Adding a variable resistor to the negative feedback loop?

[rhodco]

Looking over my Hoffman Princeton Reverb build I got an idea. Could I add a potentiometer to the negative feedback line in series between the speaker output of T2 and R51 (2.7K ohms)? Has anyone ever done this? If so, what value pot did you use?

I'm thinking that it wouldn't hurt anything because all the way down it would just be zero resistance and have no effect on the circuit, but as I increase the resistance it would reduce negative feedback and increase gain in that stage to achieve a little more natural distortion.

Please let me know if you have ever added this to one of your builds.  Thanks.

[Ed_Chambley]

Do it all the time.  I have used a 10K pot by itself, like the fender bias pot as it doesn't turn easily.  Actually I have one on all my builds.  I like a couple of things on a Princeton.  A Raw and a pot on NFB.  There are more like a Dwell for the Reverb and lower the 1 meg in the tremolo, I use a variable resistor to find where I prefer the slowest speed and measure that to get what I prefer. 

I have done many mods to the 1164 Circuit, but the NFB, RAW and swampy tremolo are my favorites.

[rhodco]

Thanks Ed. I've done the Tremolo mod, swapping out the 1Meg for a 470K, but I'm not familiar with "RAW". what is that?

[Ed_Chambley]

Thanks Ed. I've done the Tremolo mod, swapping out the 1Meg for a 470K, but I'm not familiar with "RAW". what is that?

RAW is simply adding lots of mids and there are different ways. I prefer 250KA pot coming from the mid cap/bass pot through a 4.7K resistor to ground.  Do a search on forum topics and you will find many versions.  It is really nice on a Princeton.  Sort of turns a pretty small sounding amp to a crunch machine.  Also, I have a 6 position rotary switch with different cap values to bypass the tonestack.

[dude]

I would assume adding a pot in series with the negative feedback resister to the speaker, that turning it down would be the same as an off/on switch turned off?

al

[sluckey]

I would assume adding a pot in series with the negative feedback resister to the speaker, that turning it down would be the same as an off/on switch turned off?

Turning the pot to zero ohms would be the same as turning the switch on.

[dude]

I would assume adding a pot in series with the negative feedback resister to the speaker, that turning it down would be the same as an off/on switch turned off?

Turning the pot to zero ohms would be the same as turning the switch on.

What would turning the pot to max do? 

From what I take is that a pot here, is only going to change the resistance to the speaker (from zero to the max of the pot) and cannot be used as an additional "on/off switch" for negative feedback, true? You'd need a switch with the pot if you wanted to turn on/off NF?

Edit:
Would using a 250K pot here, turned to max (250K) the high resistance would be the same as basically turning off NB?

[pompeiisneaks]

Not gone, no, its never off unless the voltage is low enough and the resistance is high enough... .Ohm's law, current equals V/R so as that resistance goes up, the amount of current going through goes down, but to use a resistor to turn it off, it would have to either be really low voltage or really high resistor, or both.  and even at that, a resistor by definition means that some will bleed through, its not a switch.  That being said, you would also need to determine the resistor level that cuts the current down to a point where the NFB loop is insignificant in its impact to the amp tone.  If you want real 'off' you should add a switch as well as the pot. 

Some of the gurus can likely say if that level of pot is enough to shut it down completely for this purpose.

[rhodco]

Using a 10K pot would be about right if you want to try this. I used a 100K and nothing much happens to the tone between 15-100K. Taking the lead off completely at that point makes no difference either. So, my experience is that the tone starts thin and clean from zero. Increasing the resistance seems to increase gain a little and thicken the tone slightly. Once you get over 15K it might as well be open because there is no difference in the sound. The difference is not huge, it's just a little something to tweak.

[Fresh_Start]

Sort of off topic:  Dwell pot is useful. A 1 meg linear taper pot wired like a volume for the input to the parallel 12AT7 reverb driver.

Raw:  25K Mid pot in place of the 6.8K resistor gives me medium-rare. Turn it all the way up and the Treble and Bass controls do very little while gain and mids go up substantially. Down at the bottom part of the range you get normal blackface tone control.

I tried a "crossline" post PI master volume. Didn't like it at all. However, a volume pot in place of the 1meg grid leak resistor for the cathodyne triode might be useful. Still need to try that one.

Cheers,
Chip