U47 hand wired with VF14 and BV8

[rodabod]

Indeed. Dropping off about 100V at, let's say an underheated 400 ma, ballpark, R=V/I gives 250 ohms, W=I^2R gives 40 watts... That's one big resistor and one hot microphone.

Yes, I neglected to add that option 1 doubles as a radiator for the studio. 

On the subject of bias, without looking into the numbers yet, I think it might be worth following the U47's biasing scheme in case the small amount of NFB formed across the 29R resistor has any noticeable effect. I will try and get a figure for this at some point compared to if it were not there (or bypassed!)

 

[mad.ax]

@Speakercoil:
I suggest you read those treads:
http://recforums.prosoundweb.com/index.php/mv/msg/22971/0/0/0/
http://recforums.prosoundweb.com/index.php/m/345325/0/?srch=u47+drop+resistor#msg_345325

You will find there a few answers and more information about r4 and the heat it can generate.

C2 and c3 were paper and tar capacitor. Short of finding the exact type that Neumann used, my advice is to try some film caps, and use what sounds best.
(this also stand for resistors btw...)

R1 & R2 are the most unlikely to benefit from a wattage upgrade because they don't see any significant current. At 0.1W, they are already way over-rated!

Axel

 

[speakercoil]

As always Axel excellent stuff... I think the most interesting part of the thread was reading how different capsules tend to like different polarization voltage... So this might take a little tinkering as I have not decided on my final capsule choice yet... I am leaning more and more toward a Thiersch PVC capsule as I have been hard pressed to find an original PVC M7... I feel that a new PVC might be closer to an original U47 than a vintage Mylar M7 may be. Also it will last a little longer than an original PVC M7 if I did find one... Which would probably end up on Thiersch's work bench not too longer after installing it! I have read some good things about his capsules and he seems to be doing the best job... So He might be a winner.

As for the resistors... I am awaiting a reply from Mills still. However I am once again debating what to do with the other resistors... I am not going to tinker too much with the Wattages, however I am debating on type... Carbon Comp will give a more vintage kind of charm, aesthetic, and arguably a more vintage tone. However for keeping things spot on dead accurate and preserving the life of the microphone I am playing with the idea of using Vishay Bulk Foils... I cannot use this for the wire wound however, because they don't make them in 3 watts... I believe it's only .2 and .4 watt configurations. However this would work fine for the rest of the resistors... That's up in the air right now. Obviously the upside is that the values will be dead on balls accurate and won't fluctuate much, so the mic will be more reliable and consistent in it's sound. However the Carbon Comps might give it more of an authentic sound, and the drift obviously is part of that. I am sure I will come up with a solution.

I agree with the two paper and tar comps.... I am wondering if those were Bosch or Siemens as well, or if they were another company. I have to do some poking around to figure that out. If I cannot find the exact type Neumann used, I will substitute them for film caps... Possibly still NOS siemens if I can find them or if not then something high end and modern.

It's hard when wanting to do a clone, because I want the highest possible quality of build, but I also want to stay extremely close to the original... It's a juggling act.

I e-mailed Giles over there on that forum for more info on the resistor and the heat sink... Hopefully he can help sort things out!

Who knew such a simple design could have so many complications... I mean for something that has 8 resistors and 3 capacitors... You would think this thing would be a cinch to build!

-Grant

 

[speakercoil]

WOOT WOOT!!!!! MILLS MAKES THE 1780 IN A 3WATT WIRE WOUND DESIGN!!!!! JACKPOT!!!!!

What a great e-mail to find buried in my spam filter!!! Sure like 50 ****, insurance, credit card, scam, and school loan e-mails get through... But of course the one I am looking for gets filtered?!?!

Alright! So I am in business... Now for the other parts.

If I cannot find a Siemens in the correct values I am going with an Audionote 1uf copper foil, and a polystyrene for the .5uf. I already found Siemens NOS foil caps in .01 so that's fine.

Now to decide on the rest of the resistors.

Anyway I have to go!!!

-Grant

 

[dangeruss]

Thanks for the response Roddy!

a couple questions. I don't have an electrical engineering background. I'm really just getting into the science of this stuff. I understand a bit of what you're talking about, but need to fill in a few blanks.

1) massive 105V supply like original but more current, 2) independent 5V heater, but dummy 105V filament supply for bias, 3) small 105V supply with small 5V heater which doubles as a bias supply.

