Preamp valves vs power amp valves changing

Jetfire

So coz im abit skint right now, I wondered aboit changing the valves in my amp. It's been gigged hard recently so there's an element of maintenance and an element of wondering if the sound will change as its not had new tubes in the time since I've had it. 

I must say that I don't think it sounds bad and nothing has gone pop (id defo notice that) or microphonic that I can tell. 

So if you could only afford to do one, would you change preamp or power valves first?

richardhomer

You should ideally have (at least) a spare set of output valves if you’re gigging.

Preamp valves generally last well - having at least one spare with you is a good idea.

jpfamps

Always useful to know what the amp is.......

Jetfire

Hughes and Kettner Grandmiester 36 

jpfamps

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.

Twinfan

Having just had the EL34s start to pop and crackle in my Mini Jube I'm looking at swapping the power valves annually from now on.  Tone was starting to suffer just before the failure (less punch) so they were definitely on the way out.  I've actually changed all the pre-amp valves too - there are only three ECC83s - so at £30 I'll probably swap them every year too.

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.

DJH83004

I don't have any personal experience of the H&K 36, but looking at the spec. it is 4 x EL84, I suspect in cathode bias, so they will be running pretty hard, suggest that is your priority. I also suspect the 'Tube Safety Control' monitors the current through the valve via the cathode resistor (actually the volts drop across it), and hence can determine the 'best bias, overcurrent and no current' conditions via the rear panel LEDs. 

rawk100

When I was rehearsing and gigging regularly with my Peavey Classic 50 it used to need new el84 output valves every 10 months or so. It would eat any "weak" valves quicker. Generally you could tell when they were on the way out could sound would deteriorate and sometimes volume would drop a bit.

crunchman

rawk100 said:

When I was rehearsing and gigging regularly with my Peavey Classic 50 it used to need new el84 output valves every 10 months or so. It would eat any "weak" valves quicker. Generally you could tell when they were on the way out could sound would deteriorate and sometimes volume would drop a bit.

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

crunchman said:i

rawk100 said:

When I was rehearsing and gigging regularly with my Peavey Classic 50 it used to need new el84 output valves every 10 months or so. It would eat any "weak" valves quicker. Generally you could tell when they were on the way out could sound would deteriorate and sometimes volume would drop a bit.

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.

I think peavey run the plate voltage on the el84 higher than most valves are designed for hence they tend to wear out quicker. All el34 Marshalls I've had / got don't tend to wear out the valves nearly as much. Same for any 6l6 amps I've had.

crunchman

@ 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.

jpfamps

DJH83004 said:

I don't have any personal experience of the H&K 36, but looking at the spec. it is 4 x EL84, I suspect in cathode bias, so they will be running pretty hard, suggest that is your priority. I also suspect the 'Tube Safety Control' monitors the current through the valve via the cathode resistor (actually the volts drop across it), and hence can determine the 'best bias, overcurrent and no current' conditions via the rear panel LEDs. 

It's actually fixed bias and the bias for each valve is monitored and set but a PIC microcontroller.

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.

Maynehead

In my experience preamp valves either go bad, or they work fine. You don't get the gradual degradation in tone you get with power valves.

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.

crunchman

Maynehead said:

In my experience preamp valves either go bad, or they work fine. You don't get the gradual degradation in tone you get with power valves.

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.

Phase inverters can start to sound rough.  I've had phase inverters in two different amps start to sound a bit nasty (especially with drive sounds) but still work.