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There are several modes of failure but these are the main ones I find.
1) Internal short.
This happens between the high voltage and low voltage pins.
In a well designed amp the likely outcome is that the HT fuses blows. Replacing the fuse and the valves will be all that's needed to bring the amp to life.
However there is opportunity for collateral damage, which will need sorting out before the amp can be used again.
Components in the firing line are screen grid resistors, power supply dropping resistors, cathode current sense resistors (not in every amp, but used to aid biasing), cathode resistors and caps (in cathode biad amps), filiament hum balance resistors / pots (can be destroyed of the HT end up on the filament supply), control grid resistors and bias supply (if the HT is imposed on the control grid).
2) Valve stops conducting current.
This can be a consequence of 1) but also mechanical failure in the internal connections.
3) Valve emission too low.
As valve age they experience reducing capacity to emit electrons from the cathode, and power loss will be noticeable.
Most valve expire due to 1) before this happens.
Is it just the design of the amp that stops No.1 taking out more than the HT fuse? (Mines a Mesa 5:25+ Express FWIW)
Quick visit to the tech with a pair of 6L6s, and a couple of days later (and about £45 lighter), all was well with the world again.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
I would not generally change old power valves unless there is some fall-off in power or tone, or a noticeable drift in bias which can be a warning sign.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
So I powered up my 5E3 Deluxe at the recent Water Rats meet. Big hum, motorboating, varying with the volumes and adjustable power. After much poking around and head scratching at home it turned out one 6V6 had failed - heater was open circuit. The heater must have flapped around inside the bottle and hit cathode/anode or screen and the two 100 ohm heater 'centre tap' resistors had blown. Visually you couldn't tell but they measured open and the heater supply was floating up to silly amounts of AC.
Not had that happen before!
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein