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Peavey Classic 30 - screen resistors (or their lack of)
#1
Hello, 

I have this Classic 30 that's been sitting in a corner for quite some time and I thought I'd upgrade it a bit. I had been altering some capacitor values before, but thought the circuit could be improved in some way safety-wise. I'll be adding individual bias pots for each tube as well as measuring jacks, but what struck me right from the start was the lack of proper screen protection. TUT suggests providing individual 2,2K/2W screen resistors for each tube in those low-wattage EL84-driven amps which is exactly I'm going to do, but one EL84 in each pair has a 100R resistor on the screen supply. I gather that's a protection against oscillation between doubled up tubes, but will they still be neccessary once the 2,2K resistors are in place? 

Tomislaw
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#2
I'm pretty sure you'll want to remove those since they won't be necessary anymore.
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#3
Hi Guys

Yes, the 100Rs are to protect against oscillation given that otherwise there are no screen-stops. Once you install individual screen-stops, there is much less risk of oscillation.

You can use the 100R positions for two of the new Rs, but you'll have to slash the trace that goes to the other tube so that both screen-stops on a given circuit side can terminate at Vs. Alternatively, you can leave the 100s in place and do the other slash required to give the inboard tube a screen-stop and effectively have both tubes sharing the screen-stop.

Traynor did what PV did in his version of a Twin Reverb. That amp has a quad of EL-34s with a shared screen-stop overall and 100R to the outboard tubes from the inner ones. This is a hifi approach, since Pete was into using hifi techniques. PV was just saving $$
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#4
Thank you.

I will go full-bore here and install 4 individual resistors. There'll be some more trace slashing anyway to allow each power tube to be biased individually.

Tomislaw
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