Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: 100decibels on December 14, 2022, 08:43:27 pm

Title: Somebody try a variable negative feedback on Hoffman Stout TMB
Post by: 100decibels on December 14, 2022, 08:43:27 pm

Hello.
I built a Hoffman Stout TMB for a friend and put a switch on the NF and it makes noise if I remove  the NF. I previously built a few Fender Champ clones with variable negative feedback with good results. Anyone try it? Regards.

Title: Re: Somebody try a variable negative feedback on Hoffman Stout TMB
Post by: sluckey on December 14, 2022, 09:23:25 pm

Many people have put a variable NFB control on their amps. Works OK.

Try this... Put your NFB switch across R19. Open = NFB. Closed = no NFB. Any noise with this configuration?

Or you may be interested in a NFB switch and presence control like this...

     http://sluckeyamps.com/november/november.pdf

Title: Re: Somebody try a variable negative feedback on Hoffman Stout TMB
Post by: SILVERGUN on December 15, 2022, 08:05:12 am

Quote from: 100decibels on December 14, 2022, 08:43:27 pm

I built a Hoffman Stout TMB for a friend and put a switch on the NF and it makes noise if I remove  the NF.

Removing NFB increases gain and as a result adds some noise.
What kind of noise and how much?

Title: Re: Somebody try a variable negative feedback on Hoffman Stout TMB
Post by: JPK on December 15, 2022, 09:34:07 am

I had it on an 18 watt plexi and it didn't make any "bad" noise when the NFB switch was off. Or on. When I started to play guitar through it, it didn't always sound that good but it wasn't the amp.  :dontknow:

Title: Re: Somebody try a variable negative feedback on Hoffman Stout TMB
Post by: 100decibels on December 15, 2022, 08:01:48 pm

Finally I put in a 39k resistor with a 100k pot, the original NF resistor is 82k so in the middle I have this. A little hiss at maximum and not at minimum. Me too when i play the guitar it doesn't sound good but it's not the amp or the guitar.

Title: Re: Somebody try a variable negative feedback on Hoffman Stout TMB
Post by: JPK on December 16, 2022, 08:10:08 am

Slucky's schematic had a list of suggested resistors based on OT tap. Not sure this will fix it but good to know.