Vintage Club 30 Distortion problem

[ejd]

I'm getting some unpleasant distortion on the distortion channel on my Vintage Club 30. The amp sounds great until it gets hot (15 minutes playing at moderate volume) and then the distortion gets really loose and undefined, especially on chords and low notes. The preamp tubes are newer JJ the power tubes are GT. I've had problems with this amp in the past with the footswitch jack issue. I was hoping someone could point me in the right direction as to what to look for.
Thanks in advance.
Eric

[Jazz P Bass]

The easiest way would be to swap the preamp tunes around.
V1 is the clean channel.
V2 & 3 are the dirty channel. (V2/ gain, V3/ tone)
V4 is the power tube driver (phase inverter)

[ejd]

The easiest way would be to swap the preamp tunes around.
V1 is the clean channel.
V2 & 3 are the dirty channel. (V2/ gain, V3/ tone)
V4 is the power tube driver (phase inverter)

Thanks, ill try that.