Variable NFB pot

[madditch]

I want to add a pot between the 16ohm tap and the NFB resistor on my JTM45. I was hoping to us a 16mm alpha with 11 detents for a rotary feel, but I believe those are only 1/4 or 1/5 watt. Do you think that would work? If not what is the lowest wattage pot I can use for this?

[jjasilli]

The wattage rating of the pot or resistor is not a factor.  You get a "free lunch" here.

NFB is all about voltage.  There is very little current, so absolute resistor value is not the issue.  The NFB circuit is a voltage divider. So the ratio of the resisitors is the critical factor.  EXCEPT that the value of the shunt resistor is usually important -- not to the NFB circuit itself  -- but to the circuit being fed by the NFB.  See:  http://www.aikenamps.com/NegativeFeedback.htm

The point is that with near -0- current, no watts are dissipated.

BTW:  Where do you source pots with detents?  (I can't find them.)

[madditch]

Great thanks for the help, I got the pot from Small Bear Electronics http://www.smallbearelec.com/Detail.bok?no=1030

[jjasilli]

Smallbear!  Good to know, thanks!

[stingray_65]

I had read that article at Aikens, very informative!

I was building a 5F2A at the time and did some experimenting with putting a tone stack in line with a variable NFB.

I used the tone stack from a Gretsch 6162. really weird stack, lots of losses, but that's not a factor in the NFB loop.

No kidding, I plugged an SM 58 into the instrument jack and twiddle the 2 NFB knobs and hold the mic less than a foot away on center of the speaker and not get feedback.

of course change the room or move the mic and you'd have to change your settings.

Funny how it responds to the tone stack, as you cut the bass in the NFB loop the over all tone of the amp got boomier. cut the treble and it brightened up.

It was just an experiment, I didn't implement it in the amp, but I always thought it might be useful in a harp amp.

Ray

***EDIT***

speaker went to the "volume" pot and the "tone" to the board
used a 50k B pot wired as a variable resistor for the "Volume"

[Platefire]

I did the Varible NFB pot on my SE EL84 Practice amp. IMHO is a waste of time and effort--I very seldom use it. To me it's a much better set up to find your most efficent NFB fixed resistor and leave that permanently in place and then make your NFB switchable to go to no NFB but if your like me, you want be happy until you try it, so go fur it.