MV abrupt taper change

[vibratoking]

I would be in for some, but they would have to be true 30% taper and not some piecewise linear thing that approximates 30% taper.

[heisthl]

I think Gary/GlasWerks has a connection to get specific spec pots ordered from Alpha, not sure what the quantities are (10000?). Maybe he can chime in....

[overtone]

The pot buying party for true 30% taper pots would be great. I prefer the quality of the PEC RV4 to the usual Alphas, but still use a set of either one or the other.

There is apparently a 30% taper CTS pot out there, but it is probably only good for guitar use, I don't know it's voltage rating:
Fender® 250k ohm 30% Taper Split Shaft (part#0028531000)
I have no experience with it, just pointing to it.

[boldaslove6789]

Does anyone know or have custom ordered pots through RS pots? I have RS super pots in my Les Paul and have used the 250k & 500k Audio pots in a few amp builds even. Compared to the vintage CTS pots I had in there prior to, they are considerably alike with a similar taper. (In fact, dare I say better ) @ $25 a pop though I'd say they're a bit too overpriced.

[greiswig]

Er...okay, silly question time again. I got into the amp to try to measure some values to see what I could figure out. It's an Alpha pot. Measuring between the wiper and the grounded lug in the circuit, it measures as follows:
Noon - 130k
1:00 - 155k
2:00 - 270k
3:00 - 375k
4:00 - 369k
Full - 309k

Only when I disconnected the lug from ground did I get readings like I'd expect:
Noon - 140k
1:00 - 170k
2:00 - 310k
3:00 - 630k
4:00 - 900k
Full - 1030k

Does this mean I've got a leaky cap nearby, or is that first set of readings what I should expect if I try to measure it in the circuit?

[overtone]

I am guessing that you have a Lamar type MV in there and that, for each pot, a paralleled high value resistor is going from the wiper to to ground. The resistors may not actually be on the pots themselves.
When you disconnected the pot lug to ground you have no longer had the paralleled resistor in circuit. The resistors being in parallel with each pot gives approx. the value of the grid leak resistors of the power tube in your circuit when the MV is fully open.

Can you post a snippet of the schematic for this D'lite so we know what we are discussing?

I keep on thinking that there is a trick that you can do to shift the "1 o'clock setting", maybe use a different taper, but I have not the time right now to think it through.

[vibratoking]

The RS pot subject has been discussed many times. There are some detailed threads at the mylespaul.com forum and there are also some here regarding pot taper and RS pots. There is a good thread at mylespaul that includes plots showing the taper of various pots. I have measured several of mine and they are not 30% taper. I have also used the RS superpot in several of my guitars and it does not solve the taper issue. It is overpriced marketing hype and does not have the correct taper to solve the 'issue' related to volume control smoothness. It's interesting how this topic often seems to quickly turn into a cork sniffing discussion. I hope that doesn't happen here. The RS is the wrong taper. I believe Glen K, geetarpicker, has said the same thing. I suspect that many of the positive opinions on the other forums about the RS pot are related to the "I paid 25 bucks, I always make good decisions, so they are better" phenomenon. I am in for a group buy. Alpha is good enough for me, as long as the taper is correct.


EDIT to add the following:

You have to measure it out of circuit due to parallel resisitance. I like visualizing data. Here is a plot of the out of circuit measurements. That pot is about 14% taper. It also looks like it has a two piece resistive element so it is not true audio taper.

[greiswig]

I am guessing that you have a Lamar type MV in there and that, for each pot, a paralleled high value resistor is going from the wiper to to ground. The resistors may not actually be on the pots themselves. (SNIP)
Can you post a snippet of the schematic for this D'lite so we know what we are discussing?

Schematics can be found https://tubeamparchive.com/viewtopic.ph ... ht=tweeked. I'm not seeing the parallel path, which is what was confusing me. I see the wiper "bounded" by the .02uF cap into the PI on one side, and the .02uF cap at pin 6 of V1.

