Hiwatt HG50C channels volume difference

[nomorers]

Hi peeps, i acquired a hiwatt hg50 combo about 6 months ago and a couple of weeks back the high gain channel's volume got really low. I have to crank the high gain channel's volume up to 4 oclock to compete with the clean channel's volume set to 10 oclock.
The preamp tubes are 3 x 12AX7 and 1 x 12AT7, the power tubes are 2 x EL34.

I've noticed that when i switch from the clean channel to the hi gain channel, there is a big volume bump for like a split second before it goes down to the low level I was talking about.
Any idea about the problem? Is it bias, preamp tube or power tube?

Cheers

[92Volts]

Sounds like a preamp problem-- maybe a bad tube. Bias or power tube problems would have the same effect on both channels.

I couldn't easily find an HG50 schematic, but it's a good bet the 3x 12AX7s are the preamp tubes (the 12AT7 is likely the phase inverter which is used equally by both channels). You could try switching the 12AX7s with each other.

Hopefully the 12AX7s are used in different channels in a way that will allow you to detect a bad tube with this method. For example, switching two tubes might cause the clean channel to be quiet and the gain channel to work normally, then you know it's a bad tube not another component.

Without a schematic it's hard to guess ahead of time which tubes are involved in which channel (therefore which one is responsible for this problem), but if the problem moves when you move tubes you'll know at least one is bad. In that case you can replace them all or use trial-and-error to replace only the bad one.

[nomorers]

Very logical thinking... I will try that. Thanks a lot.
You reckon the big bump in volume when switching channels comes from the same problem as well?

[shooter]

the big bump in volume when switching channels

good bet, without a schematic you can only get so far

is it a 1 jack n channel switch affair? or can you jack either channel?
is it a shared TS?

[nomorers]

Sorry, i'm a real novice to all these technical terms, what do you mean by shared TS?
There's only one input, each channel has it's dedicated volume and there's a master volume, the eq is shared by both channels though. The channels are exclusive, no jump is possible.

[Willabe]

shared TS = eq is shared by both channels though.

TS = tone stack. 

[shooter]

without a schematic there's not much left, swap tubes n see.

other than new volume settings the HIgain side sounds like it did?

what's your skill/comfort level, are you comfortable measuring deadly volts, comfortable tracing wires from one place to another, etc?
from what little I know of HIwatt, they can be somewhat over engineered