Estey T12 guitar amp

[johnhoef]

A friend asked me to try and fix the hum on his Estey T12 amp. Replacing the filters was the first step, and that did help a lot. But, there is still more hum than we would like, and I have not been successful in reducing the hum any more. Hoping you all would give me some ideas. I made two modifications to the amp. I installed an isolation transformer, since this amp ran off the power line without any power transformer. Since the amp was now safe, I also removed the input transformer. I also installed a three prong power cord, connecting the chassis to external power ground. Other than these three mods, the rest of the amp is original. The amp hums without a cable plugged into the input. I did a few things in attempts to reduce the hum further:
1. Added more filtering. That helped a little, but not enough. It did bring the ripple down to practically zero!
2. Took a metal plate, grounded it to the chassis, and laid it on top of the open part of the amp - no change!
3. Replaced the tubes - no change
4. Disconnected the tremolo circuit - no change
I wonder if mounting the isolation transformer in the chassis is my mistake? It's way over on the far side where the AC wiring is. Perhaps another test would be to remove it temporarily to see if that helps? Grounding the grid of the output tube eliminates the hum completely. This tells me the hum originates in the driver tube. I then shorted the grid of the driver tube to ground, the hum persisted. Then I shorted the cathode of the driver tube to ground, the hum vanished, but of course, the amp no longer worked properly either! So, at this point, not sure what to do next. I'm including the schematic here, which I modified to reflect the changes I made....

[johnhoef]

One more thing I did was to create a ground bus, and redirected all grounds to that bus. Still no improvement in hum.

[johnhoef]

here's the schematic:

[The Dude]

Just hit the "Upload Attachments" box in the lower left corner, find and select the file. Choose Open

Edit: I see you got it while I was typing.

[The Dude]

When you say you "replaced the filters" did that include the cathode bypass caps? They are likely to be dried up as well on an amp of this age. A single ended amp will be noisier by nature, but it shouldn't be ridiculous.

[johnhoef]

Dude,
Yes, I finally did figure out how to post the schematic...thanks for trying to help me.
Yes, I replaced both bypass caps.
I have a feeling there isn't much more I can do with this to get the hum out.

[Chuck H]

Can we assume the posted schematic is modified to match the current circuit? You mention that you added an iso transformer and removed an input transformer which is represented in the posted schematic. But the posted schematic does not show a three prong AC wall plug or an earth ground. This should be implemented for safety. And, small chance of it but if there's dirt in the wall AC or external EMF this could only help.

EDIT: Also, since grounding the driver cathode eliminated the hum you might try a larger cathode bypass cap there. Maybe 220uf.

[Helmholtz]

What do you mean with "input transformer"?

The amp uses half-wave rectification. This has several drawbacks.

1) Means more ripple, requiring better filtering than full-wave. Try to add another 47µ across the filter cap after the 5.6k dropper.

2) The half-wave rectification draws a net DC current from the isolation transformer. Transformers don't like to be loaded with a DC current as this pre-magnetizes the core.
Consequently it might get warm, produce mechanical noise and will radiate a stronger stray field.
The stray field can couple into the preamp tube.
See if it helps to rotate the isolation transformer by 45°.

[g1]

It would be helpful if you can also post the original schematic.

[Chuck H]

What do you mean with "input transformer"?

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

Since the amp was now safe, I also removed the input transformer.

A three prong power cord is also indicated in the opening post but I don't see that in the posted schematic.?. Which We still don't know to be representative of the current circuit. We need to know what we're dealing with for any real evaluations.

[The Dude]

............ I'm including the schematic here, which I modified to reflect the changes I made....

The OP says this in post #1, so my assumption is that the schematic in post #3 is modified and that the lack of grounded AC cord in the schematic is an oversight.

[johnhoef]

Thank you, everyone for your thoughts. Yes, the thought did cross my mind to temporarily remove the power isolation transformer, and set it far away from the circuitry. I'll do that tomorrow and let you all know the results. If that does help, perhaps I can mount it on top of the chassis, instead if below where all the circuitry is. As I mentioned earlier, I did temporarily clip in more filtering, bringing the ripple essentially to zero! The result was no change in the hum. The green ground wire in the new power cord is simply connected directly to the metal chassis of the amp. Concerning the input transformer, it's purpose was to isolate the hot ground from the chassis. I eliminated it since the chassis is no longer hot. Here's the schematic of the original circuit:

[The Dude]

If grounding the preamp tube grid didn't negate the hum, I think we can eliminate anything before that grid as causing the issue. And, by the way, the volume control does just that in your modified version. So, if it's working and wired as the schematic shows, turning the volume control all the way down already grounds the grid of the preamp tube.

[The Dude]

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.

[Chuck H]

Moving the iso transformer occurred to me. But you said that grounding the cathode of the driver tube eliminated the hum. While that certainly renders that tube in less than ideal operating conditions it does seem to imply that the hum is in the filament circuit and maybe not caused by the iso transformer being in proximity to anything. That's why I suggested the big bypass cap in the preamp.