Peavey Mace 320T Screen Resistors

[Tom Carlos]

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

Why-oh-why did Peavey use 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. 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 resistors are still within specs, even the 47K grid resistors. But I am asking out of curiosity!

Thanks in advance, Tom

PS and Output.JPG

Output.JPG

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[Stevem]

Good question.
I guess that they wanted to limit the total current thru each side of the OT and those two screen resistors let them do that while still having only one bias setting on pin 5 for all of them.

[Roe]

Look a the big shared screen resistor that reduces voltage and current. Looks like 400R here

[Tom Carlos]

Look a the big shared screen resistor that reduces voltage and current. Looks like 400R here

Roe: Peavey uses a similar 400 Ohm resistor in many of it's amps. But unfortunately, that does not explain the inconsistent usage of the 100 ohm resistors. Thanks.

[Tom Carlos]

Look a the big shared screen resistor that reduces voltage and current. Looks like 400R here

Roe: Peavey uses a similar 400 Ohm resistor in many of it's amps. But unfortunately, that does not explain the inconsistent usage of the 100 ohm resistors. Thanks.

[Tom Carlos]

Good question.
I guess that they wanted to limit the total current thru each side of the OT and those two screen resistors let them do that while still having only one bias setting on pin 5 for all of them.

Steve.... But it is odd how the resistors were used. And why not pick a different value and connect each screen to the +490 voltage point as done in the Mace VT? I guess this will be one of life's big mysteries.

[Roe]

Look a the big shared screen resistor that reduces voltage and current. Looks like 400R here

Roe: Peavey uses a similar 400 Ohm resistor in many of it's amps. But unfortunately, that does not explain the inconsistent usage of the 100 ohm resistors. Thanks.

no wait a second. you're discussing the individual screen resistors. I am discussing the shared screen resistor. 100r individually works if you have a big shared resistor. some of the el34 datasheets even recommonend this kind of solution

[martin manning]

Screen resistors have two main functions. One is to limit current at high power, and the other is to suppress high frequency oscillation. For the latter, it's best to locate the resistors as close to the screen pins as possible, and the resistance can be relatively low, i.e. 100 ohms. In this amp the shared screen resistor is the current limiter and the two-per side are the HF suppressors. Evidently it is sufficient to place the 100R on two of the three tubes on each side, so in the end only five resistors are required instead of six in the more usual practice of placing one per socket with no shared resistor. Note also that having the PP output share a screen resistor may have some benefits of its own.

[Helmholtz]

Yes, those 100R resistors are screen stoppers. Because of their low value they don't do much for screen current limiting.
From tube literature (e.g. RDH4) it is known that directly paralleling power tubes invites RF oscillation.
The screen stoppers provide some tube separation.

The 400R in the power supply can't act as a (common) screen resistor because it sits between 2 filter caps, making its response slow.
It's just a (screen node) dropper.

[Roe]

Yes, those 100R resistors are screen stoppers. Because of their low value they don't do much for screen current limiting.
From tube literature (e.g. RDH4) it is known that directly paralleling power tubes invites RF oscillation.
The screen stoppers provide some tube separation.

The 400R in the power supply can't act as a (common) screen resistor because it sits between 2 filter caps, making its response slow.
It's just a (screen node) dropper.

still, it will reduce voltage and current somewhat?

[Helmholtz]

still, it will reduce voltage and current somewhat?

Like any other screen node dropper or choke it lowers the the screen supply voltage when the average screen current increases - with a bit of delay due to the filter cap.
A real screen resistor immediately reacts to screen current peaks by increasing its voltage drop, thus causing NFB and instantaneous signal compression.

[Tom Carlos]

Like any other screen node dropper or choke it lowers the the screen supply voltage when the average screen current increases - with a bit of delay due to the filter cap.
A real screen resistor immediately reacts to screen current peaks by increasing its voltage drop, thus causing NFB and instantaneous signal compression.

Thank you Helmholtz.... and good to see you here. I miss the MEF !!