1940s Oahu Amp with 60hz hum

[misterc57]

6N6 plate wiring does not look right to me. I may be wrong but I see the supply going directly to the plate pin 4 (as returned from the choke and f.coil).

HV --> 33uF/field-coil --> field-coil output --> 6N6 pin 4 plate in parallel with HV --> choke --> choke output --> 6N6 pin 4 plate

6N6 pin 3 plate connects to OT.

6N6 pin 3 is where the noise is (also where the .005 cap connects to ground).

6N6 pin 4 has no noise to speak of.

There is 429 ohms resistance between plates 3 and 4. If I remove the harness to the OT, there is an open between plates 3 and 4.


Pin 3 plate hum is around 200 Hz with volume at zero. As I turn the volume up it maxes out at a 60 Hz hum.

Amp produces sound and is playable at lower volumes but as the volume rises so does the hum.

Not sure how to calculate the power on this amp.

Hum stops with input tube V1 pulled.


I have updated the schematic (attached). I have updated plate resistors on V1 and V2.

The new R designations next to the original ones indicate what I actually see in the amp based on the old color codes.

[Helmholtz]

What you state in your text above doesn't correspond to the schematic shown.

In the schematic both 6N6 plates are connected, so they must have same DC and AC voltages.
Both should connect to the same end of the OT primary that doesn't connect to the B+.
It wouldn't make sense to not have the 2 plates connected.

If your actual amp wiring differs, it's probably wrong.
Please show your actual wiring.

[misterc57]

This is what I see for the actual wiring

[misterc57]

I am reading about the 6N6 at http://www.nj7p.org/Tubes/PDFs/Frank...veral/6N6G.pdf

Looks like only one half of the tube is actually the output section at the pin 3 plate?

[Helmholtz]

Wow, that's the strangest output wiring I've ever seen.
But with the explanation in the datasheet it makes sense. (I had looked at the 6N6P data before, but that's obviously a different tube.)
So the first triode is wired as a cathode follower directly driving the grid of the second triode which is the actual output tube.

Now we know that the "original" schematic is wrong.

[misterc57]

Looks like the original wiring configuration in there. Should one side of the OT primary be connected to ground?

[Helmholtz]

. Should one side of the OT primary be connected to ground?

Definitely no!

As said, the schematic of post #110 looks correct and should work. OT primary is always between output tube plate and B+.

Why do you want to change the wiring if the amp works?

If volume is too low, it's probably due to an open field coil. Try a different speaker.

[misterc57]

Definitely no!

As said, the schematic of post #110 looks correct and should work. OT primary is always between output tube plate and B+.

Why do you want to change the wiring if the amp works?

If volume is too low, it's probably due to an open field coil. Try a different speaker.

There are 4 wires on the harness connecting the OT and field coil. I may be confusing one of them. I thought the one going to ground is on one side of the OT primary. There is no exposed wire at the speaker and OT to measure continuity back to the harness connector pins. I am going to take another look.

4 wires at the harness. 2 for OT and 2 for field coil? Should any of them be connected to ground? One of them is.

The amp can get pretty loud, but the hum gets loud also.

Thank you

[Helmholtz]

If the hum stops with preamp tube pulled, the hum is coming from the input stage only.

[misterc57]

If the hum stops with preamp tube pulled, the hum is coming from the input stage only.

New tube should be here soon. Thank you

Updated schematic. Do not understand the ground connection on the OT primary.

[Helmholtz]

Updated schematic. Do not understand the ground connection on the OT primary.

I already said, that's wrong. Should go to B+ instead.
I'm sure it does. Otherwise you wouldn't get sound.

[misterc57]

I already said, that's wrong. Should go to B+ instead.
I'm sure it does. Otherwise you wouldn't get sound.

Okay. For some reason I just do not see it.

[Helmholtz]

As OT primary and field coil (should) have on common connection, it takes only 3 wires from the amp-
A fourth wire might be used to connect the metal parts and/or on side of the voice coil to ground.

[misterc57]

Excellent! Thank you

[g1]

Also, you show a short from pin3 of the 6N6 to ground. You have noted the cap value (.005) but you need to add the cap symbol in that line.