1) Are you suggesting I use the original u47 schematic, but deliver more current (470mA vs. original 50mA) from the PSU? I would need some way to drop the 36V to 4.95 for the heater, correct? Would I then also use the "dummy load" I initially suggested to divert 31.05V to ground?

2) Are you suggesting creating a parallel, independent power supply of 5V with 470mA from within the PSU, down one of the unused cable wires (i.e. pins 3 or 6) and deliver that to the heater while the filament gets the supply from the original supply coming in on pin 5? What happens to that pin 5 current and voltage when it's no longer being used by the heater? Will I have to make other considerations there in terms of resistor or capacitor values?

3) I guess I'm a little confused as to what a bias supply is. Is it just the voltage applied to the filament in the tube? I don't understand how the bias can be either 105V (as suggested in #2) or 5V (as suggested in #3). I understand that the two areas the VF14 and the EF14 differ are in heater voltage and heater current required. So I guess it's a little fuzzy to me where and how the bias fits in and is applied.

You need to consider how the circuit works, and the implications PSU-wise in order to decide. Remember that the EF14's heater sucks around half an amp at full heat.

I'm building the PSU from scratch, so whichever solution would be the easiest to implement with that in mind would probably be the best way to go. Thoughts?

By the way, I'm still completing my underheated VF14 curve tracing tests which I will compare with EF14s at different temperatures, but as I understand it, around 5V is said to be a reasonable figure (and remember you may get reasonable voltage drop in your cable at these currents!)

Oh if only I understood the importance of that statement in paranthesis!  Does it mean that the length of the cable itself will increase or decrease the voltage with currents at 470mA?

Again, Thanks so much for your help. I apologize for being a little dense when it comes to this stuff. I'm still not really sure how to practically apply Ohms law in different situations if that gives you an idea of where I am. But I'm real good at populating PCBs and soldering

-Russ

 

[mad.ax]

Quote: "Mills make the 1780 in a 3Watt wire wound design!!! Jackpot!!!"

Does this resistor looks like the one in the pic I posted? In other terms, can it be heat-sinked with the U47 body?
If yes, jackpot indeed... If not, I suggest you read the linked threads one more time, especially the part where JJ Blair said that he measured internal temp. 91°F with the original heat-sinked resistor, and 160 with a new wire wound... There are also explanations about possible consequences of such an increased temp...

"polystyrene for the .5uF". Check that again... A .5µF in polystyrene would be huge, and probably won't fit... Polypropylene seems a better bet.

You are right. 8 resistors, 3 caps, one tube, and a single rail PSU... They don't come any simpler... But still, nobody ever managed to build a prefect clone yet!

Don't let this discourage you. What you will learn trying is worth more than the mic itself.

Axel

 

[dangeruss]

Don't let this discourage you. What you will learn trying is worth more than the mic itself.

hear hear!

 

[speakercoil]

Quote: "Mills make the 1780 in a 3Watt wire wound design!!! Jackpot!!!"

Does this resistor looks like the one in the pic I posted? In other terms, can it be heat-sinked with the U47 body?
If yes, jackpot indeed... If not, I suggest you read the linked threads one more time, especially the part where JJ Blair said that he measured internal temp. 91°F with the original heat-sinked resistor, and 160 with a new wire wound... There are also explanations about possible consequences of such an increased temp...

"polystyrene for the .5uF". Check that again... A .5µF in polystyrene would be huge, and probably won't fit... Polypropylene seems a better bet.

You are right. 8 resistors, 3 caps, one tube, and a single rail PSU... They don't come any simpler... But still, nobody ever managed to build a prefect clone yet!

Don't let this discourage you. What you will learn trying is worth more than the mic itself.

Axel

Hmmmmm I guess I missed that part. I read the thread, but was focussing more on building it, etc... Because honestly I figured mills wouldn't have that at the right wattage, as I had only found 10+watts listed on retailer sites. I guess I could e-mail Mills back and find out what the core temperature is of the resistor. I have no idea what it looks like. I mean I have seen pictures of their 12 watt ones, and that would most certainly NOT fit... Probably not even inside the mic. lol... I guess it might be back to winding it myself... Here is another question though. If I increase the wattage will that reduce the temperature? I am not used to having to deal with resistors being super hot... But then again everything else I build has been in chassis with ventilation, and lots of space!

