Weber PT in new Marshall 2203 build behaving badly

[Steve Conner]

I do think of this as a kind of crossover distortion. As JM says, it happens when the power tubes are overdriven to the point of grid current. The grid current charges up the coupling capacitors with a polarity that adds to the bias voltage. The tubes are now biased too cold, so you get crossover distortion. But when the overdrive is removed, the crossover distortion goes away.

The new OT was probably just more efficient, so it could make 120W without overdriving the tubes to the point of grid current.

[jamesmafyew]

I finally got a handle on it after researching the Paul Ruby mod -- I needed it in different words. It's the duty cycle variation caused by grid current that sets up a "DC offset" on top of the bias voltage, so yeah, it's crossover distortion. I'm not necessarily interested in doing that mod, but I get how it keeps duty cycle at 50% and therefore eliminates the DC offset.