Am done with PEC for good.

[Champ_49]

Ok. I'm probably opening a whole new can of woms. But I am really going to stick with Alphas from now on.  I guess I'm either going insane or just not getting enough sleep for me to be repeating the same mistake over and over again, expecting a different result. 

So about a year back when I first built a stock jcm800 I put in PEC pots for all controls. First when firing on the amp I noticed really low sustain and no gain at all with the preamp gain all the way up.   On a hunch I had some alpha pots lying around so I swapped the preamp PEC pot for an alpha. Interestingly there was much more gain now.  I did this swapping back and forth a couple of times and it was the same result. Low gain sustain for the PEC and higher gain sustain for the alpha. I measured the pots and they were both close to 1M. This was strange.

So I ended up putting in all alpha pots for the time being.
For some stupid reason I couldn't get over how great the PEC pots looked. Lol. Much bulkier and looked like a much higher quality pot. And I felt like I was losing out on high quality pots and felt like the amp wasn't the best because of this.  Also it didn't make any sense that this anomaly should happen as they all measured similar and the only real difference would probably be the wattage rating. Also I believe alpha and pec are both carbon comp. Or I could be wrong. Please clarify if I am wrong.
Whatever the case I put this new set in. This time the amp had been rebuilt with a much better layout and better soldering and such as I had gutted it out and from scratch built it again.  Same result. PEC s yielded lower gain and sustain. I had it with them at this point.

Now.... I ended up building another jcm800 now with an added gain stage. And all sorts of other goodies. This was by far the best amp I built up to date. I tried again and put in the PEC just for the preamp gain control and it seemed to have the same amount of gain as the extra gain stage made it difficult to notice any real difference between the alpha and the pec.  After I swapped in the new set of pec pots I started playing for a bit and noticed there was not a lot of sustain. As I felt like I was working harder with my hands and having to struggle more with hammerons and pulloffs and string attack was taxing. 

I swapped back in the alpha just for the preamp gain and the amp sounded much fuller with more gain and sustain.  Although subtle it was enough for me to say good bye for good to the pecs.

Talk about insanity. Repeating the same thing and expecting a different result. Haha. Lesson learned. More money doesn't always equal better sound.  Better quality more expensive components don't always equal better sound. And higher better quality components can make a sound worse. Not always.  but I'm going to stick with alphas for good this time.
At least now I don't have to feel like I'm missing out on "higher" quality PEC pots.

[plexi50]

Your post brings up a great subject matter that pertains to the response and taper of today's modern made potentiometers.
I never got into the hardcore differences between a 1950's pot and current made pot.
I can also hear a big difference in how a certain pots taper makes an amp sound.

I don't know if i said that right. To me pots made today seem to offer an all in your face tone immediately and ramp up fast. The (taper) is nothing like the 50's - 60's pots. Now that i have said all that, (What did i just say)? 

[HotBluePlates]

... So about a year back when I first built a stock jcm800 I put in PEC pots for all controls. First when firing on the amp I noticed really low sustain and no gain at all with the preamp gain all the way up.   On a hunch I had some alpha pots lying around so I swapped the preamp PEC pot for an alpha. Interestingly there was much more gain now. ...

I don't know what to tell you. If both pots were all the way up, I'd expect them to sound the same. I would, however, know that with the PEC at something less than maximum it might not have as much "gain" as the Alpha because the PEC's 10% taper will reduce signal faster than a 20% or 30% taper pot.

Said a different way, as long as I could get the same sound from both, it wouldn't matter to me where on the knob rotation the sound occurred. But it sounds like you're saying you can't get the same sound from both. Very odd...

[Champ_49]

Hi Plexi50.  Thanks for your input on this subject. 
Yes.  Me too. I have never really delved so deep into the "sound" of a pot.  But for me I couldn't not go into it.  Since I noticed a noticeable difference in sound and feel I couldn't discount this.
The Alphas and PEC's are 1M log.  So it is expected that they taper non-linearly but log.
 

[Champ_49]

... So about a year back when I first built a stock jcm800 I put in PEC pots for all controls. First when firing on the amp I noticed really low sustain and no gain at all with the preamp gain all the way up.   On a hunch I had some alpha pots lying around so I swapped the preamp PEC pot for an alpha. Interestingly there was much more gain now. ...

I don't know what to tell you. If both pots were all the way up, I'd expect them to sound the same. I would, however, know that with the PEC at something less than maximum it might not have as much "gain" as the Alpha because the PEC's 10% taper will reduce signal faster than a 20% or 30% taper pot.

Said a different way, as long as I could get the same sound from both, it wouldn't matter to me where on the knob rotation the sound occurred. But it sounds like you're saying you can't get the same sound from both. Very odd...

Hi Hotplateblues.

