KT88 amp popping

[Gosfried]

Hi all
From a new member. Morning.
I have a problem that my WD KT88 amp makes a faint random sporadic popping and occasionally scraping noise only on L channel. I have tried replacing and swapping the first valves in the chain but it is still there. Any ideas what I should try next?
Many thanks
Gosfried

[bob orbell]

Hi, and welcome, if you had not metioned the scaping sound, I would have said that the issue was static charge inside the output transformer, but, because of the scraping sound it is more than likely a poor soldered joint, or component beaking down, before we continue, are any of the KT88 valves glowing red, if so do not switch on again, if not you can connect the amp to some speaker and place it on its rear end, remove the base plate and with a PLASTIC proder try gently moving the wires and components on the left chanel and listern, this is called a wiggle test, NOTE, THERE ARE "LETHAL" VOLTAGES IN THE AMP, be caerfull. Bob

[John Caswell]

Sounds (pardon the pun) like a noisy resistor to me. If you are stuck or don't know what you are doing bring it to me I am SE of Reading.

John

[Gosfried]

Dear John and Bob
Thank you for your quick replies. The KT88s are not glowing red, and the amp is performing normally L and R despite the occasional popping noises. I cannot look inside the amp which I built btw 7 years ago, for a few days. But when I can get it on the bench I will definitely try the wiggle test. Do either of you recommend a cold spray test, on the resistors? Also the most stressed resistors in the L input chain are presumably the anode/cathode load resistors on V1 and V2 - would you agree that that would be a good place to start?
Best
Gosfried

[John Caswell]

Hi Gosfried,
Yes go for the wiggle test first, then freezer/hot air gun/hairdryer next. I use all these in tracing something like this.
Start at the first valve then move onto next one. You could also disconnect the feedback R/C, this will of course ramp the gain up but as there is no signal it won't matter too much and can help in localising the fault.

John