6L6 / KT66 types don't need that. A true EL34 does. If you don't mess around with tube types you don't need t worry about it.
When sticking EL34 types in, you need to watch heater current and also bias levels. But why would you do that instead of a KT66 ???
When sticking EL34 types in, you need to watch heater current and also bias levels. But why would you do that instead of a KT66 ???
Just look at the "getter" material on the glass. If it isn't silver anymore, the tube has issues. If the getter is good, letting them run can clean them up. Same for any tube really.
If that material is white, take extreme caution handling the tube. The seal is broken which probably means the glass is cracked or broken.
If that material is white, take extreme caution handling the tube. The seal is broken which probably means the glass is cracked or broken.
A member of another site said:
'If the glass envelopes are 'shouldered' rather than straight-sided then I think that means they have to be M-OV (Marconi-Osram Valve, who also made GEC-branded valves, all of which were identical apart from any badge).
These tended to fail either by losing cathode emission or, if they were left unused for a few years, by accumulating gas which would destroy them shortly after first switch-on.'
This seems to be exactly what happened to the valves in these amps. Again, I'm surprised both amps acted identically........
'If the glass envelopes are 'shouldered' rather than straight-sided then I think that means they have to be M-OV (Marconi-Osram Valve, who also made GEC-branded valves, all of which were identical apart from any badge).
These tended to fail either by losing cathode emission or, if they were left unused for a few years, by accumulating gas which would destroy them shortly after first switch-on.'
This seems to be exactly what happened to the valves in these amps. Again, I'm surprised both amps acted identically........
He's referring to the "ST" bulb shape. That's just an earlier style, an older tube. They could get gassy over time when not used. As tubes are used they do lose emission and that is what many cheap tube testers are based on measuring.
I don't think I have ever lost a tube due to sudden gas conduction unless the bulb broke.. That would be an arc, something hard to miss. I would think that the silver coating on the glass would have disappeared or gone brown or grey if it was that gassy.
I don't think I have ever lost a tube due to sudden gas conduction unless the bulb broke.. That would be an arc, something hard to miss. I would think that the silver coating on the glass would have disappeared or gone brown or grey if it was that gassy.