Hum - PT inducing this into the OT?

[charlesc]

I have hum while in standby (100 watt Marshall head) and I'm thinking it's the PT inducing hum into the OT and I'm waisting my time trying to reduce it?

1. Ground is good and tested amp at a different location across town, no effect on hum.
2. Volume has no effect on hum.
3. Pulled all preamp and power tubes, still hums, no effect
4. Disconnected OT primary so there's no signal in, the hum would now have to be induced into the OT secondary via wires or PT<>OT induced.
5. Just for good measure I disconnected PT bias and HT supplies so at the moment there is only 120vac into the PT and 6.3vac PT out to the heaters. Still hums same as before.

The hum is not terribly loud, a 1 on a scale of 1 to 10 but not hard to hear a few feet away.

[Enzo]

You're probably right.

Disconnect the scondary wires and connect a speaker directly to it with clip wires. That will eliminate any possibility of heater or mains current getting into it through the pc board. Disconnecting the 6vAC winding from the board would probably accomplish the same thing.

And you've come so far already, then dismount the OT and move it away from the chassis as far as wires will reach. COnnect a speaker directly to it. Does that kill the hum, and can you control the hum volume by moving the OT nearer and farther from the PT?

Those tests should make it final that it is or is not magnetic coupling. If it is, you might be able to remount something at a different orientation. You might be able to stick a faraday shield between them.

Or you might be able to ignore it.

[charlesc]

I decided the hum is so low that its a normal and certainly a non-issue with this amp. Got it all soldered back together and it down right snarls with the volume on 10 with some nice harmonics. 16 ohm THD hotplate was manditory whoa that amp is loud!

[Diablo]

I had the same low volume hum on standby with my Ampeg V4 100 watt head. Removed the power tubes - same thing. The only time the hum got quieter was when I moved the impedance switch to lower ohms. It was silent on 2 ohms. Like you, I assume this is just magnetic coupling with the power transformer.

[jrfrond]

With tubes out of the picture, it is a classic case of induction hum. This seems to be more of an issue in vertical-mount than core-mount (horizontal) transformers. My guess is that the magnetic flux lines are running tangent to each other at that point, so induction is minimized.

When designing my own amps several years back, I built two identical prototypes, one with a core-mount Fender replacement PT, and the other with a vertical-mount Hammond PT. The first one was close to dead quiet, while the second one had a low-level induction hum I just could not get rid of, no matter how I repositioned the PT. It was a pain, because I wanted to avoid cutting a big square out of the chassis, but I wound up contracting the chassis to a company that specialized in it. Vertical-mount would have allowed me to keep it in-house, and no, I don't have a milling machine.

[charlesc]

This seems to be more of an issue in vertical-mount than core-mount (horizontal) transformers.

Interesting, this is a mid 1968 100watt Plexi clone so the PT is laydown while the OT is standup but they are only like 1 inch apart. Towards the end of 1968 Marshall switched both PT and OT to stand up, turned one 90 degrees to the other and spaced them several more inches apart. I don't know if that decreased the hum but it seems logical, I seem to recall the induction falls off fast with distance.

[jrfrond]

One inch is pretty damn close. Is your chassis steel or aluminum?

[Wakculloch]

If the mounting bolts are long enough, or get some longer ones, and get some nuts to go between the transformer and chassis. If its bolted directly, the chassis can act as a transformer lamination inducing hum all over the shop.

[Enzo]

Wait, this wouldn;t be a DSL100 or a TSL100 would it?

If so, there is a small disc cap next to the power tube closest to the power transformer. I think it is C46. it is wired between pins 3 and 4 of that power tube socket. Is it shorted?

Oh wait, not what you have, never mind.

[charlesc]

If the mounting bolts are long enough, or get some longer ones, and get some nuts to go between the transformer and chassis. If its bolted directly, the chassis can act as a transformer lamination inducing hum all over the shop.

That's interesting, I'm going to pick up some fiber washers and raise it up off the chassis .060 or so and see if that makes any difference.