Another Hum-dinger Of A Question

[unclej]

i know..that was weak but it's late

ok, i finally got all of the parts for the ampeg that i've been working on. replaced the burnt resistor, installed the new hum balance pot and the new matched 7591 power tubes. nothing's overheating, no plates glowing and no smell of burning anything. and i sounds great! but..when idling with no guitar plugged in and with the standby switch in the on position there's an unacceptable amount of hum. i checked all of the pots and jacks for tightness and loose grounds and there was nothing there. there are no obvious loose connections or bad solder joints that i can see.

so i pulled the first power tube and nothing changed. i replaced it and pulled the second one and no change. then i pulled the 7199 which acts as a phase inverter/driver for the power tubes and all sound ceased. this leads me to believe that the problem is in the output circuit. unfortunately i can't test this particular tube on my tester but it heats up and doesn't make any noise when i tap on it.

now here's a weird thing..i reached across the chasis to pick up a probe and the pitch and volume of the hum changed as my hand passed over the front edge of the amp. i moved it around a little and noticed a big difference when i held it over the bias trim pot. again, louder and actually a sort of overlaying hum. i could hear the original hum underneath the newer hum. and each time i touched the chasis on the front edge i heard a tiny pop and the overlaying hum went away. i reversed the plug in the socket and the overlaying hum disappeared but not the original one.

so i'm thinking that could be one or both of the electolytic caps because when i turn the the standy switch on the hum builds up inabout the same time it would take a cap to charge and when i turn it of the volume of the hum peaks quickly and then fades.

any thoughts?

[Andy]

I don't know much about amps but it sounds like it could be a grounding problem. I get the pop and lack of buzzing on my guitar that sounds like you describe.

[lovekraft]

...i pulled the first power tube and nothing changed.  i replaced it and pulled the second one and no change.  then i pulled the 7199 which acts as a phase inverter/driver for the power tubes and all sound ceased.  this leads me to believe that the problem is in the output circuit. unfortunately i can't test this particular tube on my tester but it heats up and doesn't make any noise when i tap on it...

You're starting from the wrong end - if the hum disappears when you pull the phase inverter tube, it means that it's coming from that stage or some previous stage. If you start with the first preamp tube and move towards the power amp, you can positively locate the culprit - it's the first tube that you pull that stops the hum.

I can't help you with the other strange hum, but I do feel the need to mention that if you're running that amp ungrounded (two-prong plug), you're playing Russian roulette with the owner's health and well-being, not to mention your own potential liability. I'd strongly suggest you get that cord replaced and get the amp grounded correctly before somebody gets hurt! If the owner has a problem with it, send the amp home with him - you can't afford to be responsible for it.

[unclej]

thanks lk...i'll go back the other way with the tubes. and i actually picked up a grounded cord this morning to put on it. have'nt had time yet to install it but i'm totally with you on your concern there.

[lovekraft]

Excellent - I hate to sound pedantic, but I've developed an allergy to high voltage in my old age.

Let us know what you figure out - this has been an interesting process for me to follow, kinda like troubleshooting in the dark.

[unclej]

well i started at the other end and it was still the 7199. and i also discovered that the 7199 has become microphonic so i'll get another and see if that fixes it.

the thing that still puzzles me is that overlaying hum. again, it increases when i pass my hand over the bias trim pot and then i discovered this morning that it does it when i touch either of the 12ax7a's. none of the other tubes, just those. but the big hum doesn't stop when i remove either of those two. and i still get a slight popping when i tap the chasis. it sounds like it should be a grounding problem but so far i haven't been able to find anything that should be grounded that isn't. fixing the other hum may fix this also i suppose so i'll keep you posted.

[Paul Marossy]

Do those two 12AX7s have a tube shield? Sometimes tube shields help with weird hum problems. The other thing that comes to mind is that you may possibly have a faulty tube which is giving you an abnormal amount of hum. You could try swapping tubes and see if that has any affect at all. If all grounding has been verified to be good, then my bet is on a faulty tube.

Edited December 20, 2004 by Paul Marossy

[unclej]

hey paul...yes, the 12ax7's have shields and i think i've narrowed it down to the 7199. i've ordered another and will keep you posted.

[Paul Marossy]

Well, then, I'm placing a bet on that 7199...