Cracked Power Valve

welshboyo

So, the whole backup plan kicked into action at the weekend...it actually took less than a minute thanks to the Helix - amp went down, I managed to swear, pull faces at the drummer and get my "backup" direct preset up within a few seconds and did the change mid-song!!

Anyway, at 1/2 time a quick look over the amp showed me a blown Mains and HT fuse, changed them but it was still not playing ball so decided to leave it and check it when I got home.

So, it looks like one of the Power Valves (Ruby EL34BHT's) had actually blown out - cracked glass around the base, no vacuum left and therefore went to the big valve sanctuary in the sky...

Didn't actually notice this until I tried to check the bias last night - it was lighting up somewhat but had no readings whatsoever..

So, is this just pure bad luck or is there something more sinister that caused this?

The amp is in today for a check-over, revalve and bias but just want to check my facts before handing it over!!

John_A

Always difficult to tell if the blown valve is the cause or the symptom, hopefully the former 

welshboyo

John_A said:

Always difficult to tell if the blown valve is the cause or the symptom, hopefully the former 

yeah, that's my worry...

ICBM

The only amp fault which will normally cause a valve to crack is total bias failure, leaving the valve (or valves) running wide-open so it overheats and either cracks or the glass melts. You'll spot that straight away with a new valve - immediate red-plating. Don't let it continue for any length of time, it's very bad for the valve obviously.

welshboyo

ICBM said:

The only amp fault which will normally cause a valve to crack is total bias failure, leaving the valve (or valves) running wide-open so it overheats and either cracks or the glass melts. You'll spot that straight away with a new valve - immediate red-plating. Don't let it continue for any length of time, it's very bad for the valve obviously.

@ICBM would you notice an amp going into total bias failure - i.e would it start to oscillate or sound wonky?

This one didn't it sounded fine and I actually had it running with new fuses yesterday - its only when I decided to check the bias as it was a little quiet in comparison to what it normally is that I noticed the valve itself had cracked around the base.

ICBM

You would usually notice the amp starting to sound louder, more overdriven, even 'better', before the valve failed.

welshboyo

ICBM said:

You would usually notice the amp starting to sound louder, more overdriven, even 'better', before the valve failed.

Shit really!! - I (well the whole band) commented on how loud I was and I was running at the same MV settings!! hmm, OK, might have been playing up for a while then.

Its in being looked at now anyway, so hopefully the tech will pick something like this up.

normula1

The best sound I ever had was when my amp had smoke pouring out if it. Luckily I spotted it before too much damage had occurred.

ICBM

Apparently the Marshall DSL these came out of sounded absolutely fantastic for a couple of minutes before this…



… caused by the amp simply stopping supplying bias, due to the poor-quality jumper connector they use in the circuit. Surprisingly (given that it was a DSL!) after repairing the connector and replacing the valves, there was no other damage.

Your amp wouldn't happen to be a DSL would it…?

welshboyo

@ICBM no it's a Bogner Shiva

ICBM

I can't remember for sure but I don't think there's anything similar in those - although they do run at really high voltages, which isn't especially good for the valves in general.