volume jump on Laney VC30

jonnyburgo

Usually after about 10 mins playing on both channels, a volume bump as if it has been turned up a notch, sometimes it stays there and sometimes goes back to normal after a minute or so. I had 3 12ax7's knocking about do tried swapping them but it still does it. Ive given the speaker input and terminals a wiggle to no effect and Ive tried 3 different guitar leads.

Ideas on a postcard to Mr Jonnyburgo.

ICBM

When it's on the dirty channel, does it show up as a volume increase or a gain increase? I'm assuming volume from what you said - in which case the problem is in the power amp or FX loop areas.

Are there heating/cooling noises from the power valves after it changes volume, or any sign of them glowing unusually brightly when it's in the 'louder' mode, eg one or more of the plates glowing red?

jonnyburgo

Just trying it now, had it powered up for 30mins and typically it's not doing it.

DJH83004

I have had a similar problem on a different amp in the past, where the cathode bypass cap on one of the pre-amp valves was intermittent due to a bad joint, maybe worth tapping the board with something non-metallic (handle end of a terminal driver is quite effective) just to see if you can replicate the fault.  

Voxman

A really common issue on the VC30 is a dirty contact on the rocker channel switch, whereby the amp jumps onto the gain channel. 99% of the time its an easy fix by just squirting a little electrical switch cleaner so it trickles down the side of the switch, and then rock the switch back and forth a few times.