BRAUNER VMA

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Jesper Findshøj

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Joined
Jan 26, 2022
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Denmark
HI everybody
I have bought a used VMA, not sounding as I hoped, AND the switching between VM1 / VMX mode doesn't work
a very skilled friend, have drawn the schematics for me to understand how the switching works.
as seen on the drawing it's a relay switching between 2 capacitors who are decoupling the cathode on the 6922 tube.
unfortunately it looks like the mic was repaired wrong, or even built wrong from the start... :(
for now it's 2 totally equal capacitors with 470uf ( c1 & c2 ) on the drawing.. which means no difference in sound..
i se that a no vm1 has a 47uf as cathode decoupler ..
QUESTION:
- does anybody know what the right capacitor values would be ?
- is the 6922 the right choice ? other brauners use 6267..! can't find any documentation from Brauner which will be the right one...

best regards
Jesper Findshøj
 

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..actually, C2 parallels with C1 at closing the switch - so you have 470 or 940uF at switching. But I don't see much difference to be had there.. Should it have been 47uF and 470uF?

6922 is a horrible tube to use with a 1G grid resistor, isn't it? Data sheets mention 1M as absolute maximum value at cathode bias. Maybe it's heavily selected? (and WHY parallel both triodes? To ensure maximum leakage?)

Perhaps try with a 6072 if you can find one - or a 12AT7/ECC81 if you can't - but note that you need to wire heater differently (4+5 to 9 in stead of 4 to 5) - or supply it with 12V in stead of 6V for heating

/Jakob E.
 
Why not ask Brauner what they originally use for a ube and capacitors. I realize they are no longer in business but I am sure they would be helpful about their product.
 
I agree with Jakob about a 6922
I would not let a 6922 touch a capsule.
Can you post some pictures maybe we could tell if it was worked on.
Does it really have a 2.2uf coupling cap? that is odd.

Did a search for a replacement tube from tube sellers and found nothing yet.
 
I found the same info from Brent Jessee ! but still its not confirmed info right ?
the print layout is very fine, with no obvious changes, perfect soldering etc.
doesn't look like it have been repaired at all..
i will post pics later today.
 
This is interesting. The following is assuming the trace is correct.

I did some simple math guessed about 186VDC B+ that gave about 60VDC for the capsule voltage so the pattern control voltage is most likely 0 to 120VDC.

Don't understand why there is a 200VDC Zener at the B+ node in the schematic.
If it is for hot plugging protection or waiting for the heaters to get to heat on power up overvoltage protection it kind of makes sense however, Zeners can add noise and there is no bypass cap shown in the trace

Is the 2.2uf coupling cap an electrolytic cap? that can make some sense if it is.

https://frank.pocnet.net/sheets/049/6/6922.pdf
 
..actually, C2 parallels with C1 at closing the switch - so you have 470 or 940uF at switching. But I don't see much difference to be had there.. Should it have been 47uF and 470uF?
Yes, I saw that and wondered what is it for? According to the original litt, it switches from "a VM1 sound (natural and transparent)... to a VMX sound (soft top end, pronounced mid range and bottom end).
OK, I could admit (very reluctantly though) that switching the cathode bypass cap changes sthe sound significantly, but it is in blatant contradiction with the claim that it is "achieved by integrating two complete high quality signal paths and not by simply using frequency response correction."
So, unless the person who drew the schemo in post #1 was totally psyched at the time, I conclude that the Brauner litt contains a whole heap of BS.
 
We have one in the studio I work in, if you need gut shots or simple measurements from a working speciment I might be able to help you out.
 
I had a VM1 and it had a EF86 I have not seen inside a VMA.

So I wonder if this is a VMA that was worked on and circuit changed or if it a different version of the VMA because of TLRTs post about the tubes they saw in the microphone.

Lets guess 1mA across the 127K for a 127VDC drop
186VDC(guess) - 127VDC(guess) for about 60VDC at the plates and about .5mA a triode section
That would be about 2.5 VDC at the cathode
This is a guess because you can't trust the tube curves at low currents like in tube microphones
also keep in mind contact _____
Also note the secondary is wired in series MAYBE it a lower ratio like 3.5:1(guess)

Pictures of this microphone and some from bobtheninji would be fun to look at.
I would have guessed a different cathode resistor and cap to be switched in to change the distortion spectrum.
 
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Any updates or pictures?

I could not find an inside picture of a VMA on the web.

I was curious if the dual triode 6DJ8 circuit was stock.

Also curious if the EF86 version changed the cathode total resistor value for the tone change.
 

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