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: 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.
Quote from: J M Fahey on October 24, 2025, 07:58:52 AMJust add the 1k 5W screen resistors plus fresh tubes.
Thank you Juan...
Agree with Juan, screens were not as big a concern back when tubes were much more robust and had tighter tolerances in manufacture.
Screen resistors were sometimes just used for dealing with instability due to layout etc.
You will find a fair bit of stuff from that era with screen resistors only on some of the power tubes. The power supply dropper resistor for the screen node is considered a 'common screen resistor' for all the power tubes together. So much so that Traynor even set bias for one of it's models by measuring voltage drop across the screen node supply resistor.
Quote from: g1 on October 26, 2025, 11:09:54 AMAgree with Juan, screens were not as big a concern back when tubes were much more robust and had tighter tolerances in manufacture.
Screen resistors were sometimes just used for dealing with instability due to layout etc.
You will find a fair bit of stuff from that era with screen resistors only on some of the power tubes. The power supply dropper resistor for the screen node is considered a 'common screen resistor' for all the power tubes together. So much so that Traynor even set bias for one of it's models by measuring voltage drop across the screen node supply resistor.
Thanks G1 !!!
I have seen the common PS dropper resistor in other Peavey amps (as in the Triumph 60/120). But in those amps, each tube had its own screen resistor.
I guess it was an evolving design - as the next Mace version (the VT) included a resistor for each tube.
So far, the amp owner reports the amp is working - but he is using it at a very low volume. If the amp ever makes it to a live gig and higher volume levels, perhaps it will be time for a sextet set, modifying the screen resistors, and biasing.
And G1, good to see you and JMF here!!!
Thank you.