variable nfb - pot value

[stusixtysix]

Thinking about trying this as I can't decide between 100k/ 4 Ohm (gainy/ elastic/ easier to play with sloppy technique but outta control when plugged straight to a cab unattenuated) or 47k 8 Ohm (nice and tight on chording but loses a little bit in the scream-gain dept). I think the ideal setting for my amp ('73 50w tweaked to pseudo 12-series values/ filtering) is somewhere in between the two.

what I'm considering is leaving the currently 12-series 47k/ 8 Ohm NFB setup and adding a pot in series, the question is what value I would need in series with the 47k resistor to reduce feedback to approximately the same 100k/ 4 Ohm value. maybe a 100k pot in series with the 47k? I'm thinking the lower/ lesser NFB would suit some 6ca7's better and is a bit TOO loose with my Siemens EL34's

[rgalpin]

100K pot would give you a range from 47K to 147K if you wire it as a variable resistor which would be like one wire on the middle lug and the other wire on one of the outside lugs.

i think this is a pretty cool idea - i'd like to to try it some day - where are you going to put the pot?

the nfb circuit is susceptible oscillation type noise - so i'd be interested if the placement of the wires and pot would be critical and what would work best.

[stusixtysix]

rob

I'm thinking about temporarily removing the second speaker jack and putting it in there; see how it works...

this work day is getting long. There's amp tweaking to be done, dammit!!!!

[Roe]

I'm using a 100k pot. have posted the details earlier

[vh junkie]

Yup! You need somewhere right between a 50k and 100k pot in series with the existing 47k resistor. Use a linear pot if you can. I would mount the pot temporarily (with the amp on the bench) near the presence pot. Once you get the right value dialed in, change the 5k presence pot to one with a switch to short the proper resistor value in and out.

Something like:
Mouser #
313-1601F-5K

[rgalpin]

i think the amount of nfb has such an expressive effect on the amp that it would be cool to have the control of a pot rather than fixed. i can see diddling for hours between presence settings and nfb settings.

maybe i would replace the 47k with a 27k though so you can go all the way to 27k for super gnatsbutt tight if you want it - giving you a range from the vinatge 27k to 127k.

[Roe]

vintage varied from 27k@16 to 100k @ 4 that equals 27-200k @ 16 or 13.5-100 @ 4 or 19-141k @ 8

[rgalpin]

ah - good point - def want that 200k at yer fingertips - just in case. ROCK!!

[stusixtysix]

still getting round to this one, looking for a pot in my parts bin.

put my amp back to 100k/ 8 last night. just cant settle. gotta get the pot in there!!!!

[lidesnowi]

I use a 27k NFB res with a 250k pot at 16 OHM, giving a lot of NFB range, works just fine .

[plexified]

27k with a superbass circuit is awesome in its origional intentions .

Newer builds have to adress the additional high end . I highly suggest starting with the bass circuit as a starting point . Unless you have a metal panel spec , in which you should run the 8 ohm tap and variac down to 90ish . I'll respond to Junkies questions soon , just need more time to do so .

[Roe]

the larmar mv reduces nfb when turned down so you may want the ability to increase nfb quite a bit if you use the larmar

[cebix]

Digging up an old thread but I have a question conerning the pot value.

I have a 1987X mostly stock but I installed a LarMar and I mostly use the amp to practice at home which means 99% of the time the LarMar is at night time apartment bedroom playing levels. Thinking about installing a NFB pot to minimize fizzyness.

So my question is what is the lowest safe resistance value the NFB circuit can have? I'm thinking of installing the pot instead of the fixed resistor and nothing in series to be able to drop down say from 100k (with a 100k pot) to 0. Maybe throw in a 1k in series just in case the pot fails.

What do you guys think? What would be a desired option for these very very low volume levels?

[vh junkie]

You don't ever want to go to zero... this sounds pretty good to me...

I use a 27k NFB res with a 250k pot at 16 OHM, giving a lot of NFB range, works just fine .