Last saw this amplifier 2 years ago when the owner brought it in complaining of strange noises. From the amplifier, not himself. That was due to both the EL34 output valves being microphonic.
The problem now was that there was a lot of random background noise and other crackles, more so when the amplifier was cold. A few tests showed that the output stage did not seem to be to blame this time. Just talking about the output stage, it threw me a bit when I first saw it, as there were four output valves, 2 x 6L6GC and 2 x EL34. The circuit has 2 sets of valves and allows the user to swing between and or mix the drive to the 2 pairs. Not too sure if a guitarist will hear the difference, but that’s me. Anyway, back to the fault. With the chassis out, gentle tapping of the valves proved nothing was microphonic. As the background hiss was dependant of volume settings, the pre amp side of things was where the problem was. Turned out that one of the ECC83/12AX7 triodes in the preamp was noisy. Now, valves are going up in price. Usual racketeering, blamed on being at war with Russia, Covid, Monkey Pox, water drought, Sun Spots, lack of toilet rolls etc. I still have a good stock of valves here, around 3,000 give or take a cathode or two. Down to my last 20 ECC83s. In order to keep costs down, swapping the preamp valve with the final stage phase splitter is an option. The phase splitter circuit is far less sensitive to a slightly noisy valve. This shut the amplifier up and was all good.
Then, with the chassis upside down, had a good look around. Nice PCB, good components and so on, but messy and time consuming to get apart. Moving come cabling around produced lots of popping and intermittent audio. The amplifier is let down by the use of cheap little connectors with tiny crimped pins. These always cause problems. In this case, a little board where two user controls lived, was connected by 4 of these plugs and sockets. The wires were re-crimped, cleaned, resoldered and one pair of audio cables hard wired. All seemed to be in order. Oh, added a small 12V fan in the case to get rid of the hot and sweaty environment in the cabinet. I knows it’s a construction thing, but having an upturned chassis with 4 valves churning out some 20 Watts a piece can’t be great can it?