Volume fading in and out when boosting

ChrisCox1994

Pro Jr, turned up a decent amount, works great with unity gain overdrive pedals (timmy and overdrive side of a KoT) but as soon as I engage any level boost (clean boost side cranked up a bit) the volume starts fading in and out like theres a bad valve... 

The problem never happens otherwise....

anyone have an idea what might be the issue? 

ChrisCox1994

after playing for 20mins it seems to have stopped, but it happened a couple of times at gigs (remember those?) and i already swapped out the valves for some other ones I had that we're less noisey (did a shoot out between different phase inv and v1 to see which sounded best, also changed to the pair of el84s which had the most headroom. 

Should i just bite the bullet and order a fresh set of all valves? 

ICBM

There's a fault which can happen with these amps when they're driven hard - there's a thing called a 'Zobel filter' consisting of a resistor and a cap across the primary of the output transformer. If the cap starts to fail, which it sometimes can when it gets a lot of voltage applied to it, the volume drops. Eventually, the cap will short completely and the amp will remain very quiet.

It may not be that - there are some other possibilities, in particular you should try changing the valves first - but I've come across this one several times so it's worth mentioning as it isn't at all obvious until the cap dies completely.

ChrisCox1994

ICBM said:

There's a fault which can happen with these amps when they're driven hard - there's a thing called a 'Zobel filter' consisting of a resistor and a cap across the primary of the output transformer. If the cap starts to fail, which it sometimes can when it gets a lot of voltage applied to it, the volume drops. Eventually, the cap will short completely and the amp will remain very quiet.

It may not be that - there are some other possibilities, in particular you should try changing the valves first - but I've come across this one several times so it's worth mentioning as it isn't at all obvious until the cap dies completely.

That sounds like it as I've tried some different combinations of el84 matched pairs and 12ax7s that are lying about and it's happened (albeit rarely) with all combinations when driven hard only.

DJH83004

For what it's worth, I've had similar problems with the snubber cap, breaking down on Pro Juniors, though these had actually shorted and caused the resistor to overheat, which is the extreme case. So I suspect IC is bang on the money.
  

ICBM

If it is this - or if you want to do it anyway which would tell you one way or the other, and it's not an expensive part - you need to replace C11, which is a 2200pF 1KV cap. I would use a 1.5KV one if you can get one, it will help stop it happening again.

It's maybe surprising that they fail since 1KV seems to be overkill for a circuit position which "shouldn't" see more than 320V, but the induced voltage from the OT when the power valves clip can be many times higher, somewhat like a car ignition circuit.