AC15CC1 hum

[Mars Amp Repair]

Hello Valvehead,
This may be obvious, but I did not see the question asked: Does the amp hum with the EL84's removed? Even though this amp is most likely stock, is it possible, albeit waaaaaay remote, that the flux from the power xformer is inducing a magnetic field in the output xformer by virtue of not being 45 or 90degrees oriented to each other.
Just had to ask. glen

[Jon Snell]

Place a 47uF 450v capacitor across C132 and check with just EL84s fitted. (Assuming your EL84s are matched or you will get hum regardless).

[Valvehead]

C132 is brand new ... but I can try that

[Valvehead]

Place a 47uF 450v capacitor across C132 and check with just EL84s fitted. (Assuming your EL84s are matched or you will get hum regardless).

Tried that. It made the hum volume drop slightly but still there

[Valvehead]

I think i found it. Had to go get another meter to check bias.
with either pair of el84 , im getting 34ma / 45ma . seems way too high for plates at 358v , yes ?? should be around 20ma per tube ??

I dont think either pair is matched , and the pair that came in it look cooked .

Im thinking a new matched pair and then see what i have for bias ?

[The Dude]

FWIW: I have had to up the value of C132 in a few of these amps to quiet them down. This is after testing for nearly all potential failures. I've also had to raise the cathode resistor value to get bias in acceptable range.

Edit: First measure value of the existing cathode resistor and make sure the cathode cap isn't leaky or shorted.

[Valvehead]

FWIW: I have had to up the value of C132 in a few of these amps to quiet them down. This is after testing for nearly all potential failures. I've also had to raise the cathode resistor value to get bias in acceptable range.

Edit: First measure value of the existing cathode resistor and make sure the cathode cap isn't leaky or shorted.

Cath resistor tests good = 122 ohms. What value do you suggest ? I i might have something here to clip in--in series.
Cant really get to that cath cap = pcb mount radial. I can order 1 with the new tubes .

[The Dude]

If the resistor checks good, you're probably ok on the cathode cap. Since it's parallel with the resistor, if the cap was shorted, your resistor would also measure shorted in circuit. I honestly don't remember exactly what value resistor I used there, but it was the result of trying/clipping in several until the bias was reasonable. I would think 180-220 ohms should get you there.

[Valvehead]

If the resistor checks good, you're probably ok on the cathode cap. Since it's parallel with the resistor, if the cap was shorted, your resistor would also measure shorted in circuit. I honestly don't remember exactly what value resistor I used there, but it was the result of trying/clipping in several until the bias was reasonable. I would think 180-220 ohms should get you there.

Thanks !! Ill post back when i try the new tubes

[Jon Snell]

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

R68 if open circuit will allow 60HZ hum, not 120HZ!
No point in checking that component.

[Helmholtz]

Can you post a scope pic of the hum?
The waveform might give some clue.

[Valvehead]

I'll try to get to it sunday

[Mars Amp Repair]

Hello Valvehead,
This may be obvious, but I did not see the question asked: Does the amp hum with the EL84's removed? Even though this amp is most likely stock, is it possible, albeit waaaaaay remote, that the flux from the power xformer is inducing a magnetic field in the output xformer by virtue of not being 45 or 90degrees oriented to each other.
I had an amp, albeit a boutique amp, where I had to reorient the output xformer to eliminate that hum induction issue. I don't recall right off what brand it was.
Just had to ask. glen

[Valvehead]

No noise with no el84s. I think.the culprit is bias being way unbalanced and way too hot. New tubes on the way then might be changing that bias resistor

[Valvehead]

Thanks boys-- amp is dead quiet now ! new matched el84s and bias resistor . idle current is now 19/20 ma