Pre-Amp Gates M5530 - Loud hum even with new caps

[spunko]

Hello there! Happy new year!

I got my hands on this preamp "Gates M5530" which came with some caps already changed, but it had the two big original cans still in circuit.
When I tested it, inmediately started to hum pretty loud. I ordered some caps to replace the big cans, did the re-cap and the preamp still hums pretty bad.
Then I tested without tubes, even the rectifier out, and still makes a lot of hum, but less than with all the tubes installed.
I haven't found a diagram, just one which is pretty close from another Gates model.

What would you suggest to look for now?

[PRR]

That's certainly not a guitar amp. Not really a modern mike-amp; anyway aren't your in/out plugs' sexes switched?

How are you listening to it? Direct connect speaker? Into a Line In? (a Mike IN??)

Does the knob do anything?

[Latole]

Hum with no tubes !

You have to check if all filter cap ( and the big can) are good even the "new" can and if they have not been installed reversed.
Without a 'scope it will not be easy.

Filter caps / power DC supply issue IMO

Some AC voltage go to the OT

[PRR]

Hum with no tubes !...

A piece of cable laying on the bench feeding a high gain mixer input will hum without tubes. Especially if grounds are broken.

[spunko]

Hello, I got the virus, and everyone in my house too, so I got a time off.

It turned out to be 2 broken terminals on v1 tube socket. Replace them and now the horrible hum is gone. There is still a bit hum, but I think it is just part of this old piece of equipment.

[Latole]

Some shield wires may help.

Wiring dress are poor