Help with Plexi clone hum

[martin manning]

I agree with you, better to take the CT to the reservoir cap and then run a lead to chassis ground from there. The next best thing would be to put the lead from the cap and the CT lead on the same ground lug (and ground the screen filter elsewhere).

[framos]

ok my friends, good news! Here´s what I did tonight:

1. checked the solder joint as noted by tubeswell, but it didn´t help.
2. tightened the PT and OT bolts, input jacks. No noticeable difference.
3. replaced the main filter cap and redid the 'star' ground. Didn´t help either.
4. grounded the 32-32 screen/PI filter directly to the chassis, at the transformer bolt, as suggested by martin. It definitely helped a tad, but it was not the main cause of the hum.
5. checked the v2 grid as also suggested by martin earlier today. Chopsticked it a bit, at first nothing, but then I tried to actually move it a bit harder... until it was in a almost perpendicular plane to the chassis... BINGO! The amp got DEAD QUIET.

So I turned off the amp, shortened the both the grid and cathode wires of v2, turned in on again, success. So quiet one could sleep next to it.

The next step would be to try RG´s suggestions (I left it last since it was the most laborious), but it was not necessary at this time.

So... many many many thanks for your help, it was invaluable, both on the practical side (no more hum) but also on the learning side.

THANKS!

[drew]

Comparing the photo of the completed amp to the layout diagram, do you have the wire from V2, pin 8 connected to the wrong end of the 33K resistor?

[martin manning]

Looks that way, Drew.

Given the solution to the hum, I’m thinking the source was the filament string. Those leads should approach the sockets radially, without any wrap around. Listening to the recording again the main part of the hum does sound more like 60 Hz than 120, and we should have validated that assumption. It’s likely that there was some 120 Hz in it that was reduced by improving the power supply grounding.

Good news, though, and we all learn something from these.

[framos]

Comparing the photo of the completed amp to the layout diagram, do you have the wire from V2, pin 8 connected to the wrong end of the 33K resistor?

I will double check that tonight, thanks for the observation!

(though I seriously suspect the wire is simply tucked beneath the board)