KT88 faint crackle

[mellis]

My KT88 XL has been going for a dozen years and sounds fantastic. A few weeks ago it started a very faint crackle on both speakers equally. We recently moved house so hadn't had time to diagnose. But then the fuse for the 12v heaters for the small valves blew.

I thought if it was on both speakers it must be the ECC83 - which I replaced - but there's still very faint crackle on both channels. The fuse hasn't blown again. And annoyingly, no other components look visibly distressed! All the other valves are relatively young.

I'd be very grateful for advice on how to track down what might be the cause, or any similar experiences? Thanks!

[bob orbell]

If it is on both channels it is possibly the power supply, it is worth removing all valves one by one and replacing just to see if it is dirt / oxidisation on the valve pins or sockets, if that dose nothing, it could be a power supply resistor breaking down or even a capacitor, apart from trial and error, you will need a scope to find it. Bob

[mellis]

Thank you - I'll have a look when I've a few days off...

So mostly likely that'll be C14/17 & R30 - just the pwr supply to the DC heaters that could be causing this?

And out of interest, why is this DC heater circuit the only one fused!?

[bob orbell]

Sorry Mellis, you read me wrong, it is not the heater supply that I am suggesting may be your problem but, the High Tension supply, I don't' know the exact voltage here, but it could be around 400 volts DC or more so be very careful, this voltage will KILL you . Bob

[John Caswell]

The heater supply is fused because to protect the power transformers from excessive heater current on startup, which can h be quite high.
As far as the crackle is concerned, it could be a noisy resistor in the common HT supply feeding the ECC83 anodes, possibly a capacitor breaking down, although I doubt that.

John

[mellis]

Ah, the HT does make more sense than the heater supply (was just confused because that fuse blew)! Thank you both for your time and knowledgeable input! 😁

[LincolnG]

Bob, did you resolve your "Crackle" ?

[bob orbell]

What crackle ? . Bob