Marshall 1962 Bluesbreaker (JTM 45) Reissue Hum

[Jon Snell]

If you remove the output valves, power up the amplifier and you get mains hum, the mains transformer is interfering with the output transformer. If you have no hum, it is not the position of the transformers.
You can take that comment as fact because it is true.
So if no hum with no output valves, the hum is coming from the wiring of the amplifier.
You state that is you remove V2 the hum is gone but the clue is the crackle from the presence control.

It is not the smoothing capacitors because it is not twice mains frequency.

Is this a factory produced amplifier or a kit?

[ca7922303]

If you remove the output valves, power up the amplifier and you get mains hum, the mains transformer is interfering with the output transformer. If you have no hum, it is not the position of the transformers.
You can take that comment as fact because it is true.
So if no hum with no output valves, the hum is coming from the wiring of the amplifier.
You state that is you remove V2 the hum is gone but the clue is the crackle from the presence control.

It is not the smoothing capacitors because it is not twice mains frequency.

Is this a factory produced amplifier or a kit?

As far as I know it is factory.

[ca7922303]

If you remove the output valves, power up the amplifier and you get mains hum, the mains transformer is interfering with the output transformer. If you have no hum, it is not the position of the transformers.
You can take that comment as fact because it is true.
So if no hum with no output valves, the hum is coming from the wiring of the amplifier.
You state that is you remove V2 the hum is gone but the clue is the crackle from the presence control.

It is not the smoothing capacitors because it is not twice mains frequency.

Is this a factory produced amplifier or a kit?

Removed the 2-power tubes and there is no hum. What course of action would you suggest?

[Jon Snell]

I would look for the source of the hum, possibly a bad ground or even a leak from heater to cathode in V2. Have you checked the chassis connections between the earth buss bar through the vol control covers and then to the chassis?
Be methodical, start at V3 input. Does the hum increase/decrease on Volume 1 and/or Volume 2?
If it does, the issue is before the Volume controls. If not, it s after the volume controls.
With Treb, Mid and Bass at zero has it gone ... if it has you've got to the source of the problem. If it doesn't remove C10 and test.
If still there, the issue is almost nailed down to the phase splitter, check the coupling for the nfb loop and coupling capacitors C12 & 13.

[ca7922303]

I would look for the source of the hum, possibly a bad ground or even a leak from heater to cathode in V2. Have you checked the chassis connections between the earth buss bar through the vol control covers and then to the chassis?
Be methodical, start at V3 input. Does the hum increase/decrease on Volume 1 and/or Volume 2?
If it does, the issue is before the Volume controls. If not, it s after the volume controls.
With Treb, Mid and Bass at zero has it gone ... if it has you've got to the source of the problem. If it doesn't remove C10 and test.
If still there, the issue is almost nailed down to the phase splitter, check the coupling for the nfb loop and coupling capacitors C12 & 13.

C13(100nF)
C12(100nF)
C10( 22nF)in circuit

[pdf64]

I did swap a good 12ax7 @,V2 and hum about the same.

A problem can be that the DCCF V2 circuit there will evoke hum from valves that are hum free in other circuits.
And Russian made valves may not cope in a DCCF anyway.
That makes it difficult to advise what to do next, as it still might be due to the valves you’ve got handy having excessive heater to cathode leakage.

[Helmholtz]

A problem can be that the DCCF V2 circuit there will evoke hum from valves that are hum free in other circuits.

Interesting.
Have you found an explanation for the effect?
Does it show with new tubes?
Or is it some kind of cathode deterioration caused by high Vhk that develops over time?

[ca7922303]

I would look for the source of the hum, possibly a bad ground or even a leak from heater to cathode in V2. Have you checked the chassis connections between the earth buss bar through the vol control covers and then to the chassis?
Be methodical, start at V3 input. Does the hum increase/decrease on Volume 1 and/or Volume 2?
If it does, the issue is before the Volume controls. If not, it s after the volume controls.
With Treb, Mid and Bass at zero has it gone ... if it has you've got to the source of the problem. If it doesn't remove C10 and test.
If still there, the issue is almost nailed down to the phase splitter, check the coupling for the nfb loop and coupling capacitors C12 & 13.

Could you point me in the direction of the coupling for NFB loop? Thanks

[Chuck H]

If you have other 12ax7's available that are known good tubes please try several in the V2 position and see if any of them makes a difference.

[Enzo]

Interesting.
Have you found an explanation for the effect?
Does it show with new tubes?
Or is it some kind of cathode deterioration caused by high Vhk that develops over time?

Not directly appropriate but perhaps of interest:
In certain circuits - and I cannot recall which at the moment - a 7199 pentode/triode tube is used as a phase inverter. And the Sovtek ones, new from the box, would consistently be hummy via the heaters. My old RCAs worked fine in th same sockets.

[pdf64]

Interesting.
Have you found an explanation for the effect?
Does it show with new tubes?
Or is it some kind of cathode deterioration caused by high Vhk that develops over time?

Haha, I was just thinking of its unbypassed cathodes being susceptible to buzzy hum entering the signal path if the valve’s h-k insulation isn’t perfect.
Such hum tends not to be apparent if the valve is checked in a stage that has fully bypassed cathodes.

[ca7922303]

If you have other 12ax7's available that are known good tubes please try several in the V2 position and see if any of them makes a difference.

Before I reinstall this board, can you verify that the in circuit test of C10 will suffice. Helmholtz requested that I remove it to test and I got the correct reading without removal. This board is impossible to desolder any components without first desoldering numerous short wires attached to it. Thanks.

[Helmholtz]

Haha, I was just thinking of its unbypassed cathodes being susceptible to buzzy hum entering the signal path if the valve’s h-k insulation isn’t perfect.
Such hum tends not to be apparent if the valve is checked in a stage that has fully bypassed cathodes.

But then the effect should be much smaller than with an unbypassed input gain stage, where the hum is amplified by more following gain stages.

[Helmholtz]

Before I reinstall this board, can you verify that the in circuit test of C10 will suffice. Helmholtz requested that I remove it to test and I got the correct reading without removal.

I think Jon Snell meant to lift (one end of) C10 in order to isolate the preamp from the power amp.
Instead of physically disconnecting C10 you can disconnect the treble pot wiper instead.
If still hum with C10 disconnected, it's definitely not coming from the preamp, i.e. before the PI.

[ca7922303]

I think Jon Snell meant to lift (one end of) C10 in order to isolate the preamp from the power amp.
Instead of physically disconnecting C10 you can disconnect the treble pot wiper instead.
If still hum with C10 disconnected, it's definitely not coming from the preamp, i.e. before the PI.

Appreciate that. Could you point me in the direction of coupling for the NFB loop also? Thanks.