Twin Reverb Issue

[g1]

Its almost like these particular TungSols cant handle the plate voltage.

They aren't by chance the modern tung-sol 5881, rather than 6L6STR are they?
I think I see getter rings at the top. The TS 5881's would not be able to handle the plate voltage. (400V plate/23W rated)

[tdlunsfo]

These are 6L6GC STR's. @hemholtz, I hear you on the bias probes, I actually own one of the sense resistor types but rarely use it. I guess old habits are hard to break. I've had no oscillation issues with these probes, but they do use the pain in the neck ceramic sockets and I have to be careful not to insert the tubes a pin off.

[tdlunsfo]

These JJ's have taken every punch I've given them. I guess I need to give the customer a call and give him the bad news on his new Tung Sols.

[Helmholtz]

Old habits are hard to break.

Correct idle current readings matter for the troubleshooting here.

Try pulling the PI.

[tdlunsfo]

With the JJs installed and the PI pulled, the least hum using the bias balance pot yields 21 and 24mA at 502Vdc on the plates. It's cold but sounds okay.

[The Dude]

Many of the older Fender amps were biased cold. As long as there is no crossover distortion, it's fine.
Also and FWIW, I've had nothing but problems with the reissue Tung Sol output tubes. I had one customer who bought and returned tubes 3 times and we still couldn't get a working set. I finally just sold him a set of JJ's,...... problem solved.

[SoulFetish]

actually, I wonder if you are loosing bias because of the shitty octal sockets fender used during this era. have you pulled the output tubes one at a time to see if any single position is pulling most of the current?

[tdlunsfo]

This amp is out the door. I put in a quad of JJs and the issue never returned. Time will tell if it's the tubes.