Quote Originally Posted by Simon Barden View Post
I just managed to fix an amp of mine that went noisily wrong just before a gig. It was the Fender Blues Deluxe that had been stripped out and the PCBs replaced by a hardwired turret board Fender 5E5-A circuit (40W Fender Pro) that I had mentioned above. It would be OK, then suddenly only produce a loud mains hum type noise. Seemed to be OK when starting cold, but then very soon switched into full-on hum mode. Checked for dry joints and damaged caps, but everything looked fine. It had a Weber Copper Cap rectifier (valve emulator) and I thought maybe that was the problem, as it seemed to be more 100Hz than 50Hz (and apparently reading the web, a failed rectifier can do that), so I had to order a new one from the US. But that didn't fix it. So in desperation I thought about what might cause a lot of hum and I then thought about the V3 phase-splitter valve. I had swapped the existing V3 valve out a while ago for a more balanced one that cured the background hum (The 12AX7 as you know is a dual triode and if the triodes have different gains then you get residual hum). So if one side wasn't working properly, then there would be a big gain mismatch and a lot of hum. Plus it was a fault that showed itself after the amp warmed up. So I swapped the valve out for another one and it all worked again (albeit with a slight hum). I hadn't come across a valve which worked for a bit and then didn't until now. They either worked OK or else behaved badly all the time.

So I ordered a new balanced valve yesterday, it came in the post today and it's all back to near-silent, with just some very gentle white-noise hiss. I had used it a couple of times with the speaker not plugged in for a few seconds before realising, and thought maybe I'd damaged the output transformer, but luckily this appears to be OK. Now just got to fit the Weber beam blocker (that I ordered as the same time as the Copper Cap rectifier) to the front of the speaker and I should have my gigging amp all set to go. Not too loud, not too heavy, great sound and hopefully no piercing beam of treble coming out of the front of the amp.

That's a bit odd, I'm guessing that the 12AX7 valve that was suspect probably had an internal electrode connection-issue that was temperature-sensitive, something I've never come-across before, another possibility is that the cathode-emission could have gotten a bit low, but at least you managed to fix it in the end.