Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: spunko on January 06, 2022, 06:23:23 pm

Title: Pre-Amp Gates M5530 - Loud hum even with new caps
Post by: spunko on January 06, 2022, 06:23:23 pm

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?

(https://i.imgur.com/SJXZo53.jpg)
(https://i.imgur.com/8wVEg4z.jpg)
(https://i.imgur.com/K8QMURs.jpg)
(https://i.imgur.com/p5VZC8W.jpg)

Title: Re: Pre-Amp Gates M5530 - Loud hum even with new caps
Post by: PRR on January 06, 2022, 08:05:45 pm

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?

Title: Re: Pre-Amp Gates M5530 - Loud hum even with new caps
Post by: Latole on January 07, 2022, 03:30:10 am

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

Title: Re: Pre-Amp Gates M5530 - Loud hum even with new caps
Post by: PRR on January 07, 2022, 11:38:04 am

Quote from: Latole on January 07, 2022, 03:30:10 am

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.

Title: Re: Pre-Amp Gates M5530 - Loud hum even with new caps
Post by: spunko on January 18, 2022, 11:15:18 pm

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.

Title: Re: Pre-Amp Gates M5530 - Loud hum even with new caps
Post by: Latole on January 19, 2022, 03:53:31 am

Some shield wires may help.

Wiring dress are poor