New PT and OT, now I have hum

[HBamps]

I have an old build that was cool, quiet in the past but I always thought the transformers were undersized. The amp hasn't been played in a couple of years. This weekend I got to it and replaced both the PT (Hammond 270fx) and OT (Mercury deluxe rev job). Everything went pretty smooth until I started it up. It works, sounds pretty good but has alot of hum.

Details
- Hum is there even when a guitar cable is not plugged into the input.
- Hum is there with volume all the way down, gets louder with volume knob.
- Pulled V1, hum still there
- swapped rect and power tubes, hum still there.
- Did chop stick test, poked all the filament wires and all, hum did not change.
- Pull V2 (PI), hum is gone. Different tube in that spot, hum.
- New PT has different bell orientation (vertical vs horizontal). OT and PT still at 90 degrees from each other but only about 2 inches clearance between them.
- Hooked up a different OT (outside chassis) with clips and another jack, hum stays.

I am not sure where else to look. Not sure what to take away form the PI removal getting rid of the hum. Perhaps the filament wiring to that tube? That would be weird because it was quiet before and I didn't touch that part.

Could it be the center tap of the filament wiring (green / yellow) is not working properly? thinking of trying the 100 ohm CT trick to see if anything changes.

Any idea or hits would be greatly appreciated!
Thanks!

[g1]

You said it was sitting a few years, did you listen to it now before you swapped transformers?

[Enzo]

Any chance the new transformers were mounted with their laminations facing parallel?

Remove all power tubes, turn it on, do you still hear hum in the speakers?

[HBamps]

I should have, in hindsight, but didn't.....

[HBamps]

Any chance the new transformers were mounted with their laminations facing parallel?

Remove all power tubes, turn it on, do you still hear hum in the speakers?

I don't think so, at least it looks as if they are 90 degrees from each other.

As I stated, if I remove the PI, the hum is gone. I suppose I can try all out and place them back on at a time.

[Enzo]

If you didn't you didn't, it was just a thought.

With all power tubes out, the only way for hum to come out the speaker is for the transformer fields to couple magnetically. It isn't about the circuit.

[HBamps]

If you didn't you didn't, it was just a thought.

With all power tubes out, the only way for hum to come out the speaker is for the transformer fields to couple magnetically. It isn't about the circuit.

That's a great point! Thank you and I'll try this evening.

[g1]

I think he meant that he didn't try it recently before swapping the transformers. So it sat for a few years and may have developed a problem other than something caused by the swap.

[Leo_Gnardo]

As I stated, if I remove the PI, the hum is gone. I suppose I can try all out and place them back on at a time.

That's a good test, it points to the PI or a tube earlier in the circuit as your hum source. You clipped in an OT off-chassis, had hum there too, so it's less of a chance the transformers are "talking to each other."

Yes remove pre tubes, replace 1 at a time starting with the PI and working back. Also look for signal wires in close proximity to filament wires, especially those bringing signals to control grids. It's possible you have a pre tube that's a hummer as well. Put on your Sherlock hat, look & listen during your tube replacement session.

As you mentioned the center tap may not be really dead on center. You can try the 100 ohm resistor pair, or even better a "humdinger" pot. On occasion I find a combination of non balanced resistors, say 82 & 120 ohms makes a better solution, but it is nice to have the adjustment available.

Other solutions involve floating the filaments (via center tap or 100ish ohm pair) on a DC level, say 20 to 50 volts. Some old Ampegs floated the filaments on a 0.1 uF cap, not charged, surprisingly that sometimes works. A film cap 200V or up is recommended, and you may find some other value works best.

Once you minimize your hum from filament sources, you can see (and hear) what transformer orientation may help improve the hum rejection in your amp better. In some cases you wind up with the OT at some odd angle. There's a test involving headphones and no tubes in the amp, power up, listen to the speaker output with the cans on, move the OT until you hear minimum hum and that's the best spot for it to minimize transformer crosstalk.

[TimmyP1955]

If you want to add a hum balance pot (though I doubt it will solve the issue), and have room, I like Fender part number 0028421000 (or 028421). It's a full size pot, not a little trimmer as in some amps.

[HBamps]

further tests;
- added 100 ohm CT on filaments. still hums
- removed power tubes, no hum
- all tubes in, removed PI, no hum
- changed PI tube, still hums
- removed V1, all other tubes in still hums
- changed rect, still hums.

Maybe I got a bunk PT... Only one way to find out...

[Leo_Gnardo]

further tests;
- added 100 ohm CT on filaments. still hums
- removed power tubes, no hum
- all tubes in, removed PI, no hum
- changed PI tube, still hums
- removed V1, all other tubes in still hums
- changed rect, still hums.

Maybe I got a bunk PT... Only one way to find out...

Try power tubes in, PI in, other pre tubes removed. Now add one at a time starting with the one closest to the PI.

Another thing to check, hi voltage ct, is it brought to chassis at the same place the main filter cap negative lead is?

Are we sure other grounds are tightly connected, especially any made thru pot bodies, input or output jacks?

Bunk PT, yes I s'pose you could sub the old one back.

[Mick Bailey]

What PSU ripple do you see on V1? If the amp hasn't been used for years, you could have a bad filter cap or two.

[HBamps]

What PSU ripple do you see on V1? If the amp hasn't been used for years, you could have a bad filter cap or two.

I don't have a scope so I don't know. But I did clip in some filter caps in parallel to the ones installed and it did get quieter. non silent but quieter. I ordered some today and will install before I switch out the PT.

[Leo_Gnardo]

I did clip in some filter caps in parallel to the ones installed and it did get quieter. non silent but quieter. I ordered some today and will install before I switch out the PT.

Good to see some progress! Don't forget you can bring the lowest voltage filter cap's negative lead to ground near the input circuitry, this often results in a hum reduction.