Single Ended Master Volume Hum

[Hugo]

Hi guys, I'm new here and I hope to get some advice on how to deal with a little amp problem I'm having.

I built a fairly simple, single ended amp. There's a Marshall style preamp feeding a 1Meg master volume into a single octal output tube.

The amp sounds good, but with the master turned down below the halfway point, there is some annoying hum (80mV at the output). The more I turn the master up, the quieter the hum gets, until at wide open it's almost gone (below 10mV). I have adequate filtering, that can't be the issue.

This is idle hum, with nothing plugged in, unaffected by any control settings (gain, TMB), except the Master.

Any ideas what is going on here?


Your help is much appreciated. Thanks, Hugo.

[ThermionicScott]

Is it 60Hz noise or 120Hz? (or 50/100, depending on your area...)

[Hugo]

Good question, I forgot to add that. It's 120Hz, not noise from the tube heaters.

Hugo

[ThermionicScott]

Ah, okay. Any chance you could post a schematic? Is this amp in a chassis?

Just trying to think of ways to get more details about the build in the open.

- Scott

[loudthud]

What kind of grounding scheme did you use? It seems from the description so far that the Master Volume's ground is the source of hum. Have you tried moving it?

[Hugo]

The amp is very similar to the single ended schematic on the AX84 site. The topology is identical, CF fed tone stack into MV into single output tube. The master is exactly like the one in the schematic:

http://www.ax84.com/static/sel/AX84_SEL_080729.pdf

It is built on a sufficiently large chassis and grounded very close to the scheme described in Randall Aikens article:

Star Grounding

I have disconnected all relevant grounds and jumpered them all over the chassis, with zero reduction in hum. I tried different output transformers, no effect. I even physically removed the transformers from the chassis and put then more than a foot away, no effect. Input and output jacks are isolated from the chassis. Nothing I've tried had any effect. Massively increasing the filtering (220mF at the CT, 100mf at the screen) reduces hum by about 15mV at the output, still way to loud. Changes in preamp filter size had no effect.

With the master wide open the hum is gone. If I disconnect it altogether, the hum is gone. Master in the circuit, turned down, it hums.

I can pull both preamp tubes with no effect on the hum, but without preamp tubes the hum no longer goes away when I crank the master.

I am lost.


Hugo

[Hugo]

New finding: I was wrong about the frequency of the hum. It is in fact 60Hz, NOT 120Hz.

What's weird is that it isn't the heaters though. I tested this by disconnecting the heaters while the filament is hot. Even with no filament voltage connected at all, the hum remains unchanged (until the filaments cool off).

Where does this leave me?

It can't be the power transformer inducing hum in the output transformer, because I have removed the OT and jumpered it in a foot away from the chassis, with no change in hum whatsoever.

Do I have a defective power transformer? If so, why does the master volume affect the hum? Any ideas?


Thanks, Hugo

[Steve Conner]

Sounds like just power supply hum. Single-ended power stages have virtually no ripple rejection, and they do hum. Maybe you're just lucky, and hum induced at an earlier stage cancels it when the master is up full. Once you've fixed the power supply, you'll need to go after this hum source, too.

It should be 120Hz, is one of the diodes in your rectifier loose?

The only 100% effective cure I've found is to upgrade the filtering to a C-L-C. So you have one filter cap straight after the rectifier, then a big hefty choke, then a second filter cap, and everything in the amp runs off this second one, as if it was the first filter cap in your original design. With a choke you can make the hum completely inaudible. I have an old tube hi-fi amp with a CLC filtered supply, that has a noise floor 90dB below full output.

Of course it could also be grounding, but ground loop hum sounds more like a sharp buzz. Hum on the power supply is more of a low, muffled hum.

[loudthud]

A stack of spade lugs on a transformer bolt does not make a very good star ground. The order you stack the spade lugs will make quite a difference in the noise level. Actually, the ground scheme in the AX84 file couldn't be much better. It's the best I've seen in a long time. They've come a long way on that site since the last time I looked there.

[Hugo]

Sounds like just power supply hum. Single-ended power stages have virtually no ripple rejection, and they do hum. Maybe you're just lucky, and hum induced at an earlier stage cancels it when the master is up full. Once you've fixed the power supply, you'll need to go after this hum source, too.

