1940s Oahu Amp with 60hz hum

[Helmholtz]

1.17V across 3k means a (plate) current of 0.39mA, which will drop 117V across the 300k plate resistor.
So if supply voltage is 150V at the filter cap, plate voltage will be 150v - 117V = 33V.

Correct 220k resistors and a 3.5k (or 3.3k) cathode resistor would increase plate voltage.

[mozz]

Change both 220k. Clip out any old cap instead of just jumpering it, i don't know where that ever got started. V1 cathode cap, how is it?

[misterc57]

Change both 220k. Clip out any old cap instead of just jumpering it, i don't know where that ever got started. V1 cathode cap, how is it?

I did clip out and then jumped. Then soldered old one back in since there were no changes. Yes I need to change those 220Ks! V1 cathode cap is newish, I have not tested it. 10uF/63V, one installed at both V1 and V2.

[misterc57]

Here is my marked up schematic. resistor readings are what I am reading in circuit...

[Helmholtz]

Are choke and field coil in series?

[misterc57]

Better schematic showing choke and field coil

[misterc57]

Are choke and field coil in series?

No. See newer schematic.

As in; choke --> 33uF --> field coil --> 22uF --> 220K --> 22uF

[Helmholtz]

Better schematic showing choke and field coil

So they are in parallel. Strange wiring, as rippe suppression should be better without the choke. Wondering if they needed the choke to lower the field coil current.
The node after the field coil shouldn't connect to ground.
And, as you said , the input tube is 6SN7.

[misterc57]

Look at this schematic I just found online. Wonder if I should adapt the filtering series from it.

[Helmholtz]

Go step by step. Only one change at a time.

[mozz]

Last picture has the inductor first, that lowers your plate voltage, LC filter. That's probably a mistake in the drawing. On your last update schematic, I doubt very much they are in parallel. Your schematic has no output cathode resistor and your plates going to ground. What is the exact model and make of the amp, i will look in Riders book to find something close, what is the tube lineup also, 6sn7 or 6n7?

[misterc57]

Version 3 attached. Right, the HV line is not going to ground. Choke and Field coil are in parallel. The amp has definitely been updated before I got it.

There is no model on the amp. I think they were sold as a kit with the Oahu lap steel guitar in the 1930s.

Tube line up is 6N7G, 6C5G, 6N6G, 80

Tube sockets actually have the tube name factory printed on them. It shows 6C5, 6C5, 6N6, 80. The first tube is definitely wired as a duo triode and the label on the socket is wrong.

3rd image is not my amp, but that is what the front looks like.

Edit. I wonder why there is a .005 uF cap connected from the 6N6 pin 3 plate to ground.

[Helmholtz]

Tube line up is 6N7G, 6C5G, 6N6G, 80.

Edit. I wonder why there is a .005 uF cap connected from the 6N6 pin 3 plate to ground.

Still confused: Is the input tube a 6N7 or a 6SN7?
6SN7 is more likely as the 6N7 is a power tube.

The 5nF cap from plate to ground attenuates HF above the audible range. It might be necessary for stability.

But don't forget to focus on the 120Hz hum problem. If all your filter caps are good, you likely have a grounding problem.
Check all chassis connection for zero Ohm to chassis.
Or better use a ground bus that connects to the chassis at only one point.

[misterc57]

V1 = 6N7G

[Helmholtz]

V1 = 6N7G

Very strange to have a power tube at the input. Sure it's original?