Marshall 1962 Bluesbreaker (JTM 45) Reissue Hum

[ca7922303]

You can't hear the ultrasonic oscillation showing on the scope.
But ultrasonic oscillation may cause audible hum.
The wavy up and down movement of the HF indicates underlying hum.

As there's hum with the preamp disconnected (by disconnecting the treble pot wiper), you have a power amp problem which should be solved first.
So do not connect a signal or instrument for now.

Please post a scope pic of output with NFB disconnected.

Will be able to post scope photo this evening when off work.

[ca7922303]

You can't hear the ultrasonic oscillation showing on the scope.
But ultrasonic oscillation may cause audible hum.
The wavy up and down movement of the HF indicates underlying hum.

As there's hum with the preamp disconnected (by disconnecting the treble pot wiper), you have a power amp problem which should be solved first.
So do not connect a signal or instrument for now.

Please post a scope pic of output with NFB disconnected.

Do I need to have wiper if treble pot dusconnect d along with the NFB loop fir the oscope photo?

[ca7922303]

You can't hear the ultrasonic oscillation showing on the scope.
But ultrasonic oscillation may cause audible hum.
The wavy up and down movement of the HF indicates underlying hum.

As there's hum with the preamp disconnected (by disconnecting the treble pot wiper), you have a power amp problem which should be solved first.
So do not connect a signal or instrument for now.

Please post a scope pic of output with NFB disconnected.

Photo of oscope with NFB loop disconnected, treble wiper connected, amp in standby.

[ca7922303]

Photo of oscope with NFB loop disconnected, treble wiper connected, amp in standby.

Photo of oscope, out of standby, power on

[Helmholtz]

Puzzled by the HF hash even in standby.
I'm not familiar with your scope and not sure it can be trusted.

Are you using a real scope probe with ground wire connected on amp side?

Apart from that the main difference out of standby seems to be the large spikes.
These will be audible and actually look like a heater hum problem.

I would try some other (non-JJ) ECC83 in V3 position.

[ca7922303]

Puzzled by the HF hash even in standby.
I'm not familiar with your scope and not sure it can be trusted.

Are you using a real scope probe with ground wire connected on amp side?

Apart from that the main difference out of standby seems to be the large spikes.
These will be audible and actually look like a heater hum problem.

I would try some other (non-JJ) ECC83 in V3 position.

As far as I know it's a real scope probe with a ground wire. I have the probe connected to the positive wire coming out of the 8 ohm speaker jack and the ground wire of the probe connected to chassis ground. Is this correct? Also, have 2-marshall branded, 1-mesa branded and 1-made in Mexico 12ax7s installed instead of the 4-JJs now.

[ca7922303]

Puzzled by the HF hash even in standby.
I'm not familiar with your scope and not sure it can be trusted.

Are you using a real scope probe with ground wire connected on amp side?

Apart from that the main difference out of standby seems to be the large spikes.
These will be audible and actually look like a heater hum problem.

I would try some other (non-JJ) ECC83 in V3 position.

Here's what I'm getting on scope now. 1st photo is in standby, 2nd is standby off. AC is set at 2V/div, m is set at 1ms/div.

[ca7922303]

Swap the power tubes around and see if the higher reading stays with the tube, or the socket.

What do you make of the idea that the PT being turned is causing hum. YouTube video says PT turned to position of an original JTM45 cures the hum issue?

[ca7922303]

We may have to live with the hum on this amp. I'd like to trace the ilow distorted output on channel 1 with a signal generator and DMM. Can I get some on tracing that down? Thanks.

[Helmholtz]

What do you make of the idea that the PT being turned is causing hum. YouTube video says PT turned to position of an original JTM45 cures the hum issue?

As Jon Snell said, that would be only promising if you could hear hum with V3 pulled or in standby like the guy in the video.

[Helmholtz]

Here's what I'm getting on scope now. 1st photo is in standby, 2nd is standby off. AC is set at 2V/div, m is set at 1ms/div.

Now that looks much cleaner than before (I think you had a measuring problem).

I think what you're hearing is the large spikes.
These are typical for heater induced hum/buzz.

I would try a humdinger pot, see here:
https://www.valvewizard.co.uk/heater.html
Requires disconnecting the heater winding center tap from ground.

[Helmholtz]

I'd like to trace the ilow distorted output on channel 1 with a signal generator and DMM. Can I get some on tracing that down? Thanks.

The only difference between the channels is that Ch1 uses V1b and Ch2 uses V1a.

Measure DC cathode and plate voltages (no signal) on both as well as supply voltage where R31 and R32 meet.

[ca7922303]

The only difference between the channels is that Ch1 uses V1b and Ch2 uses V1a.

Measure DC cathode and plate voltages (no signal) on both as well as supply voltage where R31 and R32 meet.

R31/32(302vdc)
V1 pin1(186vdc)
pin 3(1.5vdc)
pin6(196vdc)
pin 8(1.5vdc)

V2 pin 1(192vdc)
pin 3(1.3vdc)
pin 6(323vdc)
pin8(194vdc)

V3 pin 1(216vdc)
pin 3(35vdc)
pin 6(212vdc)
pin8(36vdc)

V4 pin 1(118-165vdc)
pin 3(.59-.75 vdc)
pin 6(322vdc)
. pin 8(120-167 vdc)

[Helmholtz]

R31/32(302vdc)
V1 pin1(186vdc)
pin 3(1.5vdc)
pin6(196vdc)
pin 8(1.5vdc)

DC voltages look good.

See if disconnecting one end of the 0.22µ cap that connects to the tremolo intensity pot makes a difference.

[ca7922303]

DC voltages look good.

See if disconnecting one end of the 0.22µ cap that connects to the tremolo intensity pot makes a difference.

Here's plate to cathode ac voltages (photo).