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To add a little to IC's response? As with most things to do with valves and guitar amps, it all depends.
You see Cathode Bias was used universally for valves in just about every application, mainly because it is much cheaper to deploy than generating a negative bias voltage anywhere from 10 to say 100 volts. Not only cheaper but in many ways better because it is also called "Auto-bias" because it self limits the anode current of the valve to a safe level IF THEY DO IT RIGHT!
Mains radios and the sound stages of TVs of yore had CB pentode and tetrode OP stages and these lasted for decades, in the case of the 6V6 they were virtually immortal!
But as ICBM says, guitar amp makers biased too hot. To be a bit fair they probably looked at the "book", saw a value for the cathode resistor and used the nearest preferred value.Then, were not so fussy what the HT voltage was! The Book you see probably gave a value for 250V HT.
Fixed biased amps CAN run into thermal runaway, not common but can happen. You are also dependant on a rather complex collection of components to provide the essential bias whereas CB just needs one adequately rated resistor.
Paul, that looks like some form of staining I would investigate the amp's internals further for the source.
There is no reason why a good (ha!) EL84 should not last several years in a properly designed circuit that kept anode dissipation (Pa) under the 12W limit. The El was not common in domestic kit but its crappier brother, the Ul84 was used in 1000s of record players and even though its life was short compared to say the ECL82/ECL86 they did run for 4 or five years. I had a Rogers stereo hi fi amp and never changed the ELs in 3 years. AND that got used a lot!
You should check the valves for "red plating". Put the amp in a darkened room and look at the anodes. If there is the slightest sign of a dull red glow on the spine of the anodes get it to a tech. Just 10 or 20 more Ohms in the bias resistor can bring them down from RP. You will never hear the difference. N.B. Some brands will red plate, others not but that does NOT absolve the manufacturer in my book!
As an aside. The cathode resistor should be many times the wattage rating of the amp when running normally. If just on rating a faulty valve (of two) can burn it out. That puts HT on the cathode decoupling cap...Messy! Of course, proper HT fusing is also required.
'King junk design some gitamps! (bit ambiguous that. What I mean is, say the power in the cathode R is <1W when running normally? A five, better ten watt resistor should be used. Same goes for 1R in a fixed biased amp although a 1W plus diode is ok)
!! Don't you have curtains? I would never bin old valves if they still work. New bottles can blow anytime and even a worn set will get you through the gig!
One other benefit of CBias of course, you can just swap valves, no re bias necessary.