HELP! My JCM 800 is acting weird and has lost its grunt!

Nerine

Hi there.

Ok, I'll be as brief as possible.

The other night at a gig, I realised that I had to whack the master on my JCM 800 well above 2 and a half, to get alongside my band. For those of you with experience of the JCM 800, you'll know that that knob position, unattenuated, is pretty damn loud and opened up usually.

Had my Suhr IsoBoost in the fx loop. My solo volume seemed to be very loud as usual though. It's like once it was engaged, everything became normal again, and the amp jumped up to the solo volume that you would expect from adding a few dBs of boost in the loop from an already loud JCM 800. It seemed like the difference was a large Jump. Regardless how little boost I added with the Suhr.

I thought a revalve would be good.

Did that. Winged C and Tung Sol and Gold Lion mix in the preamp.

Used amp straight in with a cable. Nothing in loop, nothing in the front.

The amp has become very quiet.
I can happily stand in the room and loudly talk above it when it's set to 5 on the master.
Everyone that's familiar with a JCM800 knows that that's bloody loud and quite frankly used up all the headroom in the amp by a long way.

Here's the kicker....

If I keep wrapping the volume round to 10, the amp does actually get a lot louder and seems to be somewhere near where you would expect an 800 to be.

It's like the taper of the volume pot has changed. The sweep still adds volume noticeably so between 6 and 10 on the dial?!?! WtF? I should add that the tone seems intact, and it still sounds awesome, just very quiet....

Before hand, and how I've always known it, is that it's headroom would usually be used up at about 3 on the master and the power section starts to get involved after that.
I also know that that is very loud under normal conditions.
3 on my dial at the minute would get drowned out by a drummer easily.

I've tried checking valves etc, fiddling with the loop, trying the low input, swapping valves out and anything I can think of really, but I've sort of run out of ideas.

The only thing I'll say, is that the output valves did seem to be not the snuggest of fits in the sockets, but the amp was noticeably down on power at the last two gigs I did with it last weekend, and that was before I revalved it anyway.

What on earth could be happening here?

In short:

Why has my volume upped sticks and moved to further down the dial?

Why has the tone seemingly remained unaltered?

Is this an fx loop issue?

Shall I penetrate the loop jacks with some cleaner?

Shall I remove the chassis and have a look round?

Could it be resistors or caps etc that have gone??

If anyone needs any more info, I'll do my best to add details.