Marshall 1962 Bluesbreaker (JTM 45) Reissue Hum

[ca7922303]

Need to bias power tubes and locate hum. Not able to record due to hum. 5881s reading(30mA/21mA) w/eurotubes bias probe. See a video on YouTube that guy is rotating power transformer to be facing like original jtm45. Says marshall designed incorrectly and is causing hum. Would like some opinions on that video. Thanks.

[ca7922303]

Need to bias power tubes and locate hum. Not able to record due to hum. 5881s reading(30mA/21mA) w/eurotubes bias probe. See a video on YouTube that guy is rotating power transformer to be facing like original jtm45. Says marshall designed incorrectly and is causing hum. Would like some opinions on that video. Thanks.

https://youtu.be/UbynIuLdIuo

[g1]

5881s reading(30mA/21mA) w/eurotubes bias probe.

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

[ca7922303]

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

Swapped 5881s, (30mA) reading stays with tube.

[g1]

Do you have a better matched pair? Try the 6L6's that matched up same in the Ampeg R-12.

[Jon Snell]

If your output valves, push pull in this case, are not balanced, hum will be in the background or forefront, depending how far out of balance they are.
Always fit a matched pair.

[pdf64]

Do any controls affect the hum?
Remove the valve in V1, what effect does that have on the hum?
Repeat for V2, then V3.

Unless you enjoy stressing the rectifier valve, avoid using the stupid hot switching standby.

https://el34world.com/charts/Schemat...e_45w_1962.pdf

[ca7922303]

Do you have a better matched pair? Try the 6L6's that matched up same in the Ampeg R-12.

Installed 2-6L6s both reading (32mA).

[ca7922303]

Do any controls affect the hum?
Remove the valve in V1, what effect does that have on the hum?
Repeat for V2, then V3.

Unless you enjoy stressing the rectifier valve, avoid using the stupid hot switching standby.

https://el34world.com/charts/Schemat...e_45w_1962.pdf

With no signal to inputs, presence is only control having any effect on hum. On zero is scratchiness, at 10 it's it's full treble. Have sprayed pot with contact cleaner and worked pot.
V1 removed-no effect V2 removed-no hum but presence pot still scratches
V3 removed-no hum, no issue with presence pot

[pdf64]

Presence is always scratchy in such circuits, ie that put DC voltage across the pot. So not a repair / maintenance issue.

Has a known good valve been swapped into V2?

[ca7922303]

Presence is always scratchy in such circuits, ie that put DC voltage across the pot. So not a repair / maintenance issue.

Has a known good valve been swapped into V2?

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

[ca7922303]

Presence is always scratchy in such circuits, ie that put DC voltage across the pot. So not a repair / maintenance issue.

Has a known good valve been swapped into V2?

Filter caps?

[Jon Snell]

If you are following the schematic from EL34world, it is incorrect around the NFB tone control and more than likely, in other places.

Is the hum mains frequency or twice mains frequency. At least with that information we can determine whether mains hum or smoothing hum.

[ca7922303]

If you are following the schematic from EL34world, it is incorrect around the NFB tone control and more than likely, in other places.

Is the hum mains frequency or twice mains frequency. At least with that information we can determine whether mains hum or smoothing hum.

I believe it to be 60hz by comparing it to an audio sample online.

[ca7922303]

If you are following the schematic from EL34world, it is incorrect around the NFB tone control and more than likely, in other places.

Is the hum mains frequency or twice mains frequency. At least with that information we can determine whether mains hum or smoothing hum.

What do you make of the thought that turning the power transformer to jtm45 original position can lower the hum?