Has anyone tried PEC pots for Gain knob? Unexplainable results.

[Champ_49]

Hi all.

I was wondering if anyone here has ever tried the PEC pots specifically for the preamp gain knob or similar.
I was switching between the alphas and PECs and have opted never to use the PEC pots again especially for the preamp gain knob.

When I first built a JCM800 I put in all PEC pots.  I noticed that the gain of that amp was significantly low.  So I couldn't figure it out at first.  Maybe a cold solder joint?  Fortunately I had a hunch that the first suspect was the preamp gain pot.  I swapped it out for an alpha pot and the there was significantly more gain when turning the knob up compared to the PEC.  I was stumped.  I measured the pots with a multimeter and both were very close from low to top.  I swapped it in again and the gain was gone.  Really strange.


So on to my next JCM800.  I built this one really well and had everything soldered and organized well.  The amp sounded great with the alphas pots in place.  I really wanted to like the PEC pots so I swapped them in again just to see I wasn't imagining things.  There was significantly less gain again.  I has some more 1M PEC pots lying around so I measured them and they all measured close to spec as the alphas I had in place.  Thinking maybe it was a bad pot I swapped the other same PEC 1M pots in and again there was significant gain loss for EVERY one of them when comparing them to Alphas.  From then on I always used Alpha pots.

Anyone have experience or an explanation to this?  It's really weird.  The Alphas were 1M audio.  The PEC's were 1M audio.  Essentially the same pot.   But how could there be a significant gain loss for ALL the PEC pots I tried when comparing to the Alphas.

Anyone here try the Clarostats? I have a feeling all these "higher" quality pots are bad for using in the preamp section.  But one thing I had to note was the PEC pots are really good to use for attenuation such as the master volume, etc.

Any thoughts?  I am stumped and would like to know as this has been bugging me a bit.

[Ed_Chambley]

Resistance measured the same should give the same results.  The only thing could be the type of taper.  I have and still use NOS Clerostat, and the made in Mexico Honeywell Clerostat.  Fine pots.

I would argue that if a Alpha pot measured rotation of 750K and a PEC measured at 750K you would have the same amount of gain.  Same with CTS pots which I use mostly.  I use CTS pots for their Audio taper.  It seems I get a large jump in Alpha pots early in rotation whereas CTS seem smoother.  For instance, I have a Blues Junior with alpha pots, little 15 watter.  Between 2 and 3 is where most of the volume was so I substituted a 1 Meg CTS and to my ear it is a wider range and slower increase.  No quick jump.

I have a lot of NOS Allen Bradley pots and they have a audio taper I really like as well.

[sluckey]

The Alphas were 1M audio.  The PEC's were 1M audio.

Not all audio tapers are the same. If you set either of these pots full CCW or full CW, the sound level should be identical. Anywhere between max CW and max CCW can vary widely between manufacturers.

[Champ_49]

Thanks for the replies.

That is exactly my reasoning.  I do understand that the tapers could be different which isn't an issue.  But with the PEC pots at max and the Alphas pots at max there was a very noticeable difference in the amount of gain.  I could understand also that no two pots are the same and will have different maximum resistance, but to have that much of a noticeable effects boggles my mind.  I'd say the difference would be from going from a Marshall JCM800 to a Marshall Blues breaker with just the switch of a pot.  Really strange.

I did hear about someone mentioning this on another forum, the exact or similar result as I had.  There could be other factors such as a bad solder joint, wrong components, etc, but with one variable with all else being constant it's hard to dispute. 

Again, it would only make technical sense that the gain SHOULD be similar when the resistance values are similar.  In fact I remember measuring the PEC pots resistance to be a but higher than the Alphas.  The Alpha would be 0.98 M while the PEC would be around 1.05M.  Both audio taper and both rated for approximately 1M.

The only thing I could think of was the wattage rating as that seem to be the only difference in spec between the two.

Really really strange.


I wouldn't mind trying a clarostat although they can be quite pricey. But they don't seem to have the ones with the values I want.
Checked Digikey and mouser but the shaft is either too high or they don't have audio tapers.  Anyone know a good source that sells them specifically for amplifiers?

[sluckey]

I guess this is a case of hearing is believing. I've never heard it.   

[Champ_49]

I guess this is a case of hearing is believing. I've never heard it.   

Hi Sluckey.
Yes.  I was about to say. Just saying it won't do anything.  You'd have to hear it to believe it. But believe me there was definitely a difference. Not subtle.
The PEC pots definiely look cool and the lugs are larger and easier to solder bunches of wire. Only con is there is a problem with sound. Which is the mist important thing.  And for that price um going to stay away. Maybe if they wored better with my application  I'd go for them but they don't.  So I'll stick with the alphas
 

[sluckey]

I used PEC pots (well not PEC brand, but mil spec sealed 2 watt pots) in my TDR, 5E3, Marshall 18W, and Rockola conversion. I've had no problems with them and don't expect any problems. I have no reason to believe they are sucking any gain or behave any differently than any other pot I've used. They are much better quality than the Alphas I have, although the Alphas 'feel' smoother when you turn them. I have a limited supply of PECs left but I don't have a wide variety of values, otherwise, I would still be using them.

