Thought some of you might be interested in this. Been busy with other work but got some time yesterday to do some long needed work on my own amp. This is '71 JMP 50 (see serial number below) that I bought about 3 years for a very good price. I got the price because the power transformer had been changed at some point. It was replaced with a good quality replacement that had been fitted very well so it didn't bother me in the slightest, plus it sounded great. It still had the original caps in there and lately (for the last 18 months!!!) I'd been noticing I was pushing it harder to get the same punch plus t was getting a bit noisy. Got the time yesterday so it was time, eventually, to get in there and do something about it. Following a closer inspection the history of the amp became a bit clearer. The bias circuit had been modified and the IEC power connector and earth lug were definitely new. See photo below. My theory now is that this was an American model with 6550s that someone brought over to the UK, where I bought it. The guy who brought to the UK, my guess, plugged it straight in damaging the PT, power switch and fuse holder. The tech seems to have replaced the PT with a 240Vac model, installed a grounded IEC plug and modded the bias circuit for EL34s. Here's the preamp section as I bought it. Anyways, I decided that I wanted to replace the electrolytic capacitors, internally jumper the input channels and install a post phase inverter master volume, but do it all in careful manner that I could easily reverse at a later date. I plan on keeping this amp for the foreseeable future so I went with the best quality replacement capacitors that were available from my usual suppliers. I went with TADs. My first step was to remove the old capacitors. Notice the red jumper cable in this photo. This jumper is discharging the capacitors using the V1 plate to ground method. I first verified that the discharge path from the main cap was complete and this is very important when using this method. If there is an open somewhere along the line this method will not work. I like to leave it in place a long as is practically possible especially when working on vintage amps. Here's where the preamp filter is located. Beside it a shot of all the caps removed. Here's a shot of what, hopefully, is a nice solid solder joint. Notice that it looks nice a shiny, even with lead free solder, with not too much solder blobbed on. Here the preamp and poweramp after the cap change and mods. Hopefully it looks nice a neat!! Here's the rear of the amp after work showing the new caps and master volume. How does it sound? It sounds f*ckin' awesome!! I'd have clips made only I plugged in and started playing and only stopped when the neighbours called! The MV works great but I still like to push it too 5/6 to get the feel and that's still too loud for a three bed semi apparently!! Won't get chance to record properly 'till the weekend.