sudden hum and voltage drop - help !

[smzinno]

hello folks -

looking for some help troubleshooting a home brew amp. I've had this amp up and running for about a month and had been doing some preamp gain/tone tweaking. On the next power-up there is now a significant hum that I think is 120hz, although the amp still functions.

Upon measuring voltages I found I've lost 60-70 volts from my previous normal B+ levels (approx 400v is now 330), and my cathode voltage on the power tubes has dropped from about 25 to 7. The low B+ also seems to drop progressively lower as the amp is left on, about a volt or two per minute. I did find my cathode bypass cap used was mistakenly underrated (1000uf/16v), but does not appear to be blown/vented.

The design is a cathode-biased 2x 6v6 push-pull (russian 6n3c), with 2x 12ax7 and a 12at7 PI (all new); PT is about 350v under load and rectified (previously) to about 400v using a Russian 5u4c (I think equivalent to a 5Z4?). Filter caps are new JJ 50/50 cans. I tried a few different rectifiers with the same results, have not tried different power tubes yet in case they could get damaged.

Can anyone help with possible causes of these symptoms or a method to track it down ? Thanks much for any help.

steve z.

[paddy]

Hi Steve

just a couple of suggestions.

Replace the cathode bypass capacitor
Check/replace the main filter capacitors
Check the standby switch(should not read any voltage across terminals when switch is on)
Swap output tubes.

Best of luck
Paddy

[smzinno]

Paddy, thanks. I went over the items you mentioned and couldn't identify or correct the problem, but it was definately helpful in ruling them out as causes. I eventually found I had created a short when I mounted a small supply circuit for a cooling fan. When I removed it from the chassis, no more hum or low voltages. I guess it was basically sinking or starving the current, sure did wreak havoc (noise, voltage drop, red-plating). I may still be biased a little too hot, but at least everything is back to normal.

thanks again,
steve z.

[paddy]

Hi Steve

Glad to hear you found the culprit.It's annoying when we create faults
but you sure do learn a lot along the way.

Cheers
Paddy