Quote from: TomCarlos on October 22, 2025, 03:42:14 PMMy Question: Why-oh-why did Peavey use only two 100 ohm screen resistors for three tubes?

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QuoteIt's as if one of the tubes is not protected and seeing the full screen voltage. One 6L6 Screen goes directly to the OT. The second 6L6 has a 100 ohm screen resistor that connects to the Screen of the first tube. The third tube has a 100 ohm from the OT to the Screen. What's up with this? Why not have a 100 ohm from the OT to each screen? Or maybe use a 470 on each screen as done with Fender amps?

WAY back then tubes were very good and consistent and needed minimal protection.
Notice that contrary to 99.99% of schematics they specify Tube BRAND (RCA).
Just add the 1k 5W screen resistors plus fresh tubes.

QuoteThe 6L6 tubes are the originals. But they are nowhere near matched.  I am wondering if that is how they always were or if some have drifted because of the screen resistor setup?  I just don't know.

I have seen some videos on Peavey amps, mainly from Psionic Audio where Lyle suggests changing out the 100 shared screen resistors and using a 1K on each screen. I hate to think about altering a design and I am sure the amp will continue to work. But for the life of me, this is weird.

After 50 years, the screen resistors are still within specs, even the 47K grid resistors. But I am asking out of curiosity!

Those tubes are 50 years worn abused.
Update amp properly.

Hello Forum Members...

I posted this topic in other Forum Groups but never received a reply that helped me.

I just finished recapping a Peavey Mace 320T. I replaced some resistors as well. The amp is working fine now.

My Question: Why-oh-why did Peavey use only two 100 ohm screen resistors for three tubes? It's as if one of the tubes is not protected and seeing the full screen voltage. One 6L6 Screen goes directly to the OT. The second 6L6 has a 100 ohm screen resistor that connects to the Screen of the first tube. The third tube has a 100 ohm from the OT to the Screen. What's up with this? Why not have a 100 ohm from the OT to each screen? Or maybe use a 470 on each screen as done with Fender amps?

The 6L6 tubes are the originals. But they are nowhere near matched.  I am wondering if that is how they always were or if some have drifted because of the screen resistor setup?  I just don't know.

I have seen some videos on Peavey amps, mainly from Psionic Audio where Lyle suggests changing out the 100 shared screen resistors and using a 1K on each screen. I hate to think about altering a design and I am sure the amp will continue to work. But for the life of me, this is weird.

After 50 years, the screen resistors are still within specs, even the 47K grid resistors. But I am asking out of curiosity!

Thanks in advance, Tom

Quote from: saturated on October 19, 2025, 08:55:19 PMMan that would RULE if I could just stick another plastic disc on there

Any pot from that same Alpha series should have the same plastic part and you should be able to 'transplant' it over to your old pot.

Save the old parts in case you have trouble making the Bourns pot fit.

Yes sir thanks no it might not be long enough idk 

That pot PDB181-E420K-503A2 has a 20mm long shaft.
Is that long enough?

Maybe the PDB181-E430K-503A2 might be a better fit?

How long is the original?

What's the best way to protect a SS input stage (non-inverting op-amp set for about 5x gain, in my case) from overdriving by overwound or active pickups, or a boost pedal? 
Things I've thought of:
- Make the integral gain adjustable via the (-)input leg to ground as a "drive" or "trim" control, and attach a clipping detector circuit with an LED indicator.  I figured out pretty quickly that idea is probably a bit too complicated. 
- Run the op-amp at a higher voltage for maximum headroom.  ±15v isn't that much higher headroom, and if I ever experiment with active pickups or overdrive pedals, eek! (I went with ±15v because rated max voltage for most common op-amps is ±18v, but IMO, that's pushing it kinda hard)
- Replace the op-amp with a MOSFET or JFET buffer/booster, so when it clips, it's not harsh.  However, simulations in LTSpice show that contrary to popular belief, MOSFETs and JFETs do indeed clip hard.
- Wire up a bare-bones limiter circuit so the signal never goes over a certain level.
- Instead of an op-amp chip at the front end, build a discrete unit, so I can run it at even higher voltage, because eventually I'll be putting a power amp on the back end, and it'll be running on ±24 or 35 volts.

Or do you simply design it for maximum headroom and let it ride?

Ok I ordered some pots 

POT 50K OHM 1/10W CARBON LOG

DigiKey part number

118-PDB181-E420K-503A2-ND

Manufacturer part number

PDB181-E420K-503A2

EDIT
I figured this component was a gonner so I went ahead and desoldered it

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Thanks guys I did manage to pry the black (wiper?) off

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I thought my fingernails would let go first I knew I couldn't pry against the black stuff and mess it up


Yes sir thanks it is an alpha A50K

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Man that would RULE if I could just stick another plastic disc on there

Are there numbers/letters on the back of the metal housing?

Take a pic or post the numbers/letters.

 The whole black plastic part is just pressed on to the shaft.  See if you can pry/pop it off without damaging the wafer or carbon track.