vintage amp shows no hum with external PS

[rubli]

Hello

I am stuck with a vintage mono amp, it hums... it has a 5Y3 rectifier. I changed this tube, all power supply capacitors, resoldered the ground connection etc. then I removed the rectifier tube, and inserted an external power supply , voilá, the hum disappears. if I insert the tube while the external power supply is connected, the hum reappears.

It does not matter where I connect the negative of the external power supply, near the orginal electrolitics or far away , no hum at all. I am really desperate as I can not find a logical explanation, or a solution.

any help will be appreciated

AR

 

[AllenB]

If this is caused by inadequate filtering of the original supply, you might add an RC section to test this, unless you can measure ripple.

 

[rubli]

Ripple is about 1volt . The dc supply is 350. The hum is 120hz. (My mains frequency is 60hz)

 

[6A3sUMMER]

How about a schematic?
That might be a good place to start analyzing the problem.

1V of ripple on a perfectly balanced push pull output stage will not have near as much effect as 1V of ripple would have on a single ended output stage.

 

[rubli]

The exact model is j12T,never found the exact schematic, but this one is very close.
I tried to ground 7591’s inputs or tying them togheter to isolate the rest of the circuit.
I also tried to ground either side of the 6.3 heather supply.

Thanx !!

 

[rubli]

I also removed “the death capacitor”, the one that connects between ac and ground

 

[trobbins]

then I removed the rectifier tube, and inserted an external power supply , voilá, the hum disappears. if I insert the tube while the external power supply is connected, the hum reappears.

It does not matter where I connect the negative of the external power supply, near the orginal electrolitics or far away , no hum at all.

A few initial queries come to mind:

Are you using the original transformer to provide heater voltage?
What external power supply are you using, and was it connected across C1A?
Did the external supply have a series diode protected output - as you connected it in parallel with the original supply when you put the 5Y3 back in.
Have you tried another 5Y3?
Have you put 1N4007 in series with each anode of the the 5Y3 - just in case the 5Y3 is leaky?
What circuit voltages have you measured and do you know if they are acceptable?
What meter are you using?

 

[rubli]

A few initial queries come to mind:

Are you using the original transformer to provide heater voltage?

Yes

What external power supply are you using, and was it connected across C1A?

It’s a vintage Eico variable power supply, and Yes indeed.

Did the external supply have a series diode protected output - as you connected it in parallel with the original supply when you put the 5Y3 back in.

I don’t know, it’s an old tubed PS

Have you tried another 5Y3?

Several

Have you put 1N4007 in series with each anode of the the 5Y3 - just in case the 5Y3 is leaky?

Nope, should I try it ?

What circuit voltages have you measured and do you know if they are acceptable?

Yes they are

What meter are you using?

A fluke scopemeter