Sudden volume jump on OD channel only

[JablesII]

Hi all,

So I have an odd problem. I use my HT-5R head/ cab in an apartment. I usually have the clean channel vol around 9-10 o'clock and the OD channel vol around 7-8 o'clock (gain at noon-1 o'clock) in order to balance the relative outputs of the two channels while keeping the volume down for my neighbours.

Yesterday I was playing for a couple of hours, no problem, everything sounded good and nicely balanced between the clean and OD channels. Today when I turned on the amp the clean channel sounds exactly as I expected but there's a HUGE volume jump when I switch over to the OD channel. It sounds like someone turned the OD vol up to noon or further, but the knob itself is still set around 7-8. In fact, none of the settings have changed. Nevertheless, the OD channel is suddenly at a rehearsal/ gig volume. If I turn the OD volume and the EQ knobs completely anti-clockwise I can cut the output a bit, but it's still pretty deafening.

Can anyone think of a reason the output of the OD channel would increase so dramatically and suddenly? There are no pedals driving the amp (I checked they were all off) and none of my pedal settings have changed either. But now my amp is WAY too loud to use in the living room of my apartment.

I was considering getting an attenuator so I could try driving the tubes a bit harder, but didn't bother since I was able to get good tone at low volumes. Now I wish I had one because my amp's gone crazy.

[thephantum]

It sounds like the the OD volume pot is bad. The HT5 has no master volume, so the power stage is essentially wide open all the time. Volumes are thus controlled by varying the output of the preamp to the power amp.

If the amp is still under warranty, take (or send) it to an authorized repair center. The simple act of removing the back will void the warranty.

Assuming the amp is out of warranty (or if you don' care about voiding it) you can test the pot with a multi-meter by checking for resistance. See here - http://www.instructables.com/id/How-to- ... /?ALLSTEPS

Best practice is to isolate the pot from the amp, so that downstream components don't impact the measurement. I believe (I can't remember as I have not been inside of mine in a while) the HT5 has a type of ribbon cable going to the pots. So you might be able to unplug/disconnect it. If not, it will need to be desoldered. Just be sure to drain the filter caps before poking around inside the amp. Even when powered off and unplugged from the wall, tube amps can store lethal current levels in those caps. So they need to be rendered safe first.

[JablesII]

Thanks for the reply, thephantum!

I'll check into that over the weekend. I haven't had a chance to fire up the amp since my original post; I hope the problem will have fixed itself but I know that's unlikely.

Just wondering; how common is it for a volume pot to just suddenly go bad like that, from one day to the next? I'd imagine the pot's performance would gradually degrade over time, but of course that's just conjecture. Won't know anything until I've checked inside the amp.

Thanks!

Joe

[thephantum]

It's just like any other component...sometimes they fade over time, sometimes they go "pop" and die in a blaze of glory.