Parts for Peavey VTM Bias Mod

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Enzo
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Re: Parts for Peavey VTM Bias Mod

Post by Enzo » Mon Dec 07, 2020 11:59 pm

Pick the one that needs the easiest trace cutting.

The thing to never forget: this is just a guitar amp, nor precision lab gear. If you pick just one tube to monitor, you get to hope the other tubes are running similarly. If you monitor all four tubes, then you gat an average as you divide the numbers by four.

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Re: Parts for Peavey VTM Bias Mod

Post by Leprechaun338 » Sun May 30, 2021 1:37 pm

vincenz wrote:
Thu Oct 08, 2020 2:08 am
____________________________________________________________________

Hi,

I am reactivating this topic as I just acquired a VTM 120 and I would like to make the bias adjustable with a potentiometer.
https://www.flickr.com/photos/3rdof3/se ... 289769037/


1. I easily understand the potentiometer installation/mod...

Image




2. ...but I don't understand the output tube mod below. What is the goal of that ? :

Image


I believe I understand the intent of the output board portion of this mod, but when cut traces this also removes the zener diode CR1 and CR2 from pin 3 to ground of the respective output tubes. Anyone know the purpose this component and what the effect of removing it is?

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Enzo
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Re: Parts for Peavey VTM Bias Mod

Post by Enzo » Tue Jun 01, 2021 11:25 pm

Those diodes are there to protect the transformer in the case of an open speaker, open speaker cord, or other failure of the speaker wiring. The amp will work fine without them, but you lose that protection. Sorta like your car will work fine if you remove the seat belts, you just no longer have that safety factor.

I have done this work, but I slice the board up different and add jumpers to compensate. I also add in some 1 ohm resistors for making current sensors.

Look at your schematic, the diodes are wired from the plate ends of the transformer and to ground. You could easily move them

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Re: Parts for Peavey VTM Bias Mod

Post by Leprechaun338 » Mon Jun 07, 2021 1:41 pm

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So far on the amp I’m working I found an open screen resistor R2. Unknown condition of OT. It appears electrolytic caps have been replaced at least once before in its life. I’m planning on re-capping, screen resistors from 100ohm to 470ohm, bias mod, output board mod including 1ohm bias sense resistors, and some new output tubes. The included pictures are of how I plan to mod the circuit (schematic), and physical board layout. I did have one question on where everyone adds the 1ohm bias sense resistors. Should pins 1 and 8 be tied together and both go though the 1ohm to ground, or tie pin 1 directly to ground and pin 8 though the 1 ohm resistor to ground? (I have also included a sample schematic of the two different scenarios I just described).
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Enzo
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Re: Parts for Peavey VTM Bias Mod

Post by Enzo » Wed Jun 09, 2021 8:27 pm

Unless you want to get away from 6L6s, pin 1 doesn't matter.

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Re: Parts for Peavey VTM Bias Mod

Post by Leprechaun338 » Thu Jun 10, 2021 9:53 am

I was thinking at some point I may want that option. Thank you for the responses!

One last question before I start buttoning this thing back up. I ordered a replacement for C7 on the preamp board as both the schematic and board layout call out polarity so I ordered an electrolytic replacement. When I went to replace it I now see that it appears to be an axial film capacitor. I’ll attach an image. It doesn’t appear this cap was previously replaced, is this the stock component? What would the correct replacement be?
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Enzo
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Re: Parts for Peavey VTM Bias Mod

Post by Enzo » Thu Jun 10, 2021 3:12 pm

Either one. You need a 1uf cap there of sufficient voltage rating for that cathode follower stage.

That does look like a film cap, so it likely need not be changed. They don't really get leaky like a 'lytic. On the drawing I'd have to say it looks to me like they put the + on the wrong end. Of course there is no + on a film cp.

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