1940s Oahu Amp with 60hz hum

[misterc57]

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

No, however that is what the schematic shows also.

The fun and challenge continues. I placed the choke and field coil in series. That did not work out as I have 245 VDC before the field coil and -2 VDC after the field coil.

Power Supply/Choke (260V) --> 22uF/field coil input (245V) --> field coil output/33uF (- 2V)

[Helmholtz]

What is the DCR of the choke (one end lifted)?

I'm not in favor of blindly modifying the circuit.

[misterc57]

What is the DCR of the choke (one end lifted)?

I'm not in favor of blindly modifying the circuit.

Choke read about 500K one end lifted.

It is quick for me to return to the circuit I started with.

[misterc57]

I checked every filter cap, all good. Checked every ground, all secure.

[Helmholtz]

Choke read about 500K one end lifted.

That explains the huge voltage drop, but means the choke is defective (essentially open).

Anyway, I think the choke is not necessary.

Maybe you can repair it by resoldering the wire at the lugs.

[misterc57]

Apologies. I read that wrong. Choke reads 470 ohms with one end lifted. The V drop across the choke was 15V.

[Helmholtz]

Choke reads 470 ohms with one end lifted. The V drop across the choke was 15V.

Now that sounds good (for the choke).
But then the large voltage drop is across the fiekd coil?

[misterc57]

Bid drop was at the field coil and 33uF cap junction. This was with the series wiring that I tried.

245V entering the field coil, -2 V exiting.

[Helmholtz]

Bid drop was at the field coil and 33uF cap junction. This was with the series wiring that I tried.

245V entering the field coil, -2 V exiting.

So what is the DCR of the field coil?

BTW, minus 2V is not possible.

[misterc57]

I cannot get a stable DCR reading on the field coil. I tried to read it with the harness disconnected. Do I need to have the amp on to do so?

-2 V not possible? Hmmm. That is what I read to chassis ground at the 33uF positive side. I can try again, but pretty sure about that reading.

[mozz]

Input tube should be a 6c5, look at the pinout, 6n7 is just a paralleled version that somebody stuck in there. Go with the type printed on the socket. Wouldn't matter anyway, both wil work if they are good. Field coil is open? Happens a lot. You need a perm magnet speaker then. Look at the choke, i can bet it is not factory, pictures of the mounting please. Wire it like the Oahu on the prewar webpage. Replace both 220k and the plate resistor on V2.

[Helmholtz]

I cannot get a stable DCR reading on the field coil. I tried to read it with the harness disconnected. Do I need to have the amp on to do so?

One end disconnected and amp off(!).

-2 V not possible? Hmmm. That is what I read to chassis ground at the 33uF positive side. I can try again, but pretty sure about that reading.

A negative voltage would mean a ground problem.
Any difference when you connect the black meter lead to the negative lead of the 33µ cap?

Is your meter battery good?

[misterc57]

My meter is wall plugged. I tried with both ends disconnected and could not get a stable reading. It kept climbing above 1M.

This is an interesting read about the 80 tube and use of chokes and filter caps. It states not to exceed 20uF on the caps. Not sure if that is total or for each one. My two existing 22uF caps are reading about 30uF each on the capacitance tester. Then there is the 33uF cap that is measuring 34uF.

http://www.emissionlabs.com/datasheets/EML80.html

[misterc57]

Input tube should be a 6c5, look at the pinout, 6n7 is just a paralleled version that somebody stuck in there. Go with the type printed on the socket. Wouldn't matter anyway, both wil work if they are good.

I thought at first it was the wrong tube. But schematic said 6N7 and the amp is wired to use both grid inputs, one for each input jack as shown on the schematic. Jack #2 goes direct to the second grid, just like the schematic.

Field coil is open? Happens a lot. You need a perm magnet speaker then.

The speaker is actually working. Guitar signal does get to the speaker. But maybe the field coil is still a problem.

Wire it like the Oahu on the prewar webpage. Replace both 220k and the plate resistor on V2.

The screws holding the choke in place look the same as all the other screws on the chassis.

Bypass the choke completely? That will be my next experiment. There is no value given for the V2 plate resistor, another 220K?

[misterc57]

Any difference when you connect the black meter lead to the negative lead of the 33µ cap?

I had tried that. Same -2V reading.