Thanks for bringing up that point.  For me I couldn't really care less where in the taper the sound difference is.  The thing is I have tried this with both pots all the way up at max.  So they are at the same maximum point in the taper.  So it is very strange.  The only thing I can think of is that the track element of the PEC may in fact be different than the Alpha.  Although it states the track is in fact carbon element as is the Alpha, the PEC datasheet may be wrong or not updated.

[HotBluePlates]

I wouldn't focus on the track material. That said, I don't know what accounts for your experience.

The upside is the Alphas are cheaper, and feel smoother than most of the RV4-type pots. And regardless of the price, if it performs better for your application, then it is better.

[Champ_49]

I wouldn't focus on the track material. That said, I don't know what accounts for your experience.

The upside is the Alphas are cheaper, and feel smoother than most of the RV4-type pots. And regardless of the price, if it performs better for your application, then it is better.

Yup. I have no idea why this is the case.  I have heard others say on forums like this that the pec did in fact have less gain.  So I guess I'm not the only one. But I was really just curious as to why. With all other factors being the same or similar why the very noticeable difference.

[sluckey]

How do you define gain?

[Champ_49]

How do you define gain?

That's a good point haha. Nobody really asked me that but that may be a first step to solving this mystery.
Everybody might have a different definition of gain. But what I'm really talking about us sustain really. It just so happens that more gain comes with more sustain so they go hand in hand.  So when I hit a string and a note the length of time it will ring until it dies down. 

Another way I can judge gain would be by trying out to see how easy it is to do some pinch harmonic squeals  as some would like to call it.   

Just how would you define gain

[sluckey]

My definition of gain is the simple electronics expression...

Gain = voltage output/voltage input, or V1/V2, or power out/power in.

Since hanging out in these guitar amp forums I've learned that many people (especially non electronic people) have many different ideas about what gain means. The meaning of 'gain' is almost as elusive as the meaning of 'tone'. There's overdrive, distortion, hair, grit, sustain, compression, etc, etc. For 40 years the word gain meant the same to me as it did to my co-workers. It was easy to mention gain and know exactly what the other person was talking about. It's not so clear cut in these amp forums, especially to someone like me who has had blinders on all these years.   

[shooter]

sustain really

being electronic and not musical, I understand sustain as signal decay, how long it takes a given signal to go from max through the 5 time constants.  my  guess is the R's and C's has a lot to do with that?

[HotBluePlates]

... I understand sustain as signal decay, how long it takes a given signal to go from max through the 5 time constants.  my  guess is the R's and C's has a lot to do with that?

That is nothing to do with gain, or even signal strength, really. That is a measure of how long it takes to charge/discharge a capacitor, as more R slows current and lengthens charge/discharge time.

But you should note the examples used in textbooks when defining this phenomenon show a cap charging from a source of d.c., like a battery. Applying an a.c. signal is a different matter, though RC defines voltage division among the R and C and how high- and low-pass filters perform.

[shooter]

with gain, or even signal strength

ya, I understand gain as Sluckey defined it, just trying to *understand* sustain.  In my old life we measured the time it took a signal to go from max to min, and cal'd accordingly

[punkykatt]

Champ 49 is not alone.  I have experienced the same  effects with two 5E5A  head builds about 5 years ago.  I am still trying to get that amp with the PEC pots back in the shop to try the ALPHA swap. It freaked me out too. I remember changing the 12AY7 to a 12AX7 and boosting up the 1st filter cap to get more umph. His band is gigging quite often, hopefully he will be in the area soon.
Punky

[HotBluePlates]

with gain, or even signal strength

ya, I understand gain as Sluckey defined it, just trying to *understand* sustain.  In my old life we measured the time it took a signal to go from max to min, and cal'd accordingly

Yes, max to min, but nothing to do with R & C. Don't mix the two separate issues.

Your signal source determine sustain more: I can play the same guitar to get very short or rather long-sustaining sounds, while R & C stay constant in the amp.

Electronically-added sustain is the result of either compression, distortion, or both.

[shooter]

Your signal source determine sustain more

Thanks HBP, still learnin to translate electronic to music (sorry for the hijack)

[Big_Mike]

I have heard good things about using PEC dual ganged pots for PPIMV.  Supposedly, the PEC pots have much higher tolerances than the Alpha pots and work better in that application.  I have used Alpha pots most everywhere else in the circuit, however.

[Ed_Chambley]

Ok I checked as I use a lot of Mil Spec like NOS Allen Bradley and PEC and Clerostat and Ohmite.  I have am amp with all PEC and has a 1 meg audio (Log) Volume.  With the pot at 470K I replaced the pot with 2 others both set to 470k.  An Alpha and CTS and could not hear any difference.  Measuring across all the pots from lug 1 and 3 the Alpha measured good at 1M1, the CTS was 996K and the PEC was actually the lowest measuring 982K.  All close enough where tolerance should not be an issue.

