1940s Oahu Amp with 60hz hum

[misterc57]

I have an 4 tube Oahu. Tube line up is 6N7G, 6C5G, 6N6, 80. This is a one knob (volume) amp with two inputs.

I have a 60 cycle hum that is only present when the 6N7G tube is inserted. Hum gets louder as the volume pot is turned up.

I clipped in a new filter cap that supplies that tube but no change. I do not have another 6N7G to try, I will be ordering one today.

Tube tested good on my tube tester, but that does not mean it will sound good. Can a tube cause the 60 cycle hum or is there anything else I can look for?

Both plate voltages on the tube are 253 VDC. Cathode is -0.04 VDC. Heaters are 6.5 VAC.

Closest schematic I could find is attached. It shows a different rectifier tube but the rest looks the same. Oddly input #2 bypasses the volume pot, not sure why.

Thank you. MC

[Jon Snell]

Mains hum is normally caused by a bad earth/ground. This is what I would do;

Connect test leads to short circuit the grids to the cathode ground, as shown in yellow.
Hum gone ... remove the links one at a time to reveal the bad ground.

Still hums? ... connect the blue link.
Still hums, replace the 6SN7.

[Helmholtz]

[ATTACH=JSON]
Still hums, replace the 6SN7.

Or try a humdinger first.
Always a better solution than a grounded heater CT as it allows to vary the cathode to ground potential in amplitude AND phase for best heater hum compensation.

BTW, input grid pin 4 doesn't is missing a grid leak resistor.
Make sure input jacks are actually shorting to ground.

[misterc57]

This is getting more interesting. There are two layers of 60 Hz hum.

I grounded the two grids at V1 (6N7), pins 4 and 5. This left a less dominant 60 Hz hum that is not affected by the volume pot. Pin 5 is the offender which produces a louder 60 Hz hum affected by the volume pot. Pin 4 was not a problem.

The grid leak resistor for pin 4 is actually at the input jack # 2, however is is shorted to ground. Not sure of the original value, big old yellowish resistor in picture. I will try a 68K here?

Thank you. MC

[Helmholtz]

So the most likely explanation is heater hum.
A scope picture from the output could help to verify heater hum as it has a typical pattern.

Otherwise proceed as advised.

[misterc57]

I need to recalibrate what I am seeing...

There are no grid stoppers to either grid and also none shown in the schematic.

I tried the blue line jumper from pin 7 (heater) to ground. This made the hum louder and increased the current flow about .3 amps through the entire amp.

[Helmholtz]

I tried the blue line jumper from pin 7 (heater) to ground. This made the hum louder and increased the current flow about .3 amps through the entire amp.

Means you shorted one half of the heater winding. (Note heater CT!).

Sorry, hadn't looked at the blue line before..

[misterc57]

Sorry, I am not getting it, bare with me please.

The heater has a CT at the PS. This CT is connected to chassis ground.

[Helmholtz]

Sorry, I am not getting it, bare with me please.

The heater has a CT at the PS. This CT is connected to chassis ground.

Because of the CT, each heater pin sits at 3.15VAC wrt ground.

[g1]

You can have the heater CT grounded, or ground one side like the blue line, but not both. If you want to try the blue line you must disconnect the CT.

[Helmholtz]

As indicated earlier: Use a humdinger and disconnect the CT.

And/or try a different 6N7 (or is it actually a 6SN7?) as Jon Snell recommended.

[mozz]

60hz? Bad tube.

[misterc57]

6SN7. I ordered one, will likely be 7 to 10 days before I see it.

Humdinger is new to me. If the new tube does not work I will go down that path. Can anyone briefly explain the steps to making a humdinger in this amp?

Thank you

[Helmholtz]

See here: http://www.valvewizard.co.uk/heater.html

[misterc57]

Here is a scope shot at the output. Nothing connected to the inputs. Amp at idle.