'74 super reverb biasing

[qwixzh]

I have a 74 super reverb. Got it about a year ago.

I was planning on experimenting on different output tubes. Actually, already bought some matched pair tubes (a chinese brand and Tungsol 6l6GCs). I was wondering how to rebias the output tubes on these amps. This amp have the hum balance pot.

It's going to be my first time to do a rebias.

I also have a diy bias probe but havent used it yet.

sorry for the newbie question.

thanks in advance!
qwixzh

[MWJB]

You need to change the bias load resistor, grounded from the bias balance pot (15K?) to rebias the amp. Smaller for more current, larger for less current.

Before installing the tubes set the BIAS balance pot so that negative voltage at pin 5 is the same for both power tubes. I'd be looking for -55vdc-ish before installing tubes.

The hum balance pot is hooked up to the heater supply and has nothing to do with bias.

[vintagekiki]

Install 1 Ohm/2W resistor between pin8 (cathode) and chassis (ground)
Master volume or all Volume pots put in zero (left) possition
Ballance control or OTM set in neutral (middle) position
Change 15 kOhm resistor between OTM and chassis and set cathode current 6L6 on 25 +/- 2mV measured on 1 Ohm resistor with DVM.
25 +/- 2mV is 25 +/- 2mA measured on 1 Ohm cathode resistor (DIY bias probe)

When adjust bias any Fender amp, I use 10 kOhm resistor wired in series with 22 kOhm trimer pot instead 15 kOm bias resistor . When is bias adjust on 25 +/- 2mA replacement 10 kOhm resistor and 22 kOhm trimer with equivalent resistor
Deinstall DIY bias probe 1 Ohm/2W resistor

It’s entire philosophy of bias adjustments.

[qwixzh]

Wonderful!

Very fast and very clear instructions! I think I can do it anytime now!

thanks MWJB and vintagekiki!!!

another question though, regarding the bias probe resistor, why do i need 2W? voltage rating? cause for power, it seems overrated.

[vintagekiki]

Power resistor is 1/2 to 2W, I use 2W because is temperature stability, and if is the failure 6L6, or if is too large current not burn immediately.
Basically, when you set bias always set the cathode current to 25mA =/- 2mA; bias voltage varies from tube to tube (depending on the emission tube)

[MWJB]

"Basically, when you set bias always set the cathode current to 25mA =/- 2mA; bias varies from tube to tube (depending on the emission tube) " For a 25-30W tube (6L6/EL34/KT66/5881) 30-35mA is more typical. The highest plate voltages that you are likely to see in a properly functioning Super Reverb are 525-530vdc so even then you could play safe and stick to 30mA (530*0.03=16W), +/- 2mA. 13-15W plate dissipation is usually fine for most tastes, I usually aim for low-mid 30's unless plate voltage is at the high end.