300bpse

[AndyR]

Hi everyone I have a pair of 300BPSE’s built when they first came out and lovingly looked after since, one amp has developed a problem over the last year or so which I have lived with but now hoping to fix. On switch on after a few seconds as the 5U4 lights up I get a very loud crackle lasting about a second after that it settles down and all is OK and at switch off. Over the years I have had a number of 5U4’s which have occasionally flashed over but a change of valve has sorted it, have also swapped all the valves which has made no difference. Can anyone help me in a possible cause and what to check - thanks

[Richard]

Hi Andy and welcome,
Yes valves are prime suspects, then electrolytic caps then resistors. It could also be a joint gone dry. Obscure stuff too, I once had an HT choke repeatedly go short circuit about a minute after switch-on causing hum, took a while to find.
The rectifier 5U4 might seem first suspect but you only hear the noise because the HT has come on and the 300Bs have warmed up. So, it's a delayed switch on really which means you need to suspect almost everything.
It's going on 20 years now so I'd be looking to change all the electrolytic caps but in particular C7 and C8 which are seried to make the first HT cap and take the most hammering from ripple and heat.
Then, the 300B cathode caps C5 and C6 often fail in aging valve amps but after that it's pot luck and you may as well do the lot.

[AndyR]

Thanks Richard
Will have a look you’ve given me some good pointers, the caps are not from original build having been upgraded over time - most are black gates - expensive at the time - if I check them are they likely to have just short circuited or will they only fail when powered up - thanks again

[bob orbell]

Hi Andy, to check capacitors properly, you need a capacitance meter, the meter can be quite expensive, don't worry, they won't have gone short circuit, that would have resulted a firework display, , as Richard has said, it may be a good idea to change them, I am sure they are not big capacitance with a thermionic rectifier, so can be bought quite cheap. BOB

[AndyR]

Success !!
Decided before I start changing components to check the valves again and clean the sockets - the ECC82 phase splitter was the problem - swapped cracking gone and RH channel quieter
- thanks for your inputs