It should be 120Hz, is one of the diodes in your rectifier loose?

The only 100% effective cure I've found is to upgrade the filtering to a C-L-C. So you have one filter cap straight after the rectifier, then a big hefty choke, then a second filter cap, and everything in the amp runs off this second one, as if it was the first filter cap in your original design. With a choke you can make the hum completely inaudible. I have an old tube hi-fi amp with a CLC filtered supply, that has a noise floor 90dB below full output.

Of course it could also be grounding, but ground loop hum sounds more like a sharp buzz. Hum on the power supply is more of a low, muffled hum.

I used my Fluke meter on the speaker out and it shows 60.02Hz. The amp has a rectifier tube socket. When I raised the filtering to see what would happen, I used two different SS rectifier plug ins. I can rule out a bad diode.

I have tried a choke (5H, from a JTM45), in a setup like you describe. 220mfd -> choke -> 47mfd (OT screen) dropped only 10mV of hum as opposed to no choke. Still very audible.

Something else must be going on. I just don't understand why the hum goes away completely as you dime the master.


Thanks for the advice, Hugo.

[Steve Conner]

Well, tell us more about your grounding. Loudthud is right, you can't just stack up some lugs on a transformer bolt. it needs more cunning than that. Well, at least I tried it once and it was a dismal failure.

In particular, the ground wire between the HV winding center tap and the first filter cap's negative terminal carries a heavy 120Hz pulsating current, and that current must be kept out of the rest of the ground system. A star ground isn't good enough to do this, you need a dedicated wire that runs from the transformer center tap to the cap terminal, and then another wire connects the cap terminal to the rest of the ground system.

[ThermionicScott]

Is one of your grid wires hanging out too close to a heater wire? Might be time for some aggressive chopsticking.

I wonder if 60Hz noise (maybe from the PT) is getting coupled onto the chassis, and that's why the gain stage is so sensitive to it when the grid is closer to ground.

- Scott

[loudthud]

Thanks for the kind words, Steve.

How does turning the Bass and Treble controls influence the hum with and without the 2nd preamp tube installed?

I'm thinking the hum enters on the ground end of the Master Volume and it is more or less shorted out by the driving impedance of the preamp tube and tone controls. So as the Master Vol is turned up, the hum is attenuated.

[Hugo]

Well, tell us more about your grounding.

If you're looking at the chassis upside down on the bench, upper left hand corner is guitar input, lower right hand corner is AC in. Power transformer is by the AC in, as far away from the signal input as possible.

Input jack and preamp grounds are grounded right by the input jack on a ground lug with a star washer digging into the aluminum chassis.

Main filter and HV CT are grounded by the power transformer, again, on a dedicated ground lug with a star washer.

In particular, the ground wire between the HV winding center tap and the first filter cap's negative terminal carries a heavy 120Hz pulsating current, and that current must be kept out of the rest of the ground system.

Yes, but the hum is 60Hz.

Is one of your grid wires hanging out too close to a heater wire? Might be time for some aggressive chopsticking.

I wonder if 60Hz noise (maybe from the PT) is getting coupled onto the chassis, and that's why the gain stage is so sensitive to it when the grid is closer to ground.

That's not it. Remember, the hum doesn't change even when I disconnect the heater voltage completely while the filaments are hot.

I wonder if 60Hz noise (maybe from the PT) is getting coupled onto the chassis, and that's why the gain stage is so sensitive to it when the grid is closer to ground.

That would make sence, except it's a non magnetic aluminum chassis and even if I physically remove the power transformer from the chassis and connect it with flying leads a foot away, it hums exactly the same as with the PT installed on the chassis.

Do you guys see why I'm frustrated?


Hugo

[Hugo]

How does turning the Bass and Treble controls influence the hum with and without the 2nd preamp tube installed?

I'm thinking the hum enters on the ground end of the Master Volume and it is more or less shorted out by the driving impedance of the preamp tube and tone controls. So as the Master Vol is turned up, the hum is attenuated.

Bass and treble controls have no effect on the hum, only the Master.

Your explanation makes sense, now why is the hum entering there and what do I do about it? The grounding of the MV is not the issue, I've moved it all over the chassis with no effect. Wire from the MV to the power tube is shielded.


Hugo