Potato Cakes

Hypothetical Attempt at V76M boards
« on: February 17, 2021, 02:51:38 PM »
Hello, everyone,

I have been thinking about making boards for a stereo V76M preamp for sometime but have yet to give it any serious thought until recently. The idea is to do a layout with both preamps and the PSU (not the transformer) on the same board. Here is the schematic I am using:

http://audio.kubarth.com/rundfunk/getfile.cgi?f=A%2C38Q%2CS8P%2CC%3CT-%25%5D%22%3CF%25U%3BF%29U8V%40O%3D%26%25B7U8W-FTN%3C%261F%0A

The 2x12 gain switch would stay as it is. I would do away with the fine gain switch (0 to +5dB) and replace that with a switch that just does +3dB. Also I would do away with the 1dB gain control as well. If you need precision gain control you would just do that in a DAW. I have been reading over on an old V76 build thread here and noted that the choke (BV Nr 517) can be omitted as it affects HF response. A DI section would be nice as well as some way to do a simple variable HPF, but I am fine going without one.

I am curious about the capacitors labeled 35 and 37 being electrolytic. Is there a reason that these would not be film besides the fact that when this circuit was new that a 2uF and 4uF 300V film cap would have been too big? Also I am curious why there is a trimmer for the heater voltage just for V1 and separate transformer winding for V2-4. Does this also have to do with the components that were available at the time? I also saw on CJ's V76 build that some of the inductors were replaced by resistors. I am wondering if that could apply here, especially for BVNr 515. Lastly, I would want to have the transformers mounted on the board. I am considering the Carnhill 9045M and/or Lundahl 1578 for the input and haven't decided on an output option. Thoughts?

Thanks!

Paul
« Last Edit: February 17, 2021, 05:59:34 PM by Potato Cakes »


Re: Hypothetical Attempt at V76M boards
« Reply #1 on: February 17, 2021, 04:01:47 PM »
Hey Paul

No reason to not change electrolytic capacitors to film, size and price notwithstanding.  There's more chance the value will be closer to what's written on the tin, and they're unlikely to ever need replacing.

If you use quiet regulated DC for the heaters then one winding for all valves is fine. 

Replacing the plate load inductors with resistors would work, but you'd need a higher B+ to feed those resistors to maintain the same operating point. 
Replacing BV515 especially would require another 200 volts give or take, otherwise headroom would be diminished.  All the data you need for the operating point is on the schematic, almost 12mA on the E83F anode and 170V DC.   
An inductor, besides the stored energy of swinging twice the voltage, also shifts the load line.  A constant current source does too, in a similar way.  Except a pentode's anode is already a CCS, so then you have 2 CCS's possibly fighting each other so you need think about adding a shunt resistor. 
None of this is insurmountable and you may end up with a better performing/testing unit.
Just food for thought.

On choices for transformers and chokes, I used all Cinemag on the Mercury M76m.  But I bet there are lots of good choices out there. 
I like the Lundahl anode chokes as you can specify DC current vs signal voltage vs inductance, but they can be pricey.     
D. J. H.

The standard way to reduce much of the noise and distortion in audio gear in 1955 was to have a couple of beers.
 Anything else was too fiddlesome and too expensive.

Potato Cakes

Re: Hypothetical Attempt at V76M boards
« Reply #2 on: February 17, 2021, 06:44:56 PM »
Hey Paul

No reason to not change electrolytic capacitors to film, size and price notwithstanding.  There's more chance the value will be closer to what's written on the tin, and they're unlikely to ever need replacing.

If you use quiet regulated DC for the heaters then one winding for all valves is fine. 

Replacing the plate load inductors with resistors would work, but you'd need a higher B+ to feed those resistors to maintain the same operating point. 
Replacing BV515 especially would require another 200 volts give or take, otherwise headroom would be diminished.  All the data you need for the operating point is on the schematic, almost 12mA on the E83F anode and 170V DC.   
An inductor, besides the stored energy of swinging twice the voltage, also shifts the load line.  A constant current source does too, in a similar way.  Except a pentode's anode is already a CCS, so then you have 2 CCS's possibly fighting each other so you need think about adding a shunt resistor. 
None of this is insurmountable and you may end up with a better performing/testing unit.
Just food for thought.

