Adding screen resistors Peavey Triumph 60

[nashvillebill]

Amp doesn't have any screen resistors. 500 volts on plates and screens now.

I'm thinking of adding 470 ohm screen resistors, 5 watts. Also will add a trim pot for bias adjustment. And maybe, just maybe, replacing the 470 ohm resistor on the power supply with a MM choke...

Thoughts? Advice? Disadvantages?

Thanks in advance!

[guitician]

That depends on were the whole circuit has the tubes running at. If the amps been fine without them, it should be fine still, or not?

[nashvillebill]

The screens are running at full B+ now (500 volts).

Sure the amp works, yet I've read countless threads claiming Peavey has scrimped on screen resistors, either sizing them too small or leaving them out.

Adding them is relatively easy, I'm trying to learn from other people's experience here.

[Prairie Dawg]

The screens are running at full B+ now (500 volts).

Sure the amp works, yet I've read countless threads claiming Peavey has scrimped on screen resistors, either sizing them too small or leaving them out.

Adding them is relatively easy, I'm trying to learn from other people's experience here.

That's running the screens at their maximum value. Prolly wouldn't hurt to put some resistors therein.

Replacing the 470 ohm resistor with a choke may raise the voltage a little.

Both things sound like pretty good ideas to try out. If you don't like em it's easily reversed.

[jrfrond]

It can't hurt, but they are probably not necessary. Peavey used 100/5W screen resistors on the Triumph 120, but eliminated them on the 60, probably finding that screen current wasn't an issue. However, limiting screen current is not a bad idea.

[mooreamps]

It can't hurt, but they are probably not necessary. Peavey used 100/5W screen resistors on the Triumph 120, but eliminated them on the 60, probably finding that screen current wasn't an issue. However, limiting screen current is not a bad idea.

For the 6L6GC, I use 1K 3 watt. True, the tubes I get from my vendor are checked for screen grid current before I get them, but if that same tube should take a little "bump" that might get the screen grid wires knock out of alignment, that when the resistor would come into play.

-g

[nashvillebill]

Thanks for the input! And after carefully going back through the schematic and looking at the pictures I took of the board, I found an error on the Peavey schematic.

The screen voltage is actually being dropped to 500 volts, versus 508 on the plates, by resistor R59 (400 ohms, 10W)--the schematic had the plate voltage being tapped off after R59 when in fact it's before.

So I no longer see a burning need to replace R59 with a choke--but I believe I will still install screen resistors.

Thanks again!

[Enzo]

Go ahead and add the resistors if you like.

You may have read lots of people discussing how Peavey "scrimped" on screen resistors, but I bet those were people theorizing, and not people having tubes blow up for lack of resistors. Those amps will run all day just as they are. After all, a 5w resistor costs the same regardless of value.

You are correct, the plate B+ node is taken off before the 400 ohm resistor. The schematic is mis-drawn.

[nashvillebill]

The schematic also shows pins 4 and 5 as one side of the heaters, so I guess the drafter was having a bad day.

[Enzo]

Hah!!

I never even looked at heater wiring.