 120 V primary

    * Secondary 1 - 140 V, 100mA
    * Secondary 2 - 28 V, 20mA
    * Secondary 3 - 10.5 V, 2A


Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on January 23, 2011, 11:04:19 AM
yeah, that's the one I was looking at.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on March 01, 2011, 04:33:48 PM
OK!  I completed this PSU.  It came out great so far.  This weekend I'll go pick up an oscilloscope to make sure everything is quiet and ripple-free, but for now I'm optimistic.  For the most part, all my math was correct thanks to the help of people on this thread/forum.  I had to change one resistor from 5k1 to 470R to get the right adjustment range on the B+ side.

Here's my final schematic (unless there are problems with ripple that necessitate further changes). 

(http://img.photobucket.com/albums/v283/jazz347/47-inspired%20mic%20build/Screenshot2011-03-01at42222PM.png)

On the filament side, my adjustment range with the pot, while hooked up to an actual filament of an EF12 tube, is 3.5V to 6.6V.  This seems like a pretty good range... there's some 'excess' on the ends but the range is narrow enough that I can fine-tune the sweet spot between 5 and 6.3V in the middle.  It's not "idiot proof," but there's nothing there that's too dangerously far out of range, either, for short-term trimming.

On the B+ side, assuming a current of .55mA and a target voltage of 105V, I estimated a 200K resistor to simulate the load of the mic.  With a 200K load resistor, my adjustment range on the B+ side is between about 101V and 110V.  This seems like a pretty safe, narrow range around 105V that will allow for very precise fine-tuning.

I went total overkill on the build.  I even added knobs on the internal pots... what the hell, why not... I had some extra-ugly ones laying around.  I still am waiting on a 7-pin Binder connector to use for the tube mic cable connection.  Here are some pics:

(http://img.photobucket.com/albums/v283/jazz347/47-inspired%20mic%20build/DSC00615.jpg)
(http://img.photobucket.com/albums/v283/jazz347/47-inspired%20mic%20build/DSC00617.jpg)
(http://img.photobucket.com/albums/v283/jazz347/47-inspired%20mic%20build/DSC00621.jpg)
(http://img.photobucket.com/albums/v283/jazz347/47-inspired%20mic%20build/DSC00606.jpg)
Title: ec
Post by: gary o on March 01, 2011, 07:59:58 PM
Cool loking good :)
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: mad.ax on March 02, 2011, 09:27:21 AM
Your voltage adjustment range is a bit short for my taste. For instance, the B+ is a little short from +/-5%. Now what happens if your mic ends up in a place were AC mains is 10% higher or lower than whatever it is now?

Axel
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on March 02, 2011, 09:50:44 AM
true, and duly noted.  I'll keep an eye on that for the future.  I was trying to strike a balance between adjustability, fine control of adjustability, and not having anything so far outside the range as to be immediately dangerous for the mic should it get bumped out of adjustment or whatever.  But your point is very well taken.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: soapfoot on March 14, 2011, 05:53:37 PM
Just for an update: after discussing this with lots of bright people, I've chickened out on the EF12 and will be using an EF800 instead.  After these discussions, I was wanting to use a higher plate voltage with the EF12 than my supply was designed for, and rather than starting all over with a different supply, I just decided to use an alternate tube.  At first I wanted to run the EF14, but the heater current used by that tube would've dropped my heater voltage too low, and there was no easy way to modify my supply without this short of regulation, which would've been a big re-design.  The EF800, with its heater current draw of about 275mA (compared to 200mA for the EF12 and 470mA for the EF14) seemed to be a good choice-- and Oliver Archut already has a schematic for a 47-inspired mic using that tube.

Once I get the build underway, I will start a more appropriately titled thread.  Thanks everyone for your input here.  I've learned a lot that has helped me throughout this process already.


Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: gyraf on March 16, 2011, 04:47:12 AM
Please note, that for a power supply stabilized like this, it's VERY important to have the microphone plugged in BEFORE turning on the power.

Otherwize high voltage will build up in the last reservoir capacitor and discharge with a shock when you plug in your mic...

Jakob E.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: Phrazemaster on October 16, 2012, 10:00:30 PM
Hi Brad, I know this post is old but I had a question on your PSU. I decided to make a U47 clone like you, and I'm going to go with an EF12 tube. I actually had the privilege of speaking with Andreas Grosser today and he suggested this tube. I'm going to buy it from him, tested, for a good price.

I wanted to ask what you would do differently on this PSU - I know you originally had envisioned using an EF12 as well.

Additionally, I had a question about the orange symbol that looks like a circle with a cross in it. I'm not a total newbie, but I've never seen that symbol before.

So, if you had to do it over and use the EF12 after all, knowing what you now know, what would you do differently? I'm reposting your last schematic.

Also, had you considered adding some kind of soft-start to the psu?

Thanks for your time, or anyone else who wants to chime in.

Mike
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: bockaudio on October 20, 2012, 05:47:13 PM
Right.  This is actually not my first tube microphone, but I believe you.  My experience with audio circuits (mostly guitar and hi fi amps, a couple microphones, a couple mic preamps) has ALWAYS been that the power supply is anything but an afterthought.

In a tube mic PSU, what are some of the "critical details" in your opinion, Gus?
Remember, an amplifier is really a power supply modulator.
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: btown2009 on June 27, 2013, 01:08:03 AM
Quote
From what I've read, the EF12 was sort of a precursor to the EF86.  Can anyone validate?

That's what I understood too. . That's where I started out with my STU-47 - I used Max's EF86 circuit and then tweaked (paying attention to heater current and max voltages). Best thing is to look at the data sheets.

Where is the circuit at???
Title: Re: Insight into "U47-inspired" with EF12 tube?
Post by: zebra50 on June 27, 2013, 02:16:51 AM
Well, the one I used for my 'Stu47' is in the thread which I linked in my first post...
http://www.groupdiy.com/index.php?topic=39541.0

By "Max's EF86 circuit" I meant IOaudio's MK7 or MK47 project. I get confused by his numbering but the earlier one that uses the EF86.

Cheers!
Stewart