On choices for transformers and chokes, I used all Cinemag on the Mercury M76m.  But I bet there are lots of good choices out there. 
I like the Lundahl anode chokes as you can specify DC current vs signal voltage vs inductance, but they can be pricey.   

Thanks for the info, Winston! Perhaps I will leave the BV515 and BV514 in there for now as I would have more investigating to do to figure out the math and how to implement it. I'm fine with formulas but I have difficulty figuring out which values from the schematic to use if they are not clear for someone like myself. I do recall from that thread I mentioned that BV517 was to be omitted as it would affect the HF response. It also does not seem to be used as a plate load. It is also feeding the grid of V3 and the tube preamp schematics I have studied do not have an inductor in series with the input path.

DC regulated heaters will definitely be used.

I'll keep looking at transformer options. I'd like to do a footprint for 2 or three options for the input transformer and just go with a quality and probably end up wiring the output transformer off board. Right now I'm not worried about layout as much as I am getting the schematic correct. I'll post my proposed edits in the coming week. I hope.

Thanks!

Paul
« Last Edit: February 17, 2021, 06:56:09 PM by Potato Cakes »

Re: Hypothetical Attempt at V76M boards
« Reply #3 on: February 17, 2021, 07:29:25 PM »
Heya, yep you can lose the little series inductor that's part of the clc low pass before V3.

I think you picked the right V76 in building from the "M" version, mainly because I'm not aware of a 1:30 input transformer that you can buy that would measure worth a damn.
I'm sure Oliver Archut could have wound one to original spec but, otherwise?

You can get some tips from the "S" version too.  I didn't bother with the + 1dB steps or the fine trim either, life's too short :D

I did put a fader where the 1dB per step switch is, for what that's worth.   I think I just copied what was on the "S" schematic as far as send and return, and used an appropriate value pot - 100K log?   

On plate load chokes - it's worthwhile remembering that it is both the DC current and the AC signal voltage which contribute to saturation.  What I found was that it was acceptable to find a single choke that worked for both positions if it saves on cost or hassle.

The output tube is working with higher current and signal voltage, but needs lower inductance.  Whereas the 2nd tube is fairly low current, lower signal voltage, but needs more inductance because it's a higher impedance.

I don't remember what my exact specs were but it ended up being the same Cinemag choke in both positions.  Maybe not ideal and I'd likely do things a bit differently these days but, worth a thought. 

The DI I did myself was a simple J-fet buffer (kinda like a 'white follower) with an appropriate pad (-18dB) after it which then fed the IPT primary.
It's on here somewhere, I posted the basic thing in some thread about a year+ ago.   It ran from the 48V phantom rail which is all I had in lower voltages without reinventing things.





 
D. J. H.

The standard way to reduce much of the noise and distortion in audio gear in 1955 was to have a couple of beers.
 Anything else was too fiddlesome and too expensive.

Re: Hypothetical Attempt at V76M boards
« Reply #4 on: February 17, 2021, 07:44:17 PM »
Here's a link which shows you the relationship between DC current, AC signal, and inductance.   It pertains to the Lundahl LL1668, but similar relationship rules apply to any plate choke.

Anyway, good luck dude :)

https://jacmusic.com/lundahl/applications/How-to-bias-the-LL1667-LL1668.pdf 
D. J. H.

The standard way to reduce much of the noise and distortion in audio gear in 1955 was to have a couple of beers.
 Anything else was too fiddlesome and too expensive.

Potato Cakes

Re: Hypothetical Attempt at V76M boards
« Reply #5 on: February 17, 2021, 08:35:02 PM »
Here's a link which shows you the relationship between DC current, AC signal, and inductance.   It pertains to the Lundahl LL1668, but similar relationship rules apply to any plate choke.

Anyway, good luck dude :)

https://jacmusic.com/lundahl/applications/How-to-bias-the-LL1667-LL1668.pdf

Thanks for the link. I will give this a thorough read.

