Express build - weird note decay

ledvedder

ledvedder

Well-known member
Hi everyone. I finished up an Express build last week. When testing it, I noticed this strange noise while notes are decaying. I've chopsticked everything, but no change. It seems to be more prominent when the 500pf bright cap is selected. I've also tried different tubes. Any thoughts what to check? Is this something I could track down using my oscilloscope?

https://on.soundcloud.com/WX9rH
 
It stops when I turn the bass pot all the way down. None of the other controls stop it. But it does get worse the more the treble is turned up.
 
You could check the signal with your scope, starting at the input and see where it goes awry. Seems like the signal would be smooth-ish with decaying amplitude up to a point, then jump around a little as that noise/signal came in and out. It might take some trial and error to get it set up right, be careful in there.

I’d also move grid wires and then plate wires and also the NFB wire and see if that makes any difference.
 
I think I'm gonna tear it apart and start over. For the life of me, I can't find what's causing this issue.
 
ledvedder said:
I think I'm gonna tear it apart and start over. For the life of me, I can't find what's causing this issue.
Click to expand...
I had something similar with an 800 build. I believe either the NFB was the culprit or I had the leads from the OT backwards going to the power tubes.
 
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