Volume Pot Resistor Mod?

[Tom Phillips]

...They also have different speakers, so that may contribute to it also...

A suggestion - You could put the amps back to back and switch the speaker plugs between the amps. (Nice to do it that way so you don't need speaker extension cables)
This test will clarify the differences due to the speakers. Would be interesting to know.

[Chuck H]

What Tom said...

In my (humble) experience the Celestion speaker should be a little more efficient than the Jensen. So it could be that OTHER control pot tapers are different between the two. Which is still nothing to sweat, really. Because as I mentioned earlier, pot taper has nothing to do with circuit function. Only knob number. The division will always be the sum of a pots value regardless of WHERE that occurs in it's rotation.

[ES225]

Yes, that makes sense. To be accurate, my volume observations were with the original speaker, the way I got this BF PR it had a Rola Celestian 50W and it was lower in volume at the same knob setting than the other identical amp with a new C10R. If I previously referred to this as a P10R, which is wrong. I misspoke. This is a ceramic Jensesn P10R and is louder than the previous Rola 10. I did not compare it with the new Celestian V10 60W. Regardless that is not my concern. Having removed that resistor from the volume pot this thing is sounding sweet. They both are actually but with a little difference in tonality, as expected. I changed the NFB resistor on both to 5.6K. I think these both sound as good as possible.

[ES225]

I have another mod question, this time regarding a brown face Princeton I own. I did several mods on that one also. Increased the NFB res to 100K and I read about changing the plate resistor from 100K to 220K.
Here is my dilemma. I did not quite know which of the identical diagonally mounted resistors is the plate resistor, or if I should replace both of them with 220K. So I only changed the one closer to and pointing to V1 tube and left the other one stock at 100K.
Can someone tell me what I achieved by doing this and if both are in fact plate resistors and I should be replacing both of these resistors with 220K.
Any negative effects to the amp by changing one or both of these to 220K.
The amp is sounding very good with what I did.

[Tom Phillips]

I have another mod question, this time regarding a brown face Princeton I own...

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[nevetslab]

I looked to see if I had some Allen Bradley Data sheets on their Type J pots, and while I didn't find those, I did fine on my Bourns PDB18 Series 17mm Rotary Pots, their Taper Curves, which gets to the 10A and 30A Audio Taper pots we find listed in Fender's service manuals as well as on their schematics, such as on a Fender Twin Reverb's Tone stack. Treble pot...250k 30A, Bass pot 250k 10A, Mid Pot 10k 10A, Volume pot 1M 30A. I've printed as PDF files the tape graphs on Allen Bradley Mod Pots (Series 70) which had one of the simple taper charts, and the Bourns PDB18 Taper charts which reveals much more useful data.

I haven't stopped to play with any of the pots in question in this thread to see what the change in taper results with slugging the top of the pot's curve achieves. But, thought these curves would be useful to see what's out there.

Allen Bradley Mod Pot Resistance Tapers.pdf

Bourns_PDB18__pots_Resistive Tapers.pdf

[SoulFetish]

Adding a resistor across one of the outside lugs to wiper is a great way to make adjustments to how a control works throughout it’s range of motion.
If your looking to modify a circuit this way using the existing potentiometers values, you just need to keep in mind that this will put an additional load on the driving circuit. In some cases, it could have an adverse effect. In other cases, the additional loading is inconsequential.
In one recent example, we used this technique with great success for a customer who asked us if we could do something about his tremolo control. The effect would increase to it’s highest intensity abruptly as soon as you started to turn the knob. Off the top of my head, I forget the values of both the existing pot and resistor we added, but it ended up being a simple fix but a huge improvement. (I hope I made a note of what I did on the schematic)

[sparkies]

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.

Some tube gear designers do that on like a volume control into a stage Wiper to ground, where the wiper is connected to the grid so the tube doesn't lose bias when the potentiometer gets dirty and the wiper temporarily looses connection (the scratchy sound).

Signal input to wiper connected resistor would change the contour of the control.

[Chuck H]

Some tube gear designers do that on like a volume control into a stage Wiper to ground, where the wiper is connected to the grid so the tube doesn't lose bias when the potentiometer gets dirty and the wiper temporarily looses connection (the scratchy sound).

Good observation. I never worry much about preamp stages but I always include (or add) a fixed resistance on bias modulated tremolo intensity control pots since losing bias to the power tubes is more likely to be an expensive problem.

Signal input to wiper connected resistor would change the contour of the control.

Yes. This was somewhat covered earlier and Helmholtz also made note that it changes the load as well. Changing the load can alter circuit behavior in a couple of ways. It alters the operational load line for tube playing into that potentiometer and it also lowers circuit impedance which raises the knee frequency of any associated coupling capacitor. But at the high-ish values discussed niether effect should be significant.

[dai h.]

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.

(IIRC) on Youtube tone-talk channel?)) Bruce Egnater related a story where (quite long ago, apparently prior to Cesar Diaz(sp?)) SRV's tech (Renee Martinez?) requested that he tweak the controls to meet some particular number setting (some weird superstitious thing on SRV's part which was causing a problem with too much bass if memory serves). Bruce initially suggested to do just that above (loosen and re-set the control positions), but the tech thought SRV would figure out the ruse so requested a more invasive alteration. (Maybe the mod has nothing to do with that, but) wonder if it was that somehow mis-interpreted to be some 'magic mod'.

Also, as far as tapers, there can be ones that are more or less suitable in providing more or less control over the range involving types of taper (A, B, C, W, etc.) and percentage of total resistance at half rotation (10A = 10% of total R@50% (pointer physically turned halfway up) rotation) and number of sections. For audio the minimum (of sections on the resistive track) is two, but on some it may be as much as 4 or 5--I don't know if more is always necessarily better, but more in some cases can provide finer control over the rotation (and two pots that are both (same value) "15A" may behave differently because of this). Also some pots have (something like) silver-bearing epoxy or solder on the ends to reduce end resistance (it would matter where you are trying to resolve very small resistances (which affect gain--if there is significant (for that circuit) end resistance this could mean inconsitency at max. gain between channels).

[sparkies]

Good observation. I never worry much about preamp stages but I always include (or add) a fixed resistance on bias modulated tremolo intensity control pots since losing bias to the power tubes is more likely to be an expensive problem.

It was one of the building practices I learned in college decades ago. Its sad to see the art of tube design has fell somewhat to the wayside seeing newer gear in how its constructed. What I think is worse was those mesa boogie amps, but marshall has some great construction losers as well.


But volume control, I could see someone modding them in guitar amps so they don't hear a scratchy control.

[Chuck H]

Well they don't make 'em like they use to. But they do make them cheaper and that puts tube amps into the hands of a lot of players. And that's not all bad. But yes, there'a a reason the old amps are revered. Once snuffed up they can be very reliable.

Funny note. Leo Fender was always trying to make the amps better. He was also a cost cutter. But when he said "We don't make 'em like we use to. And we never did." he meant it in a different way. I always liked that quote so I've never even fact checked to see if it's real I think I read it in the Teagle and Sprung book.