AC15CC1 hum

[Valvehead]

I have this amp up and running and it sounds good except for a low hum . Its not any of the tubes. This is with master vol on 0. .some parts are new in the ps. Ill post a scem with the new parts / tested parts highlighted in yellow . Voltages in this area test normal . Also posting a video of the hum . D:\Guitar\Mods\AC15CC1_Schematic.pdf

any ideas???

[Valvehead]

vox vid

[Jon Snell]

I would look at pin 3 of the EL84s. Monitor the voltage.
If you remove the phase splitter, does the hum stop. If not, I would suspect a faulty EL84 bias as it sounds like either the main smoothing is not coping with the current draw or one valve is unbiased causing a ripple effect on the output transformer.

[The Dude]

Agree. Remove the PI tube to see if noise is coming from preamp or power amp section. Also, check output tube screen resistors. If one tube is not conducting, you might get this symptom.

[g1]

It will be helpful if you can tell us which parts were actually found to be bad, and which were replaced 'just in case'.
Just for example, if R69 and R70 for heater grounding were burnt, that would be worth pursuing further.

[Valvehead]

I would look at pin 3 of the EL84s. Monitor the voltage.
If you remove the phase splitter, does the hum stop. If not, I would suspect a faulty EL84 bias as it sounds like either the main smoothing is not coping with the current draw or one valve is unbiased causing a ripple effect on the output transformer.

Pin3 is 11.41v both tubes. Ive swapped out the el84s with a known good pair also. Pulling the PI has no affect on the hum

[Valvehead]

It will be helpful if you can tell us which parts were actually found to be bad, and which were replaced 'just in case'.
Just for example, if R69 and R70 for heater grounding were burnt, that would be worth pursuing further.

r69/70 test good (49ohms parallel)
1 cap was leaking so i replaced C132 / C31 . Also R72/73 5w because they looked like they been pretty hot , and 1 measured way hotter with my laser thermometer . The rest of the highlighted parts were checked and good. The hum is there with no PI and master vol on 0.

[glebert]

Is the hum 60Hz or 120Hz?

[Valvehead]

120 Hz

[glebert]

R68 is highlighted so it checked as good, did you verify that it has a solid connection to ground on the one side?

[Valvehead]

R68 ..d3...d4. Are grounded on BOTH sides . 1 ohm or so to ground. I can't get to c 56

[Jon Snell]

120HZ is a main smoothing issue, nothing else.
You state you checked pin 3 of the EL84 ... without the valves in what is the voltage on each one?
You cannot test individual EL84 cathodes as they are both connected together! One EL84 at a time and listen to the difference.
C56 has nothing to do with the smoothing, C56 functions as an RF shunt to earth from the chassis.

[Valvehead]

El84s out I have 0mv at each pin 3 socket

[Jon Snell]

0v on the grid pins, that is good, proves the coupling capacitors are good.
I presume with no EL84s, the amplifier is dead quiet? Please confirm.

[Valvehead]

Yes sir. Quiet