1484 Hum

[Smokebreak3]

Sluckey, it's funny that was on my list to lift the couplers to the grids but it's such a PITA to get down there I backburnered it. When lifted at the grids, the hum stops. I was getting some mV DC reading from one, but alas when I replaced them both(.02) the hum returned.

This indicates that the hum is generated in the PI (or prior to the PI) and passes through the .02 coupling caps. I would definitely pursue this thought.

I guess this begs the question though, what would cause hum before the .02 PI caps even with the preamp and PI tubes pulled?

[Astronomicum]

... the other is green. I dismissed it as another CT, but now I have no idea what it is.

It is the ground for the bias winding.

[nuke]

Also, when I pull the PI tube, the hum gets LOUDer.

The hum isn't mechanical, its coming from the speakers

Thanks, just had to ask.

With the PI out, what happens with the standby switch in the “standby” position?

[Smokebreak3]

... the other is green. I dismissed it as another CT, but now I have no idea what it is.

It is the ground for the bias winding.

Oh ha, yes, that's right

[Smokebreak3]

Also, when I pull the PI tube, the hum gets LOUDer.

The hum isn't mechanical, its coming from the speakers

Thanks, just had to ask.

With the PI out, what happens with the standby switch in the “standby” position?

In , or out, there is a very little amount of hum in the standby position

[Smokebreak3]

So I pulled C12, the splitload coupling cap feeding the grid of the stage driving the PI.
Hum remains.
Given that , and that the hum disappears when I lift C14 and C15 couplers to the 6L6 grids, we can say without a doubt the hum is generated in the driver/PI tube.

Edit :
And further, lifting C13 from the grid of the PI, the hum remains, so it HAS to be in the cathodyne node, eh?

Does that sound right?

[sluckey]

And further, lifting C13 from the grid of the PI, the hum remains, so it HAS to be in the cathodyne node, eh?

Sounds good to me. I've cleaned up the PI/PA schematic to make it easier to follow. I suggest checking all four resistors in the PI. Be sure R30 has a good connection to ground. Check for correct resistance readings from pins 2 and 3 to ground. Also check voltages and compare to the schematic.

I think you're almost there!

[nuke]

If hums with the PI (V3) removed, and it goes away on standby and returns when standby is on "operate" then it is coming from C14, C15, and R33, R30, R31.

The resistors you can easily measure in circuit. R33 goes to the screen power rail. R30 and R31 go to ground.

I would tend to suspect C14 and C15.

If one of those capacitors is leaky, the small amount of current will unbalance the pair of 6L6's enough to cause hum. Could be either, or both capactors.

I'd just replace them both with known good 0.02uf 400v (or 600v) film caps. I think it will fix it.

On point-to-point wiring like this, I recommend snipping the old part out near the body, bend the existing leads into a j-hook and then j-hook the new part onto the lead, then solder it into the circuit. Generally, they would have wrapped the leads around the tube socket pin before soldering. Difficult to remove those without causing more harm in the process.

[Smokebreak3]

If hums with the PI (V3) removed, and it goes away on standby and returns when standby is on "operate" then it is coming from C14, C15, and R33, R30, R31.

The resistors you can easily measure in circuit. R33 goes to the screen power rail. R30 and R31 go to ground.

I would tend to suspect C14 and C15.

If one of those capacitors is leaky, the small amount of current will unbalance the pair of 6L6's enough to cause hum. Could be either, or both capactors.

I'd just replace them both with known good 0.02uf 400v (or 600v) film caps. I think it will fix it.

On point-to-point wiring like this, I recommend snipping the old part out near the body, bend the existing leads into a j-hook and then j-hook the new part onto the lead, then solder it into the circuit. Generally, they would have wrapped the leads around the tube socket pin before soldering. Difficult to remove those without causing more harm in the process.

Fixing these old rats nests is fun ain't it ?!?

I certainly have been guilty of J- hooking, but I've also found that on the socket pins, I can solder suck first at the pi, then snip right at the pin, and I can usually get enough of a hole to get the new lead in. Sometimes the old wrapped lead end falls out! Bonus points for that. But yea, sometimes it's just not possible.

I suspected C14 and C15 too, and changed them yesterday, yet the hum remains...

[Smokebreak3]

And further, lifting C13 from the grid of the PI, the hum remains, so it HAS to be in the cathodyne node, eh?

Sounds good to me. I've cleaned up the PI/PA schematic to make it easier to follow. I suggest checking all four resistors in the PI. Be sure R30 has a good connection to ground. Check for correct resistance readings from pins 2 and 3 to ground. Also check voltages and compare to the schematic.

I think you're almost there!

Thanks! I'm close for sure.
I rebuilt ALL of the driver and cathodyne. Every single component. removed ground lug for that node's term strip, cleaned, retightened.
Still the hum remains. Swapped 6GC7s just in case (I don't have another one around)
At this point I'm looking at the choke, as I substituted a fat resistor for it, when I learned it was bad, early on.
Voltage doublers seem to be known for excessive ripple, and I have 4VAC at the top of the totem.
I mean, why else would they use a 7H choke in a budget amp in the 60's .
That doesn't explain why hum isn't injected into the screens and audible(possibly?) when output couplers are lifted, but certainly would explain hum when injected into the offending stage , eh?

I rummaged through my iron stash (found a few unused Classictones from the ol build days!) and only have a 1H 250mA choke.

So I need help specing the correct current capacity for this choke. Currently lloking at the Hammond 156LA : https://www.mouser.com/ProductDetail/Ha ... 9M2Q%3D%3D
Will 6H , 75mA be sufficient? This whole voltage doubler is throwing me off. Looks like the screens and the PI will draw through choke

[sluckey]

Did you replace the cap can (C25A, B, C)? And the original cap can is no longer connected to anything?

A 6H 75ma choke would be sufficient. Much better than a resistor. Don't overthink the stacked voltage doublers too much. I like to think of it as a four cell flashlight.

[Smokebreak3]

Did you replace the cap can (C25A, B, C)? And the original cap can is no longer connected to anything?

A 6H 75ma choke would be sufficient. Much better than a resistor. Don't overthink the stacked voltage doublers too much. I like to think of it as a four cell flashlight.

Yep I replaced the capcan also, rebuilt it internally on term strips.

I forgot I had an abandoned Twin chassis kickin arround here, and so I subbed in that choke (4H , 90mA), and that totally did the trick. Hum is gone.
Time to start keeping a spare choke around for troubleshooting!

Thanks for the help y'all!!

[Smokebreak3]

So my choices are the
Hammond 156L, 5H 75mA,
Hammond 155H , 5H 50mA

The 155H would be an EXACT drop in on the chassis, given the center to center mounting holes already there, which is 2" .
The 156L is 2.4" CTC, so would require shoehorning.

is 50mA enough?

[martin manning]

If a 90 mA choke works for 4x 6L6, a 50 mA should work for 2x, no?

[Smokebreak3]

Martin! Good to hear from you. As always, that makes perfect sense