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Preamp valves generally last well - having at least one spare with you is a good idea.
Always useful to know what the amp is.......
OK, most amps that use EL84s run them quite hard so these generally need changing more often than other valves eg 6L6s.
The main danger with waiting until a valve fails before replacing them is co-lateral damage to the amp. Of course a valve failure at a gig could be very inconvenient, especially in the absence of spares.
The HK 36 has a quite sophisticated biasing / fail safe system that (should) prevent any damage to your amp in the event of a valve failure.
From memory it will shut the amp down and indicate which valve has failed, so swapping it out should be easy.
I use the amp at home, two or three rehearsals a month and a gig a month. Regular but not excessive use. I'd say if you're proper hammering it you'd want to look at swapping power valves every six months.
I second the recommendation to take spare valves to a gig. I take a complete set, plus a backup pedal-based 'amp' rig for emergencies.
I had a Classic 30 where I had to replace the power valves in well under a year. They would develop a rattle before they dies though. That one used to eat phase inverter valves (the preamp valve nearest the power valves). I got 10 months out of a NOS Brimar once, but modern valves would need replacing every 6 months. The drive just started to sound really nasty when they were on the way out.
I must say that 6L6 power valves last a lot longer. I've had two 6L6 amps that I've kept for a decent length of time. I'm probably managing 5 years out of a set of 6L6s.
@ rawk100 6L6 amps vary. My Lazy J is probably pretty low by 6L6 standards. I'm only on my second set of power valves in that in around 10 years, and I've kept the original set as spares as they still work fine. I used to have a Hot Rot Deluxe before. That showed 430V plate voltage on the schematic, but with the mains voltage in my home I measured it at 456V. That's still not that high for a 6L6 though, and the power valves lasted years. The Hot Rot Deville is a similar circuit to the Hot Rod Deluxe but runs the valves hotter to get 60W instead of 40W. Don't know how long the valves would last in that.
The other problem with EL84s is that you normally need more of them. A lot of EL84 amps use 4 power valves where you could get away with 2 6L6s. If you have an amp that really chews them up, it can get very expensive.
This also provides over current shut down.
In theory this should be foolproof, but I've had a couple of HK amps in where the monitoring circuitry had failed.
http://jpfamps.com/hughes-kettner-tubemeister-18-repair/
The bias, from memory, is quite conservative.
I've replaced old power valves in amps that were working fine but they ended up sounding noticeably better after the revalve.
So, power valves first.