Distorting reverb tube

[Tomislaw]

The reverb driver needs decoupling from the screen node, or from whatever node it is powered from in any build. The SE driver seems more problematic in this regard, being sensitive to layout and where its power comes from, where the push-pull driver does not have that issue.

I provided additional caps, but in this particular case their presence didn't prevent the tube from distorting the signal. But that's not the end of the story as I've got more amps on my bench right now. A friend of mine is a fan of American boutique amps and he's got several dozens of them, they're mostly still wired for 120V and I've been going over them changing taps on PTs and checking their overall condition while I'm at it. The ones with reverb tanks seem to need more attention as the reverb circuit usually introduces a lot of noise into the signal. One example - another amp with a typical Fender style reverb would start humming/buzzing when the Reverb knob was turned up. First of all, the input grid on the reverb recovery tube was floating and would pick up all sorts of noise. I added a typical 220K grid leak and that already made a big difference, but there was still some buzz to get rid of. What was causing it was the return RCA jack grounded at the chassis even though it was directly wired to the PCB ground. Isolating the jack from the chassis fixed the problem completely. That was a quick win but at first the amp looked like a nightmare to troubleshoot - two sided PCB and carbon comp resistors in all signal paths. At least plate load ones were metal oxide.

There's next one from another builder that I'm working on right now and this one will keep me busy for some time. First of all its EL34's were idling at 60mA each and they must have been doing so for quite a long time as their bases already changed colour. Then after an hour of testing the 5AR4 rectifier gave up with a flash, I installed another one and I can sometimes see the arcing inside when the standby switch is being turned off. I guess it's time to add protecting diodes on the rectifier as the first filter cap is 20 or 40uF, no more. The amp even has a ground switch with a death cap even though there's a 3-prong cable installed. I snipped the orange drop cap off immediately, what's the point of putting it there? Pure nostalgia or vague noise suppression method maybe? Weird tube arrangement as well, probably messed up big time by the previous owner. 12AX7 as the reverb driver and ECC832 as a reverb recovery tube. I could see one the scope how the second V1 triode is going nuts when the volume knob is past 9. By the way, all potentiometers are wired CCW which is very annoying! ECC832 in V1 position works fine and that's how it was probably designed. Newer versions use 12AX7 there, but - of course - there's no tube chart in the amp. As usual - the 12AT7 is distorting at the plate and probably the Ck bypass cap will have to go again, like in the case I described above. But wait, there's more! Carbon comp screen resistors are so nice to see and they even used those on the entire B+ line. There are no grid stoppers, no voltage decoupling, but there's plenty of random chassis ground connections. There's not even a single shielded cable inside the chassis and there's only one fuse for an entire amp. Not bad for a pricey build which is nothing more than a Princeton Reverb clone with a TMB tonestack and EL34 power section. Anyway, I'm really looking forward to making it usable again - it's a good lesson in troubleshooting. 

Tomi