The S version schematic I have is a bit illegible but I do see the send/return lines that you mentioned. If I was going to add a variable HPF in front of V3, I'm not quite sure how to deal with the insertion loss. I noticed there is a different value of the resistor that connects V2 grid 1 and ground for V76 (1M) and V76m (500k). Is there where the V76 is accounting for the HPF that comes before V3?

I really appreciate the help.

Thanks!

Paul

Re: Hypothetical Attempt at V76M boards
« Reply #6 on: February 17, 2021, 09:38:18 PM »
Thanks for the link. I will give this a thorough read.

The S version schematic I have is a bit illegible but I do see the send/return lines that you mentioned. If I was going to add a variable HPF in front of V3, I'm not quite sure how to deal with the insertion loss. I noticed there is a different value of the resistor that connects V2 grid 1 and ground for V76 (1M) and V76m (500k). Is there where the V76 is accounting for the HPF that comes before V3?
 

The copy I have for the "S" is bad too, I've not actually seen a good clean version.

The 500K vs 1M won't do much since there's a feedback loop around it but, just use 1M.

As for a HPF, a simple 6dB per octave could be done just by switching in various series caps as the switch part no. 93 in the "S" schematic does.  If you want a steeper roll-off then you'll additionally need a tapped inductor (or separate inductors) arranged just as it is around switch no. 93.   

If you put a fader pot before V3, then part no 8 on the "M" schem (2uF) will keep DC of the pot's wiper.  I think I also included part 64, the 15K R.

I can't remember exactly what I did now.  But it was a juggle between M & S schems in terms of the parts values used inbetween V1 + 2 and V3 + 4 sections. 
For instance, I don't believe I used a 100pF cap across the 80K series resistor in the 'S', nor the 50pF shunt right after switch no. 92.  But I do believe I used 80K along with a 100K log pot.    Then again, maybe it was a 50K series like the 'M' and a 250K pot   :o

Small details really, and playing around with and measuring stuff will get you there.       
   

 
D. J. H.

The standard way to reduce much of the noise and distortion in audio gear in 1955 was to have a couple of beers.
 Anything else was too fiddlesome and too expensive.

Ricardus

Re: Hypothetical Attempt at V76M boards
« Reply #7 on: February 17, 2021, 10:19:48 PM »
The schematic won't load for me.
Audio mastering for hire..

Potato Cakes

Re: Hypothetical Attempt at V76M boards
« Reply #8 on: February 17, 2021, 11:20:14 PM »
The schematic won't load for me.

http://audio.kubarth.com/

You'll have to go through the menus. Apparently the direct link is only good once.

Thanks!

Paul

Ricardus

Re: Hypothetical Attempt at V76M boards
« Reply #9 on: February 18, 2021, 11:37:25 AM »
Thanks!

Well, I think many people would be interested in this project.
Audio mastering for hire..


shabtek

Re: Hypothetical Attempt at V76M boards
« Reply #10 on: February 18, 2021, 06:00:32 PM »
CJ?



"really fine players do not use stomp boxes or master volume, they match the amp to the room and turn it up to 11.  Stevie Ray, BB King, Albert King, Duane Allman, Dicky Betts, Louis Armstrong"
   -CJ

Potato Cakes

Re: Hypothetical Attempt at V76M boards
« Reply #11 on: February 19, 2021, 06:55:43 PM »
Here is my redraw for the schematic to use a fader per Winston's suggestion. I added the 15k to the 50k resistor (64 and 60 from the original schematic) so that I could get -infinity to +5dB when fully CW, however I don't know is if the value of the pot will be high enough. I also took into account the resistor and potentiometer from the 1dB adjust and added that to the 80k (75) and then rounded to 100k for simplicity and ease of sourcing parts, but I don't know if that needs to be changed at all.

I also noted that on the V76 schematic, the grid 1 to ground resistor (61) is 1M which also has a passive HPF/LPF section and the M version has this resistor value as 500k, which doesn't have the filter section. My thought is if one wanted to add a HPF then that resistor would have to be changed to 1M per the V76 schematic.

