Hum in power amp

[Rattler66]

New guy here...............

I have a homebrew EF86 preamp-12ax7 PI-KT66 push pull UL.....I'm getting a hum sounds like a 60Hz, the hum remains even with EF86 and 12ax7 removed. So it makes me think the power amp, volume has no effect with PI tube out. any suggestions, thanks

[mykey]

New guy here...............

I have a homebrew EF86 preamp-12ax7 PI-KT66 push pull UL.....I'm getting a hum sounds like a 60Hz, the hum remains even with EF86 and 12ax7 removed. So it makes me think the power amp, volume has no effect with PI tube out. any suggestions, thanks

a shorted diode in the power supply

[Rattler66]

Thanks Mykey,

I forgot to mention though, I'm using a 5AR4 rect tube, I'm starting to wonder if my cap can is leaky, what's the best way to check without a scope?

[Amp Kat]

I'm thinking you have a grounding issue somewhere or maybe a pot or something that is either not grounded or grounded that shouldn't be. Is it motorboating hum or just pure 60 cycle hum ? If the cans are old it could be that but if new I'd have my doubts. The only test without a scope would be a leakage test but I've seen caps that pass this and still hum so swapping may be your best bet.

[Mark Black]

And don't forget to double-check the match of the power tubes, even if they are new. If one is pulling significantly harder than the other you won't get the benefits of phase-cancellation for the ripple in the first stage power supply node feeding the OT.

Good luck!

[R.G.]

I have a homebrew EF86 preamp-12ax7 PI-KT66 push pull UL.....I'm getting a hum sounds like a 60Hz, the hum remains even with EF86 and 12ax7 removed. So it makes me think the power amp, volume has no effect with PI tube out. any suggestions, thanks

Fixed bias?
Look for high ripple on your bias supply.

With a P-P power stage having no PI on it, the only effective sources of ripple are:
1. Grounding issues between the power stage and the now-inoperative preamp/PI. Try shorting the grids together. Still hum?
2. Heater short to grid or screen
3. High ripple or grounding problem with the bias supply. That's another input that we always think is DC. When it's not DC, it confuses us.
4. Hum pickup by the OT.
5. B+ ripple; however, it is hard to get much B+ ripple through a PP output stage. More likely to be ripple on the screen supply, which in turn means a problem with choke or screen supply filter. Take your voltmeter, set it to AC and measure ripple on the B+, screen supply and bias, taking note of the fact that the voltmeter is now floating on deadly voltages, and you might just die if you touch the wrong thing. Wear latex gloves, leather gloves over those, then rubber dishwashing gloves over those and only touch the amp with a 4 foot wooden pole.
6. Ghosts.

[Rattler66]

I'm thinking you have a grounding issue somewhere or maybe a pot or something that is either not grounded or grounded that shouldn't be. Is it motorboating hum or just pure 60 cycle hum ? If the cans are old it could be that but if new I'd have my doubts. The only test without a scope would be a leakage test but I've seen caps that pass this and still hum so swapping may be your best bet.

i'm going to recheck my ground scheme and see if there's any loops I've missed

[Rattler66]

And don't forget to double-check the match of the power tubes, even if they are new. If one is pulling significantly harder than the other you won't get the benefits of phase-cancellation for the ripple in the first stage power supply node feeding the OT.

Good luck!

i got another set that were matched at 41/42ma respectively, I'll throw those in rebias and see if difference.

[Rattler66]

Thanks a lot guys, will try all these things and report back

[mykey]

Thanks Mykey,

I forgot to mention though, I'm using a 5AR4 rect tube, I'm starting to wonder if my cap can is leaky, what's the best way to check without a scope?

use an AC volt meter across the cap with the amp on. how much AC is across the cap?
with all tubes removed except the rectifier, how much AC is across the cap?
if there is very little AC, the cap is still good.
now set the meter for DC and tell us how much DC is across the cap.
427 volts? don't get shocked, oh brave one.
is one or both of the KT66s turning red? like a bath heater?
can you replace the KT66 output tubes? if one is bad, this will cause a lot of hum. another possible cause at this point is a shorted output transformer.

[mykey]

And don't forget to double-check the match of the power tubes, even if they are new. If one is pulling significantly harder than the other you won't get the benefits of phase-cancellation for the ripple in the first stage power supply node feeding the OT.

Good luck!

I think Mark is right about that, if one tube is weaker or one is good and one bad, that can cause a pretty loud hum. The output stage needs to be in balance.

[Rattler66]

Fixed bias?
Look for high ripple on your bias supply.

With a P-P power stage having no PI on it, the only effective sources of ripple are:
1. Grounding issues between the power stage and the now-inoperative preamp/PI. Try shorting the grids together. Still hum?
2. Heater short to grid or screen
3. High ripple or grounding problem with the bias supply. That's another input that we always think is DC. When it's not DC, it confuses us.
4. Hum pickup by the OT.
5. B+ ripple; however, it is hard to get much B+ ripple through a PP output stage. More likely to be ripple on the screen supply, which in turn means a problem with choke or screen supply filter. Take your voltmeter, set it to AC and measure ripple on the B+, screen supply and bias, taking note of the fact that the voltmeter is now floating on deadly voltages, and you might just die if you touch the wrong thing. Wear latex gloves, leather gloves over those, then rubber dishwashing gloves over those and only touch the amp with a 4 foot wooden pole.
6. Ghosts.

if i connect my meter postive to positive (grounded on bias circuit) i get .5vac, if i set it pos-pos neg-neg i get 150vac and 124vac (two caps), from my psu filter caps nodes to ground 1000 vac, 800vac etc... very high?

[Rattler66]

i can't figure this out.............no matter what i do the stupid hum is still present, so i guess i gotta replace the cap can, although i really doubt that's it. no matter what i do it hums, when i turn on standby it is dead silent...........

[Rattler66]

use an AC volt meter across the cap with the amp on. how much AC is across the cap?
with all tubes removed except the rectifier, how much AC is across the cap?
if there is very little AC, the cap is still good.
now set the meter for DC and tell us how much DC is across the cap.
427 volts? don't get shocked, oh brave one.
is one or both of the KT66s turning red? like a bath heater?
can you replace the KT66 output tubes? if one is bad, this will cause a lot of hum. another possible cause at this point is a shorted output transformer.

my meter doesn't seem to work properly when checking the ripple, i'm getting over 900vac, can't be

[Rattler66]

i got another set that were matched at 41/42ma respectively, I'll throw those in rebias and see if difference.

i get hum no matter