Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: ylelevier77 on August 18, 2021, 06:09:31 pm

Title: Adding negative feedback control?
Post by: ylelevier77 on August 18, 2021, 06:09:31 pm

I’m looking to convert the 27k resistor used for NFB in my SF vibro champ to a 25k linear pot. Figured I’d pull the 27k resistor and go from the speaker into the pot and then back into the circuit at the 47ohm resistor. I’m going into lug 1 and out of lug 2 back to the 47ohm and grounding lug 3. But it’s not really working. The taper ramps up quickly and then plateaus if that makes sense. What’s the proper wiring? Feeling like a total noob here. 

Title: Re: Adding negative feedback control?
Post by: pdf64 on August 18, 2021, 06:17:15 pm

You may find that an audio taper control is more to your liking.
However, having the series feedback resistor adjustable down to 0 seems like asking for malfunction when set very low.

Title: Re: Adding negative feedback control?
Post by: sluckey on August 18, 2021, 06:19:31 pm

First off, it's not a 27K resistor. It's a 2.7K resistor. I would experiment with a 10K pot on series with a 1K resistor.

Title: Re: Adding negative feedback control?
Post by: thetragichero on August 18, 2021, 06:51:18 pm

a no load pot might be put to good use for this too, no? that way not only do you have an adjustable nfb resistor but also the option for no nfb (although i suppose this would make any presence pot not very useful)

Title: Re: Adding negative feedback control?
Post by: PRR on August 18, 2021, 08:56:30 pm

You want zero to 100%. This is a job for a pot.