[kevster]

The RS pot subject has been discussed many times. There are some detailed threads at the mylespaul.com forum and there are also some here regarding pot taper and RS pots. There is a good thread at mylespaul that includes plots showing the taper of various pots. I have measured several of mine and they are not 30% taper. I have also used the RS superpot in several of my guitars and it does not solve the taper issue. It is overpriced marketing hype and does not have the correct taper to solve the 'issue' related to volume control smoothness. It's interesting how this topic often seems to quickly turn into a cork sniffing discussion. I hope that doesn't happen here. The RS is the wrong taper. I believe Glen K, geetarpicker, has said the same thing. I suspect that many of the positive opinions on the other forums about the RS pot are related to the "I paid 25 bucks, I always make good decisions, so they are better" phenomenon. I am in for a group buy. Alpha is good enough for me, as long as the taper is correct.


EDIT to add the following:

You have to measure it out of circuit due to parallel resisitance. I like visualizing data. Here is a plot of the out of circuit measurements. That pot is about 14% taper. It also looks like it has a two piece resistive element so it is not true audio taper.

Gotta love Excel... Easy graphing.

[vibratoking]

Schematics can be found https://tubeamparchive.com/viewtopic.ph ... ht=tweeked. I'm not seeing the parallel path, which is what was confusing me. I see the wiper "bounded" by the .02uF cap into the PI on one side, and the .02uF cap at pin 6 of V1.

After actually looking at the schematic, I would agree with your assesment regarding the cap bounding if Relay 2 was really in the NO position during your measurment. My next guess would be that the relay was in the NC position and the Drive pot was in parallel. The numbers indicate that the NC position might be the reason.

[greiswig]

After actually looking at the schematic, I would agree with your assesment regarding the cap bounding if Relay 2 was really in the NO position during your measurment. My next guess would be that the relay was in the NC position and the Drive pot was in parallel. The numbers indicate that the NC position might be the reason.

Okay, that makes sense. It was done with the power off. DOH! I figured it must be something simple.

I'm agreed that a decent, truly log-taper pot at 30% would be nice. A quick Google search indicated that Cotton Amplifiers had some made by CTS back in 2000 and he appeared to be offering the 1M version for sale, but they seem to be out of business or have changed their name.

[glasman]

If we could get 30% taper I would be prepared to try Alpha or CTS again.

Alphas are available in 30% taper.

What I would suggest to the forum members is to make a group buy of Alpha pots from Taiwan. You are going to have to buy 1000 per value. But there appears to be enough members that you might be able to pull it off. And the price is right. Please note that the importer WILL have to pay sales tax on the imported items if they are not used them in a commercial, for sale, build.

The only issue is the tolerance. You can request +/- 10% for pots up to 500K but the 1Megs are +/- 20%. I use a 5% spec and I don't throw that many away.

Gary

[greiswig]

Thanks, Gary. I suspect that 1.000 items per value (1M, 250k, 500k and 100k?) would be a tall order to organize on this forum. If there were someone here who already had a number of these and who could part with some for a bit of profit (say 2x purchase price plus shipping?), I think it would be welcomed.

[M Fowler]

I'm going to wait for Gary to list his throw away pots

Thanks for all the information regarding tapers and % something I don't watch close enough.

I couldn't organize such an effort at this time but will keep this in mind.

I need to get in touch with some sales reps regarding components, maybe in February I will finally have the time to do that!

Mark

[greiswig]

I got to thinking: I've got a carcass of an old Twin. Looked in there, and sure enough there's a 1M pot in there. So I pulled it and measured it, and it sure acts like 30%. So I put it in my D'Lite.

It definitely gets rowdy earlier in the taper! But there is now no point in the sweep where it suddenly just takes off.

Here's the other thing: I swear it changed the sound some. The amp sounds like it got stronger in the bass, and feeds back more easily. Probably just my imagination, though: my brain keeps telling me that a pot at any point in the sweep is just a pair of resistors in series with a tap between them, so this change shouldn't change the sound.