Estey T12 guitar amp

[Helmholtz]

Is the hum 60hz or 120hz? I'm wondering if the hum is filament hum due to the old series widow maker filament arrangement which lacks a grounded CT or artificial CT as exists in more modern amps with a 6.3V filament supply.

No 120Hz source in this amp. PS ripple is 60Hz.

I still think the main problem is the iso being driven into single sided (partial) saturation. Saturation strongly increases radiated field.

Also the distorted AC voltage will increase heater buzz..

At least it should not be mounted inside the chassis. Easy to test.

[Helmholtz]

"I" don't mean anything by "input transformer" It was indicated in the opening post.

I meant to address the OP.
Had missed your post before mine.
All clear now from the schematic.

[g1]

I still think the main problem is the iso being driven into single sided (partial) saturation. Saturation strongly increases radiated field.

How about just changing to a simple SS rectifier?

[Helmholtz]

How about just changing to a simple SS rectifier?

Yes, I considered converting to a SS bridge rectifier.
Problem is the heater supply which requires the tube rectifier heater.
Also with a bridge rectifier the PT secondary cannot be grounded, probably increasing heater hum.

A solution could be using 2 separate isos.That would even allow for heater elevation, but.....

BTW, the heater supply requires 3 heaters to be wired in series.
Wondering what happened to the (necessary) tremolo tube heater?

[The Dude]

........No 120Hz source in this amp. PS ripple is 60Hz.....

Oh, now don't I feel stupid for not paying attention. Thanks for the correction.

[The Dude]

........BTW, the heater supply requires 3 heaters to be wired in series.
Wondering what happened to the (necessary) tremolo tube heater?

The tremolo triode is in the same tube as the preamp triode and the filaments of that tube would be series wired, so I'd guess the filament circuit is still wired as it originally was.

[35L6]

How about feeding the rectifier tube from the output of a full wave bridge ?

[Helmholtz]

How about feeding the rectifier tube from the output of a full wave bridge ?

Interesting idea.
Would restore the voltage drop of the rectifier tube and eliminate the DC loading of the iso.
Still requires to lift the secondary from ground, which is likely to cause heater hum.

[35L6]

The more I think about it the more I think the heaters have to go after the bridge . The problem with this is the voltage will be around 160V instead of 117V . That would make the dropper in the heater string mighty big and mighty hot ! It's the allure of more power and less hum that keeps me hanging on . What I've done in my single ended amp is add another filter section before power tube plate .

[35L6]

And at this point we should know what iso and rectifier we have .

[johnhoef]

I'm finally getting back to this today. I will try three ideas. First replacing that bypass cap with a larger value, as suggested 220mfd. After that, I'll try moving the isolation transformer far away. And finally, I'll be giving some thought to the filament AC wiring. Perhaps I can rig a DC filament circuit.

[johnhoef]

By the way, thanks for all the talk about the solid state rectifier idea. This amp belongs to a guitar player who LOVES tube rectification, so installing solid state rectification would make him very sad. However, solid state rectification for the heaters, yes! I'll give that some more thought. Thanks for that idea.

[johnhoef]

OK, I tried removing the isolation transformer far away. Ran a twisted pair from the secondary to the amp. No change. Hum is still at 55db, using my acoustic db meter. next I placed a 220mfd cap in parallel with the cathode bypass cap on the preamp tube. No change in hum...still at 55db. Again, just for kicks, I shorted that cathode direct to ground, hum is gone! Next, I'll try running DC through the tube filaments.

[Helmholtz]

I shorted that cathode direct to ground, hum is gone!

That renders the tube inoperable. So we know that the hum enters before or at the preamp tube.

Next, I'll try running DC through the tube filaments.

How are you going to do that? Using an external 120 VDC power supply?

[johnhoef]

I just did a test. I rigged that 200 ohm series resistor in the heater circuit so I could quick disconnect it while the amp is on. Quick disconnect of the heater AC would be a valid test to see if the heater AC is the problem. The heater would stay hot enough for that one second. So, doing the quick disconnect did change the "tone" of the hum, but the hum remained. How disappointing. SO, I continue to be at a loss here.