For the gain switch, I'm wondering if I should make use of that last position and either do 65dB or -6dB. Is either of those possible by adding a resistor in either direction?

Thoughts? Corrections?

Thanks!

Paul

Potato Cakes

Re: Hypothetical Attempt at V76M boards
« Reply #12 on: February 19, 2021, 06:57:17 PM »
Original V76m for reference.

Re: Hypothetical Attempt at V76M boards
« Reply #13 on: February 19, 2021, 08:51:05 PM »
Yep looks good for the fader spot Paul.   The 100K fader along with what you now show as a 65K series R forms a divider &, as is,  gives you a bit more than an extra 6dB gain at the fader top position so you're at the 66db total + a bit.   
Adjust the value of your 65K once you have a prototype done to dial it in.  I think I ended up with 80K but it's been about 15 years so...

I would also include a spot for where the 15K R is on the "M" schematic (part 64) as I seem to remember it being necessary.   Might be that 1K is enough and you can locate it along with the 2uF cap close to V3's grid on your pcb. 

I personally would just use 1M for part no. 61.  There are enough low end poles and enough low end phase deviations already happening in the circuit that, this one being an octave lower won't hurt, can only help.     

On part 75, originally 80K, now 100K on your schem, the difference in gain between the two is less than 2dB so not anything to worry about that much.  Again, wing it for now and adjust if necessary when you have a proto :)

Biggest issue is finding good chokes at affordable prices really.  I checked the current prices on Lundahl and it's about 123 euro whatever that is in good ol' US dollars.   

Sowter have ones that are supposed to be good (haven't tried them myself) but their situation is in flux (non intentional pun, sorry  :P )  with being bought by Carnhill. 

My vote is to just leave the switch at 11 positions for what little that's worth.  I'm actually not convinced the lower level positions which incorporate the input pad are best with the "M" schematic values.  The "S" and other V76's used values which presented a more constant lower impedance to the input transformer. 
Dunno, but check out the S and the other versions.

I personally used the values from the EMI REDD pre-set attenuator for my levels below 34dB gain, but it probably made zero practical difference to the way it was done on the V76S tbh :D


« Last Edit: February 19, 2021, 08:54:51 PM by Winston O'Boogie »
D. J. H.

The standard way to reduce much of the noise and distortion in audio gear in 1955 was to have a couple of beers.
 Anything else was too fiddlesome and too expensive.

Re: Hypothetical Attempt at V76M boards
« Reply #14 on: February 19, 2021, 09:12:24 PM »
P.S.  For inspiration, Simon just built a version which he showed here:

https://groupdiy.com/index.php?topic=76868.0

You can see an internal pic of my past attempt here:

https://www.mercuryrecordingequipment.com/products/m76m/

I'm not sure why the pcb's shown in this pic aren't at least tinned copper rather than bare copper (unless it's actually gold?) but, you can see the copious use of WIMA MKP caps rather than electrolytics.   
D. J. H.

The standard way to reduce much of the noise and distortion in audio gear in 1955 was to have a couple of beers.
 Anything else was too fiddlesome and too expensive.

Potato Cakes

Re: Hypothetical Attempt at V76M boards
« Reply #15 on: February 19, 2021, 10:31:33 PM »
Yep looks good for the fader spot Paul.   The 100K fader along with what you now show as a 65K series R forms a divider &, as is,  gives you a bit more than an extra 6dB gain at the fader top position so you're at the 66db total + a bit.   
Adjust the value of your 65K once you have a prototype done to dial it in.  I think I ended up with 80K but it's been about 15 years so...

I would also include a spot for where the 15K R is on the "M" schematic (part 64) as I seem to remember it being necessary.   Might be that 1K is enough and you can locate it along with the 2uF cap close to V3's grid on your pcb. 

I personally would just use 1M for part no. 61.  There are enough low end poles and enough low end phase deviations already happening in the circuit that, this one being an octave lower won't hurt, can only help.     

On part 75, originally 80K, now 100K on your schem, the difference in gain between the two is less than 2dB so not anything to worry about that much.  Again, wing it for now and adjust if necessary when you have a proto :)

Biggest issue is finding good chokes at affordable prices really.  I checked the current prices on Lundahl and it's about 123 euro whatever that is in good ol' US dollars.   

