1940s Oahu Amp with 60hz hum

[Helmholtz]

I just realized I am probing on the primary side of the output transformer, yellow clip. The OT is off chassis by the speaker

That makes a huge difference in voltage level.
Looks as if you're not using a real scope probe?

What is the choke thing in the picture?
The schematic doesn't have this as there is the speaker field coil.

[misterc57]

I have the clips connected to the scope probe.

Yes there is a field coil speaker. The choke is not in the schematic. I will draw up how the choke is wired. Maybe the choke was added.

[Helmholtz]

I have the clips connected to the scope probe.

I don't think extending a scope probe is good idea.
Unshielded wire is likely to pick up extra noise.
Also don't forget that you may be probing lethal voltages.
If at all you may extend the ground wire only.

[misterc57]

Observations

• The choke and field coil kind of share the same wiring. The 33uF filter is before the field coil, but not the choke.
• There is a newish not leaking death cap hanging off the fuse.
• The hot mains goes through the fuse.
• The neutral mains goes through the switch.
• The ground mains is properly connected to the chassis.
• There is a missing filter cap to V1.

[Helmholtz]

Observations[*]There is a missing filter cap to V1.

That could explain the 120Hz ripple hum.

[misterc57]

Does size matter on the missing cap? 10uF, 22uF?

And here are scope output at the speaker. First image is volume at zero, 2nd image is volume around 7. Nothing connected to amp and no grid stoppers.

Thank you

[misterc57]

I clipped in a 22uF with a 220K plate resistor for the missing cap. No change at all.

Also I get a big V drop at the plates, from 80 VDC to 20 VDC

[misterc57]

With everything back to as I found it, I am only seeing 32 VDC at the plates of 6N7G. 150 VDC before the plate resistor, 32 VDC after. Seems too low. Plate resistor is reading about 300K

[Helmholtz]

Does size matter on the missing cap? 10uF, 22uF?

And here are scope output at the speaker. First image is volume at zero, 2nd image is volume around 7. Nothing connected to amp and no grid stoppers.

Thank you

Both pics show mainly 120Hz with some additional 60Hz.

BTW, the wiring is a mess. I'm not able to follow it.

[Helmholtz]

I clipped in a 22uF with a 220K plate resistor for the missing cap. No change at all.

Also I get a big V drop at the plates, from 80 VDC to 20 VDC

Makes no sense except if the 22µ cap is bad (leaky).

[Helmholtz]

With everything back to as I found it, I am only seeing 32 VDC at the plates of 6N7G. 150 VDC before the plate resistor, 32 VDC after. Seems too low. Plate resistor is reading about 300K

Measure the cathode voltage.
See if voltages change when you disconnect the cathode cap.
Also measure the cathode resistor.

[misterc57]

The one I clipped in is a brand new F&T. The other 22 uF feeding V1 that is in there I had substituted with the same new 22 uF F&T. All no changes. I could try a different new 22 uF.

I wish I had a better schematic that showed values and voltages. I can see that every cap has been replaced before I opened it up. I wonder if there is too much filtering if possible or how much values differ from the original design.

Also the big V drop taking place at the V1 plates, from 150 VDC to 33 VDC across the plate resistor which reads about 300K ohms.

33 VDC seems way to low (could this cause noise). If I read the specs right the 6N7 is looking for over 100 VDC at the plates?

The new 6N7 tube I ordered is still 4 days away.

Edit. Will try your suggestions/

[Helmholtz]

Some more:

If the choke is original, I doubt you have a field coil speaker. Check if the speaker has a magnet (using a screw driver) or a coil instead or post a pic of speaker.

Remove the death cap.

Replace all resistors that have drifted by 20% or more.

An ecap that lowers DC voltage has too much leakage.

It would be extremely helpful to see a corrected schematic with actual cathode, plate and supply voltages.

[misterc57]

Death cap removed.
Pic of field coil speaker attached. Note the 4 wire harness.
I think the choke is original.
I am going to markup the schematic to show what values and voltages I have found, coming soon.

Another weird find. A .005 uF cap connected from the 6N6 pin 3 plate to ground.

[misterc57]

Measure the cathode voltage.
See if voltages change when you disconnect the cathode cap.
Also measure the cathode resistor.

K V = 1.17 VDC
No change when cap disconnected
K resistor read 3K