1940s Oahu Amp with 60hz hum

[misterc57]

Thank you.... Updated schematic (probably not the last one) :-)

[Enzo]

OK, so am I right in saying your field coil is open so someone put a choke in its place? So in your drawing we could remove the field coil? That raises the question: is the original field coil speaker still being used? Or did they replace it with a magnet speaker? Certainly a field coil speaker with a dead field coil will make about the same amount of noise as a magnet speaker with no magnet.

You need to connect the OT primary to the B+ somewhere, likely right out of the rectifier.

[misterc57]

I know it is a head scratcher for me. The original field coil speaker is in there and working and sounding fine (in my opinion). In an earlier test when I removed the choke there was B+ to the field coil, but none returning.

[Helmholtz]

I know it is a head scratcher for me. The original field coil speaker is in there and working and sounding fine (in my opinion). In an earlier test when I removed the choke there was B+ to the field coil, but none returning.

Pic actually looks like a field coil speaker.

It's strange the amp would sound fine and loud with an open field coil, though.

If the field coil is actually open, the amp's voltage supply is solely provided via the choke.
So if you disconnect the choke from the rectifier (leaving everythig else connected), the amp should be silent.
If the field coil is good, the amp should work without the choke.

[misterc57]

So if you disconnect the choke from the rectifier (leaving everything else connected), the amp should be silent.

It was silent.

If the field coil is good, the amp should work without the choke.

It did not work without the choke.

[g1]

I think that due to the nature of the speaker construction, we don't really know what exact connections are. Is it possible that the resistance reading of the field coil was actually something else?
A higher resistance type field coil can go from B+ to ground.
Just a thought.

[misterc57]

So the NOS Raytheon 6N7G did not get rid of the 60Hz hum. It is also not processing the signal at grid pin 4, grid pin 5 is processing the signal. Bad tube?

[g1]

And here is another field coil arrangement that would measure 0V at one end.

[Enzo]

A higher resistance type field coil can go from B+ to ground.

And here is another field coil arrangement that would measure 0V at one end

Yes, exactly that. I encountered old juke box amps that way. In my adaptor drawer, I have a large 5000 ohm wire wound behemoth with clip wires to take its place when servicing them on the bench sans speaker.

[Helmholtz]

It is also not processing the signal at grid pin 4, grid pin 5 is processing the signal. Bad tube?

Did you connect the input signal via a cable plugged to input jack?

Does the "new" tube behave different from the old one?

[misterc57]

Did you connect the input signal via a cable plugged to input jack?

Yes.

Grid pin 4 connects from jack # 2 directly. This one is not working with the new tube. If I put the old tube back in, it works.

Grid pin 5 connects from jack # 1 through the volume pot. This works with both the new and old tube.

Does the "new" tube behave different from the old one?

I see higher plate voltage, around 60 V. Same amount of hum noise as the old tube.

[misterc57]

As a test I was thinking of swapping V1 and V2 tubes to see if that does anything with the hum noise. If I put V2 into V1 slot, I will only have grid pin 5 working. Since all the pins correspond I do not think it would hurt anything to try.

Am I wrong or wasting my time with that idea?

Thank you

[Helmholtz]

Am I wrong or wasting my time with that idea?

Doesn't make much sense to me as the tubes are different.

I'd say the replacement tube is defective and the old one likely good as measured voltages make sense.
Also I don't see how a somewhat worn tube would cause ripple hum.

If your filter caps are good I assume a grounding problem.

[misterc57]

I actually found another 6N7 in another amp I have. I put that in V1 and both grids work. Still have hum. But I did find that my original 6N7 is microphonic and the others I tried are not.

[misterc57]

This morning I lifted the plus side of all 3 filter caps. The 3 caps all use a different grounding point.

I clipped in 3 new filter caps, all sharing one ground connection. Still have hum. I tried 4 different ground point connections. It could be my imagination but it seemed that at one of those connection points the hum was not as loud (but still too loud) as the other 3 connections points that I tried.