Sowter have ones that are supposed to be good (haven't tried them myself) but their situation is in flux (non intentional pun, sorry  :P )  with being bought by Carnhill. 

My vote is to just leave the switch at 11 positions for what little that's worth.  I'm actually not convinced the lower level positions which incorporate the input pad are best with the "M" schematic values.  The "S" and other V76's used values which presented a more constant lower impedance to the input transformer. 
Dunno, but check out the S and the other versions.

I personally used the values from the EMI REDD pre-set attenuator for my levels below 34dB gain, but it probably made zero practical difference to the way it was done on the V76S tbh :D

Winston, this is really great information. Thank you!

The thought with removing the 15k and adding it to 50k was that when the fader was fully CCW then it would make a short the signal going to g1 of V3 to ground, completely silencing the signal i.e. negative infinity. I didn't know if it still would do that with the 15k where it was but I will definitely leave a spot for it.

I did see Simon's build. It's very impressive.

I'm going to do some more reading and look at replacing the chokes with resistors/caps and see if I can get around needing inductors. If I can achieve very similar results and save a noticeable amount of money on building these then I think it will move this project along a little quicker.

Thanks!

Paul

Potato Cakes

Re: Hypothetical Attempt at V76M boards
« Reply #16 on: February 19, 2021, 11:13:25 PM »
I personally would just use 1M for part no. 61.  There are enough low end poles and enough low end phase deviations already happening in the circuit that, this one being an octave lower won't hurt, can only help.     


You mentioned this above. I read it and immediately forgot it!

Thanks!

Paul

Re: Hypothetical Attempt at V76M boards
« Reply #17 on: February 20, 2021, 02:58:27 PM »

I'm going to do some more reading and look at replacing the chokes with resistors/caps and see if I can get around needing inductors. If I can achieve very similar results and save a noticeable amount of money on building these then I think it will move this project along a little quicker.
 

OK got it.
Looking at the E83F load, I'll shoot from the hip and say 350V B+  feeding a 15K anode load.
That's without hitting the textbooks which I'd do anyway, but that would be a first approximation if I had a unit here and wanted to just try it.   
 
My reasoning: 
We need the capability of 120V RMS on that anode to retain the headroom - 120V reflected through the output transformer is just over 13V RMS and gives us +24dBu capability. 

So, (350V DC/2)/1.414 = 123 V RMS.   Fine.

With the same amount of anode current (12mA) on the E83F plate as the original - 12mA and a 15K resistor drops 180 V which puts us at the same 170V as on the original schematic. 
 

That 15K is a 3W resistor btw.
50K screen grid is probably still OK but you can adjust that for the best performance. 

Someone else check me on this as, again, it's from the hip.
 




D. J. H.

The standard way to reduce much of the noise and distortion in audio gear in 1955 was to have a couple of beers.
 Anything else was too fiddlesome and too expensive.

Re: Hypothetical Attempt at V76M boards
« Reply #18 on: February 20, 2021, 03:15:36 PM »
P.S.  This is right up PRR's alley, he can do this stuff while also filling out everyone else's tax returns.

I'm less of a "doing all the maths beforehand" guy as that stuff was never my strong suit.
I'm quicker just lashing it up and adjusting stuff until it fits.
If I get stuck then that's when I take a time out and do all the sums.

There are other dudes on here who excel at that stuff though so, hopefully someone comes along to offer a hand.

:) 
D. J. H.

The standard way to reduce much of the noise and distortion in audio gear in 1955 was to have a couple of beers.
 Anything else was too fiddlesome and too expensive.

shabtek

Re: Hypothetical Attempt at V76M boards
« Reply #19 on: February 20, 2021, 03:54:19 PM »
wonder if PRR has just been busy moderating the IRS forum...
"really fine players do not use stomp boxes or master volume, they match the amp to the room and turn it up to 11.  Stevie Ray, BB King, Albert King, Duane Allman, Dicky Betts, Louis Armstrong"
   -CJ


 

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