Hot Rod Deluxe drive channel issue

[lowell]

I am working on a hot rod deluxe that has just been retubed (power and preamp tubes). The issue is that while playing on the drive channel the pitch 'A' and only 'A' has a weird distortion to it. There is a distortion that sounds kinda like radio interference and it sometimes occurs with the attack and sometimes a second or so after. It happens on any "A' on the guitar excluding the b string 'A' or any 'A' above that so just the lower frequencies. Also when you push in the more drive switch the problem becomes worse.

I have already tried...
a different speaker, different tubes, tightened all the hardware

Appreciate any ideas you have!

[MarkusBass]

This preamp -> another power amp --> another speaker.
Another preamp --> this power amp --> another speaker.
Another preamp --> this power amp --> this speaker.


Mark

[52 Bill]

Filter cap in the preamp stage. Try bridging a new one across the filter caps one at a time and see if it clears up.

[Enzo]

Isolate the problem.

If you unplug the internal speaker and plug a cab into it, it still does it?

Ball up your fist and whack it (turn reverb down), does it react?

Look at the solder on the tube socket pins closely, any cracks?

Plug a signal generator into it and tune to A. Still does it?

Plug the signal genny into the power amp jack. Still do it?

Plug the genny out front and run the preamp out to some other amp. Still coming out the preamp that way?