• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Troubleshooting hum in an Elekit TU-8200

I've got an original Elekit TU-8200 that I built back in 2016. It's developed a hum in the left channel, audible in the speaker as well as headphones. I've swapped the pre-amp and power tubes to no effect. Googling led me here (well, the Elekit forum here, but alas I can't post there yet 🙂, and suggests I need to replace my FET(s).

I've measured the voltages at the documented test points, and several on the left channel are out of spec. Anything else I should check, or just order some FETs and swap them out?

And @vkung if you're watching, I suspect you'd be able to nail this one down quickly.

Thanks!

Voltages (12AU7 and 6L6EH):

1 -> 3.8
2 -> 3.5
3 -> 3.6
4 -> 3.6
5 -> 123
6 -> 119
7 -> 127.1
8 -> 116.1
9 -> 220.2
10 -> 208.9
11 -> 1.1
12 -> 0.58
13 -> -20
14 -> -17.3
15 -> 0.03
16 -> 0
17 -> 0.46
18 -> 0.43
19 -> 285.8
20 -> 271.5
21 -> 285.4
22 -> 318.3
23 -> 291.3
24 -> 279.8
25 -> 287.4
26 -> -33.2
27 -> -34.5
28 -> 5.9
29 -> 6.2
 
Thanks all! I'm leaning towards the FETs as they seem to be a known issue that sounds much like what I'm experiencing (looks like maybe it was addressed in the 8200R version). Unfortunately, as a rookie here I can't yet post in the Elekit forum until I've built up some history.

As to the cap suggestion, if I'm going to have to pull it apart to solder anything in, then maybe it's time to think about upgrades too. I built it exactly as designed when I put it together originally (and was thrilled with it).