Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: leftu2 on December 14, 2021, 10:20:48 am

Title: Sudden 120hz hum and bad ghost/modulation notes on my jtm45 (SOLVED)
Post by: leftu2 on December 14, 2021, 10:20:48 am

Morning guys,
I built a ceriatone jtm45 kit last yr. Amp worked perfectly, dead quiet. Yesterday I fire it up and get 120hz hum (checked it to a online tone generator) and get bad ghost notes or a modulation/oscillation type effect on every note played. Every control on the amp works and happens on any of the 4 inputs. I also put this amp on a tube/ss switch using a design from nik at ceriatone. The issue happens in tube or ss rectifier mode. Turning the master volume make no difference in the hum or ghost noting. Nik designed this amp with a 1/2 power switch, the hum gets louder in full power mode.

Things I've tried:

-pulling each preamp tube to see if that isolates the issue (no change)
-swapped the rectifier tube (no change)
-chop-sticked the amp (no change)
-tapped on all the tubes (no change / no abnormal tube glowing )

**I don't have a spare set of KT66s to try at this point**

I would suspect a tube but not sure that would cause the weird ghost note/modulation/oscillation note issue.
Bad filter cap possibly?

Edit: fixed subject... sluckey

Title: Re: Sudden 120hz hum and bad ghost/modulation notes on my jtm45
Post by: pdf64 on December 14, 2021, 11:35:04 am

Yes, most likely suspects are bad reservoir / screen node HT cap, or connections to them.

Title: Re: Sudden 120hz hum and bad ghost/modulation notes on my jtm45 (SLOVED)
Post by: leftu2 on December 23, 2021, 02:05:19 pm

I just wanted to update this. It was the filter cap, installed a new F&T and it's works perfectly.

Title: Re: Sudden 120hz hum and bad ghost/modulation notes on my jtm45 (SLOVED)
Post by: pdf64 on December 23, 2021, 03:06:35 pm

Great, thanks for closing that out.
At least you now know what bad HT caps on the power amp sound like  :icon_biggrin: