Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Nikolaj2121 on May 04, 2024, 03:28:07 pm

Title: 100Hz noise in Selmer Treble N'Bass Mk2
Post by: Nikolaj2121 on May 04, 2024, 03:28:07 pm

Hello everyone

I have this Selmer Treble N'Bass Mk2 amp that I am haveing a bit of trouble with. It has a quite noisy 100hz, that is comming through the PI. I have narrowed it down to the PI since when pulling the tube, the noise goes away.

I have also discovered that if I add a capacitor over the negative feedback resistor, then the noise goes away. However this causes the bias on one of the tubes to go to like 45Am, and the other is at 17Am (this can however be fixed be adding a pot to each tube)

But the thing is I would really like to figure out what the source of the noise could be. I have tried a lot of things, and nothing has worked except the capacitor method.

Im open for all Ideas, so please let me know. Also if you need more pictures.

Thanks in regards
Nikolaj

Title: Re: 100Hz noise in Selmer Treble N'Bass Mk2
Post by: pdf64 on May 04, 2024, 04:16:32 pm

Maybe the amp is unstable and tending to oscillate.
What happens to the EL34 idle currents if the valve in the V3 LTP phase splitter socket is removed?

What condition are the HT ecaps in (32+32uF and 16+16+16uF cans)?
https://el34world.com/charts/Schematics/files/Selmer/Selmer_treblenbass_mkii_50w.pdf

Title: Re: 100Hz noise in Selmer Treble N'Bass Mk2
Post by: Latole on May 05, 2024, 03:40:09 am

pdf64 is right check filter caps; how many AC volts ( yes AC not DC ) do you read on filter caps ?
 You must read millivolts, not volts                   

You may have to replace all tubes's plate resistors, they come noisy with years

Title: Re: 100Hz noise in Selmer Treble N'Bass Mk2
Post by: sluckey on May 05, 2024, 06:29:45 am

Does the noise go away when you pull V1 and V2?