I cannot hear any difference, but I did notice a huge difference in rotation position and the alpha pot to me acts more like a linear pot.  So I did the old you tube search and found this.  I thought it was interesting as it is used on a guitar volume.  I noticed the same thing on the amp I tested.

https://www.youtube.com/watch?v=RdfIZEB2rdM

[Champ_49]

That's interesting.   I was wondering if you tested it on a higher gain amp like a jcm800 or one with more gain than that.  It is much noticeable on that particular design I found or at least the ones I had built.  I think on a cleaner amp it's not that noticeable I wouldnt think.

On a stock jcm800 it's very noticeable.

[Ed_Chambley]

That's interesting.   I was wondering if you tested it on a higher gain amp like a jcm800 or one with more gain than that.  It is much noticeable on that particular design I found or at least the ones I had built.  I think on a cleaner amp it's not that noticeable I wouldnt think.

On a stock jcm800 it's very noticeable.

This amp was an EF86 to 12At7 to Push Pull KT88 and is a hotter amp than than my Fargen Ole 800 which is a JCM 800 basically.  Not sure if it actually has more gain since I did not put a compare the amps with my scope to see which has the most gain.  Also, the amp I checked is a UL design.  If by gain you mean amplification factor, the amp I tested is probably close.

The Fargen is actually not a Fargen, but a clone I built using another true Fargen.  I have a friend who has the fargen and I just built them same amp using his.

Fargen uses Alpha (or did in this amp) and I used CTS as I prefer the taper of CTS, no other reason really.  When finished I actually tweaked his to have more gain.  I did not really notice any difference, but I could not tell you for sure.  In either case, a Mil Spec pot was not used like a PEC.

I will swap my CTS volume and "gain" pot with an Alpha in my JCM 800 and see.  Easy to do.

[Amperror]

This is a really old post but I wanted to chime in.

I want to investigate as I find it interesting. I'm an electrical engineer of 40 yrs and electronics hobbyist since age nine when I built my first 6c4 single stage triode amp that ran off a 67.5 volt battery. So, all my life.

I have the instruments to do this. I think it could have to do with much higher stray capacitance thereby cutting the high freq gain. I could also have to do with the taper, combine the factors and they multiply.

The first thing to quantify the observation. Put a signal generator with the target frequency and measure the output for both types. Repeat for several harmonics, maybe 3rd and 5th. That will validate your empirical observation. If it is a frequency related thing. Next an RLC bridge is to characterize the 2 pots. Changes to the circuit can me made to compensate. Although it can be tricky tp boost hi freq gain.

The other thing to do is using the same setup measure both the signal output and the DC resistance for the pot at numbers 1 thru 10 (maybe don't have to go all the way). Plot the result on graph paper with different color pens.

I think the answer will become clear upon those results.

Using the RV4 type pots is a laudable goal as they are just incredibly better and the feel is so nice and smooth. Back when I was still only 19, I worked on a bunch of equipment for an older steel player. I heard him in the music store complaining about his Bigsby volume pedal just not holding up. I approached him outside as said I could fix it. He said I couldn't screw it up any worse than it was, so he gave he shot. I had been tearing apart old military equipment and building audio stuff for years and so I had a lot of those pots in my junk box. I used that along with a nylon gear and flat gear bar that had come out of anther pedal. He told me months later that he absolutely loved it. After that I worked on his twin and fender echo. He even tried to hook me up with he is daughter. But I wasn't ready for anything like that. So that's just a motivational anecdote.


Update: Well, I haven't gotten around to doing all the work above, but I thought up the next best thing: Measure the capacitance from the wiper to the shell with my RLC bridge for both types of pots: The PEC was actually 1 pf lower coming in at 11 pf. That is not enough to make any difference and it actually goes the wrong direction against the theory, however miniscule it is. So, I don't think that has anything to do with it. I'll have to try doing a resistance measurement at different rotation angles to see if it is the taper or starting to wonder if the original poster used a linear unknowingly? I guess after that I'll substitute the 2 types in a build I am doing, but that will be a while. The chassis needs a lot of drilling and cutting and I really hate that work. Been thinking about getting a CNC.

Final Update:
I was recently checking out the difference between some PEC pots I had and discovered one thing that might have started this thread. It may be a simple mistake of using the wrong pot type because the numbering is different. in RV4 pot A and B are both linear 10 and 20 % respectively and C and D are the audio taper pots with 10 and 20 % respectively. That is opposed to Marshall pot types A = audio and B linear. It is possible the original poster used linear pts for both audio and linear.

[joesatch]

What was the additional gain stage? swapping the second V1 Cathode's 10k resistor to a 2.7k/u68uf resistor, cap ? Did you add a snubber cap to V1 ?