I have never used polystyrene before, but they looked small in the pic... lol. Oh well... I guess I can use something else for it! I just figured I would try one out, because I know the G7 called for one, so I figured they couldn't be that big. Though I never compared the value of the G7 one and the .5. I just figured they are known to be insulated well and have great signal properties.

Yeah, this is why I am on a quest to build the best clone. I am kind of tempted to contact John McBride and see if he would be interested in doing a blind taste test of the best clones once I finish mine. Throw up his favorite originals (like 2-3 of them), a korby, a wunder, a wagner, a Tele-USA, and mine (he he). Then invite some very good ears to do a shoot out... Which would be easy with John owning the studio, George Massenburg and Korby being in the same building, and Richard Dodd being a few doors down. Get a panel... Run them through the API board pre's... If this happened I would want to document the whole thing by videotape... I think that would be a very interesting and important test for the audio community. I noticed in one of the threads you linked me to that someone mentioned this needed to be done, but no one has done it in the right manner! Of course this is somewhat dreaming!!!! I would need John to give up one of his studios for a day for free... Because I certainly would not be able to afford to rent one of those rooms out for this purpose otherwise! Though he has plenty of beautiful U47's, and I know he has a few Korbys and Wunders... I think he even might have a Wagner! That guy has everything... It's crazy... Literally... EVERYTHING.

As for the learning experience... That's the main goal. It will put me in a unique spot of knowing a ton about Microphones, and particularly the U47. Also it is already improving my ability to read Schematics properly... Hopefully though my quest will not just educate me, but the videos and Russ's blog will be a great learning tool for anyone wishing to go down this road... Regardless of if they are just looking to build an M7 style mic or an actual clone!

Cheers!
-Grant

 

[rodabod]

Hi Russ,

1) massive 105V supply like original but more current, 2) independent 5V heater, but dummy 105V filament supply for bias, 3) small 105V supply with small 5V heater which doubles as a bias supply.

1) Are you suggesting I use the original u47 schematic, but deliver more current (470mA vs. original 50mA) from the PSU? I would need some way to drop the 36V to 4.95 for the heater, correct? Would I then also use the "dummy load" I initially suggested to divert 31.05V to ground?

2) Are you suggesting creating a parallel, independent power supply of 5V with 470mA from within the PSU, down one of the unused cable wires (i.e. pins 3 or 6) and deliver that to the heater while the filament gets the supply from the original supply coming in on pin 5? What happens to that pin 5 current and voltage when it's no longer being used by the heater? Will I have to make other considerations there in terms of resistor or capacitor values?

3) I guess I'm a little confused as to what a bias supply is. Is it just the voltage applied to the filament in the tube? I don't understand how the bias can be either 105V (as suggested in #2) or 5V (as suggested in #3). I understand that the two areas the VF14 and the EF14 differ are in heater voltage and heater current required. So I guess it's a little fuzzy to me where and how the bias fits in and is applied.

1) Correct, but as Zebra50 points out, if we are running a single supply rail of 105V which is delivering ~0.5A for the heater, then by using the power law of Power = Voltage x Current we find that the power is massive and thus that the mic would be very hot if it was dissipating tens of watts (mainly in the heater voltage dropping resistor). It's just a bad idea. Plus, the PSU would be huge-mungous.

2) Sounds like you understand, although heater and filament are the same thing in this context. We have an extra parallel feed of 5V which is fed to the heater alone. The rest of the mic circuit remains the same except for the bias section** (see 3) which now contains a "dummy load" which is essentially a resistor which acts like the filament of the VF14 and series resistor together since these combined form a network along with the 29R resistor which sets the bias voltage.

3)** The bias voltage is a small voltage level which is set at the grid of the valve to control idle electron flow. It's a bit like the handle on a water tap. We want it always flowing slowly so that the signal from the capsule can vary the flow up and down in level from a medium point which is chosen carefully between fully shut and fully open. The small signal from the capsule modulates this flow, and thus we have amplification since the change in output is greater than the change in input (our water pressure supply is 105V). Anyway, our grid bias is set to roughly -1.1V with respsect to the cathode. This is achieved by leaving the grid at 0V and setting the cathode to +1.1V. The value of 1.1V is derived from the voltage divider formed between the 29R resistor, the filament resistance, and the 1780R resistor. Anyway, this voltage divider could be fed from our 5V heater supply rather than the 105V supply if it made things easier. This bias supply needs to be very low in hum/noise.

