Ac30 is borken, any help much appreciated!

ICBM

If the voltage is 340V at *both* ends of R8 and R9, then the PI valve is not drawing current. Is it lit up? (I'm assuming you've already checked that!) If yes, check the voltages at both ends of R21 and R28 - one of them will be open circuit, or there's a bad connection somewhere there.

If no and you've tried other valves, it's a broken connection in the filament circuit, or a bad contact between pin 9 and the socket.

Cirrus

Thank you both again! I did try a couple of different valves and yes they all light up. In fact, it might be my imagination but I think it's glowing brighter than the other two ecc83s in the amp (not that I would bet money on it).

Good to have a plan of attack mapped out for this evening. If I can get the ac30 fixed then all will be well for Sunday's gig. If not, Then my backup amp is the "Wangs" 15 watter so there's a lot riding on this. 

:-O

Cirrus

It was R28 open circuit! I've temporarily (or permanently if it works and I forget   ) replaced it with two 100ks in parallel. I'm going to play at volume for an hour or so after tea and make sure it doesn't break again.

ICBM

Excellent - not a blown OT!

bazxkr

Well done all yoou guys who have helped Cirrus progress with this... pats on the back to you all.
Well done chaps !
I've enjoyed following this even though I couldn't be any help to the OP
Cheers
Baz

Cirrus

Still works after an hour of loud playing, happily. I was hot swapping preamp tubes when it broke, so maybe that isn't actually as bright as I imagined. . ">

ESchap

Cirrus said:

Still works after an hour of loud playing, happily. I was hot swapping preamp tubes when it broke, so maybe that isn't actually as bright as I imagined. . ">

:-O

ICBM

If you put it on standby, it's no problem at all - assuming the standby switches off the HT, which it does in most amps - or at least the supply to the preamp if you're swapping preamp valves. There are quite a few amps where it doesn't turn off the plate HT to the power valves, so swapping those without fully turning off isn't such a good idea.

Not really a good idea to swap valves with the HT still on. Depending on what order the pins make contact in, you could at least damage the valve. I'd be surprised if it blew that resistor, but if it was a bit dodgy anyway, it's possible...