Bassman 135 with shaky output

[rf7]

This amp needed new power tubes and a few other things, but the output waveform into a load looks shaky on the scope, as if the peaks are different sizes. Would that be caused by weak filter caps? The filter caps look to have been replaced around 2006, going by the date codes.

[pdf64]

Yes, it could be 120Hz ripple modulating the signal (at high power output).
Better performing caps (or increase the value) will likely reduce it, but it's a drawback of UL operation (only 1 stage of filtering on the screen supply).
Pete

[rf7]

Thanks for the info Pete!

[J M Fahey]

Or maybe the tubes are worn un-evenly.
To check, put the 2 right ones on the left sockets and viceversa.
I weak peak follows tubes, it's a tube fault.
The amp may work very well anyway, no need to change them unless obviously very weak.

[rf7]

I think it is what pdf64 was talking about. It's that the consecutive peaks vary in size, so that they look jumpy on the scope. It affects both the positive and negative peaks. And the power tubes are brand new.