Volume Pot Resistor Mod?

[ES225]

Does anyone know about this and what the effect is supposed to be? I heard some doing this mod but don't know what it does. Can someone explain?
On a vintage style Fender blackface amp with a 1M volume pot, putting a 1M or 2M resistor across the input and center lugs.
Thanks for your help.

[Helmholtz]

It changes the taper so you'll get some more volume at lower pot settings.

[ES225]

Thank you. That makes sense. I don't think I need it. It will not actually give me any more gain, just where the pot sits in terms of the numbers. As far as I understand it.
I sent you a message regarding putting in an FX loop.

[Chuck H]

And reduces the effectiveness of any "bright" circuit on the pot.

[Helmholtz]

And reduces the effectiveness of any "bright" circuit on the pot.

Making the bright cap somewhat less aggressive might be a good thing with some amps.
If the bright cap is switchable, the additional resistor could be hardwired to the cap.

[Helmholtz]

I sent you a message regarding putting in an FX loop.

Public forum area, please.
Start a new thread.

[ES225]

I actually did this mod and added the resistor, as mentioned, to a brown-face Princeton (1M) and to a blackface PR (2.2M). I removed the bright cap from the P and the PR I don't believe has a bright cap.
In terms of this resistor mod I don't see much difference in either amp volume response change. I play at bedroom levels most of the time, so this may not be a good idea. I actually want more volume at the lower part of the rotation. Did I achieve the opposite with this mod? I noticed that the amps with the mod only start to get volume at 3. Anything below 3, the amp is not really working.
Should I remove this mod from both amps for my purposes? Or should I just keep it .
I also changed the NFB res to 5.6K on the PR and to 100K on the BF P. Would going 100K on the BF Princeton be your choice or should it be a different value. I simply doubled it.

[Chuck H]

The mod in question does actually make it so the gain is higher at lower settings. But not by a lot. I think we need some clarity on what you are trying to achieve.?. If you want the amp to be louder at lower settings what does that mean? Why is there a problem with just turning the amp up louder? In other words...

Are you looking for more distortion at lower volume? Are you looking for a louder clean tone? These would both be issues seperate from the volume pot taper. Any numbers on the knob should be ignored. They are arbitrary and related to the taper of the pot with all analagous circuit functions happening the same regardless. If you really just want the volume pot to act faster at the bottom of it's rotation then a simple pot replacement with a different taper is an easy solution. But I'm not at all sure what you actually want. I'm not sure you do either.?.

[Tom Phillips]

Many players are overly obsessed with the numbers on their volume controls.
In the old days, owners would boast that their amp was louder on 2 than their friend's was at 3. One trick of the day was to just loosen the set screw and reset the Fender knob to a different position on the pot shaft.

One thing that is nice to address is a pot with an obnoxious audio taper such that all the useful range is on a very small section of the taper. Gently touch the knob and it's too loud. Touch it in the other direction and it's too quiet. The best fix is then to fit a pot with a more appropriate taper.

If the amp is otherwise working correctly, I tell people to close their eyes and set the controls to achieve the volume and tone they want. If they can do that, and none of the controls end up at max clockwise or counter clockwise limits, then they are good to go. No worries that the numbers turn out a little different compared to another amp.

[The Dude]

I certainly don't know this for fact, so speculation. I've always believed that this is why the Fender Hot Rod series amps have such a ridiculous taper on the volume controls and are so loud with volume set at only "3". If it's that loud at 3, it must get really loud, right? Personally, I much prefer an even range of control up to fully clockwise pot position. It's much easier to adjust volume.

[Chuck H]

I totally get futzing about what number a pot is at relative to what the amp is doing. Since I've built mostly custom designs for paying freinds and associates I have to keep this design aspect in mind so the amps at least seem intuitive to adjust. And I've gone to some trouble sourcing pots with tapers that do what I want. But I think something else is at hand with this thread. ES225 closed with modifications to decrease NFB. I have to interpret that the amp isn't performing as desired in some way not related to the numbers on the volume knob. Maybe I'm wrong.

[ES225]

Just to clarify, I was given a set of mods for my 66 BF PR. One of them was this resistor either 1M or 2M on the volume pot but I was not told what it will do exactly. I tried it and it seemed to me that the lower part of the rotation did not work, until the number got to 3. I mean silence to 3. Al also noticed the the amp got darker, loss of highs. Not sure if that is a byproduct aside from the taper change or if it was just my imagination. My intention is to make this amp have more gain, useable, musical gain, at lower volumes. Like a Boogie in some respects. I know that cannot be achieved without a master.
I since removed the resistor and the Volume is back to stock and sounds better actually. It seems to have more highs a bit also.
I like the NFB mod and the amp is sweet sounding but it does still need an OD pedal in front of it. I transparent OD does the trick. Its as good sounding as this thing will ever bee. I also changed the trem depth resistor to 330K and all three trem caps to .022. Speaker Celestian V10 60W. Its quite sweet.
One thing I did notice and could not correct. The Treble pot is a replaced 250K Audio and I have to have it up on 7 to achieve what should be in the middle. The correct pot for it is a J taper, which I will get and replace it. I predict that the amp will then sound the same with the pot on 5 at noon. So everything is good.

[Chuck H]

Well then improving overdrive and bumping the gain is a good start. Reducing NFB (as you have done) can be the first avenue for this sort of amp. But if you want more clipping at lower volume that gets more tricky (as you've already recognized). In your last post you mention that you like the sound of the speaker you have now. If it's not a deal breaker a less efficient speaker would cut how much volume you have relative to what the amp is putting out. In other words, you could overdrive the amp more and it would be less loud by a little.

As to pot settings and the existing treble pot vs. the J taper pot... Honestly, try to get over it. YOU already know what's up and a different pot taper will not change the tone of the amp at all. It will only change the number on the knob for a given tone but ALL THE TONES WILL BE THE SAME because you still have a specific division of 250k with either taper. Seriously, all that changes is the number on the knob and not the tone. And I don't see how this matters if you already know the score.

I mentioned earlier that I have gone to some trouble sourcing pots of the desired taper. But honestly, on my own amps I don't sweat it because I know that the circuit function is exactly the same even though the number on the knob is different.

That is to say that you could save yourself some trouble if you can tolerate the number on the knob. Because NOTHING else will change with a different taper pot. The amp will not be brighter sooner or brighter cleaner or anything like that. Seriously!!! Only the number on the knob will change.

[Helmholtz]

The resistor also has a counterproductive effect of additional tonestack loading.
At low pot settings the resistor acts in parallel with the pot and lowers the load resistance seen by the tonestack.

[ES225]

I am not obsessed with the numbers. I do have another identical BF PR with the original J taper treble pot and is at 5 while the one with the replaced treble pot with an A taper is at seven if the same high end response is to be achieved. They also have different speakers, so that may contribute to it also. The one with the original pot has a Jensen P10R and the one with the replaced A taper pot has the Celestian V10 60W. I also noticed that the Jensen PR is considerably louder than the Celestian PR at the same number settings of the knob. The knobs also mounted equally on both amps.