I'm building the PSU from scratch, so whichever solution would be the easiest to implement with that in mind would probably be the best way to go. Thoughts?

For an EF14, I'd personally go with option 2. I have no doubt that some would disgree for various reasons, but there is no simple solution in my opinion.

(and remember you may get reasonable voltage drop in your cable at these currents!)

Oh if only I understood the importance of that statement in paranthesis!  Does it mean that the length of the cable itself will increase or decrease the voltage with currents at 470mA?

The cable will decrease the voltage arriving at the valve since it has series resistance. It will form a voltagbe divider with the resistance of the valve filament. Look up Ohm's law, do some simple examples, then figure out how to calculate voltages in a voltage divider. Hopefully that'll help.

Cheers,

Roddy

 

[emrr]

I have pretty good stock on NOS American hermetically sealed paper/oil caps.  I'd bet the construction and materials are 95% the same as the German types.  Can you regurgitate some exact dimensions/mounting styles/values?  If so, and you're in need, I'll take a look in the boxes. 

If I were building one, I'd be tempted to source American parts for flavor contrast, and go with USA caps and Western Electric or Cinema Engineering precision wire-wound resistors.  Someone probably already said it, but if you go with vintage carbon resistors, use the largest wattage that will fit, as noise improves with increased rating.  That's why so many pro broadcast pieces were built with 2W types when 1/4 or 1/2 would suffice electrically. 

 

[strangeandbouncy]

Hi Doug,


  I never realised that noise was greater in smaller Wattage packages! Stands to reason , I guess, as a function of heat, no?



      Thanks!



      ANdyp

 

[zebra50]

Hi ANdyp,

As I understand it, it's to do with contact between the conductive (carbon) particles in the composite - bigger resistor = more granules, and so better contact. It even gets called 'contact noise'.

http://www.aikenamps.com/ResistorNoise.htm

Stewart

 

[emrr]

It was considered critical with carbon comp years ago.  Modern parts fare much better. 

 

[speakercoil]

Wow... Interesting... Meanwhile I was suggested to keep the carbon comp ratings almost exact, because the drift would sound different at a higher wattage. As for the capacitors... Let me get back to you on the dimensions needed.

I know I don't want the square type anymore, because it changes the way I have to mount the transformer. So it would be cylindrical. Also I need to see if I win this bid I have on some old nos siemens for the .01uf cap.

As for going american instead of german. I had already debated that and looked at some GE caps. After all they did make very good parts in the day. I will let you know if I am in need or not!

Thanks!
-Grant

 

[speakercoil]

So I feel like someone just smacked a giant pile of casino chips out of my arms while I was carrying them to go cash in...

The Mills Resistors (which I am still waiting on the temp) are 10 bucks a pop with a 10 piece minimum... I need 1. Well maybe 2-3 to test for the most accurate. However I do not need 10... Making one resistor cost 100 dollars plus shipping. Screw that... I think it's back to making my own.

-Grant

 

[speakercoil]

OK a little off topic... But not much considered I rambled on about the M50 for a little while...

Another mic I am interested in building in the future is a CMV 563... Looking at the schematic (which I attached this time)... It appears as if there is no transformer... Is the transformer in the psu or something? If so this may be the simplest mic design I have looked at to date.

7 resistors... 3 capacitors... an easy to find tube... Crazy.

-Grant

 

[zebra50]

Look again, it's in the usual place for a tube mic!

 

[Johan]

the transformer is there..200 Ohm winding going to pin 2 and 5 on the connector
if you want simple, how about this..

SELA T12. Neumann M7 capsule and an 6ak5 ( you find these for less than a Euro here in Sweden all the time...the tube that is..)

j

EDIT..Oh, Zebra beat me to it..

 

[speakercoil]

lol oh yeah... sorry it just looked like a giant ******* capacitor. I am used to seeing it placed further toward the right, and with a squiggly lines instead of straight ones. You sir are correct... How I missed that is beyond me. I need to lay off the mid-day cream liquor in my coffee. lol.

-grant

 

[emrr]

I'm pretty sure all the NOS American hermetic caps I have are variations of square/rectangular.  The only round stuff I'd have at all would be used Vitamin Q, and similar.  And not much of it. 

On the Mills resistors, you wouldn't even need a few to check for "most accurate"; they'll all be well within tolerance and more than acceptable.    Buy 10 and offer 9 to other builders?