When I order pots, Alpha is my goto pot because of price and feel, not because they are superior.

[Champ_49]

I used PEC pots (well not PEC brand, but mil spec sealed 2 watt pots) in my TDR, 5E3, Marshall 18W, and Rockola conversion. I've had no problems with them and don't expect any problems. I have no reason to believe they are sucking any gain or behave any differently than any other pot I've used. They are much better quality than the Alphas I have, although the Alphas 'feel' smoother when you turn them. I have a limited supply of PECs left but I don't have a wide variety of values, otherwise, I would still be using them.

When I order pots, Alpha is my goto pot because of price and feel, not because they are superior.

Thanks Sluckey.

I would agree with you in that they are very high quality. Although Alphas do feel better.

There is one explanation that may have something to do with whats happening.  Below is a pic.
I bent the tab outward that is supposed to lock the pot in place.  Could this cause some kind of unintentional grounding from the track to the chassis?
From the looks of it it doesn't look like it would do that but now I'm wondering if that is the case.

http://www.studioelectronics.biz/sunshop/images/products/large_1147_100k_LA3A_1176LNF.jpg

[sluckey]

I bent the tab outward that is supposed to lock the pot in place.

I do that too. Even with the Alphas. There's no danger doing so unless you're bending them with a big hammer.   

[punkykatt]

Hmmmm! Several years ago I built two 5E5A Pro heads. One of the customers wanted PEC pots and  I used the alpha pots for the other head.  The amp with the PEC pots  did have less gain which made me scratch my head as to why. I always measure every resistor, pot and cap before it goes in the amp.    I ended up changing out the 12ay7s for 12ax7s and bumping up the first filter cap from 16uf to 32uf to get a little more gain.  I know it dosent stand to reason. You got me thinking now. Im going to try and contact the owner to see if he still has that amp.

[Champ_49]

Hmmmm! Several years ago I built two 5E5A Pro heads. One of the customers wanted PEC pots and  I used the alpha pots for the other head.  The amp with the PEC pots  did have less gain which made me scratch my head as to why. I always measure every resistor, pot and cap before it goes in the amp.    I ended up changing out the 12ay7s for 12ax7s and bumping up the first filter cap from 16uf to 32uf to get a little more gain.  I know it dosent stand to reason. You got me thinking now. Im going to try and contact the owner to see if he still has that amp.

Hi Punkykatt

Yes. It really boggles me as to why.I'm glad there is someone out there who heard what I heard and had a chance to compare both type so pots.
I'm going to try and measure the pots again while it is bolted to the chassis and see if the resistance changes.  Possible ground fault?

I'm also going to check the resistance from one of the lugs to the back of the pot to check for resistance while bolted to the chassis. If it's open circuit it'll baffle me more. If there's resistance then there was my problem.

You and I are apparently not the only people to experience this.   There is another forum where someone  described the exact same thing.   Pec pots = less gain.

But then there are the people who are fine with the pec pots. Either they didn't do a side by side comparison so they don't know there is less gain or it just plain worked for them.
But the reduction in gain is VERY noticeable.   The first time i built the jcm800 with the pec pots I was thinking isn't this amp supposed to have gain?  In fact there was hardly any gain with the preamp all the way to max. Although output was good. A jcm800 is not supposed to sound like a fender twin so I knew something wasn't right.

[sluckey]

I'm also going to check the resistance from one of the lugs to the back of the pot to check for resistance while bolted to the chassis. If it's open circuit it'll baffle me more. If there's resistance then there was my problem.

If the pot is connected as a volume pot there will be resistance from every lug to chassis.

[HotBluePlates]

... But with the PEC pots at max and the Alphas pots at max there was a very noticeable difference in the amount of gain.  ...  In fact I remember measuring the PEC pots resistance to be a but higher than the Alphas.  The Alpha would be 0.98 M while the PEC would be around 1.05M.  Both audio taper and both rated for approximately 1M.

No telling what the issue is/was; with the conditions you described, the PEC should have resulted in higher gain.

I've never had that experience with PEC or any other RV4-style pots, so I'd have to actually see the amp in question to sort out what's happening.

I would have believed gain-difference anywhere except the top of pot rotation, because I'd trust the PEC to be a 10% audio taper, where other brands might be a 20-30% taper (which get louder, faster).

[John]

The only thing I could think of would be the PEC pots had little whiskers going from the carbon track to the pot body. That would feed some of your signal to ground, even though your ohm measurements would be as expected.


I run into that with the adjustment pots in Heathkit VTVM. The first time I drove myself nuts trying to figure out why I couldn't get the meter to zero after replacing all components. Finally thought to measure from wiper to pot body, and voila.

[Champ_49]

The only thing I could think of would be the PEC pots had little whiskers going from the carbon track to the pot body. That would feed some of your signal to ground, even though your ohm measurements would be as expected.


I run into that with the adjustment pots in Heathkit VTVM. The first time I drove myself nuts trying to figure out why I couldn't get the meter to zero after replacing all components. Finally thought to measure from wiper to pot body, and voila.

Thanks John

Wondering what you mean by whiskers. I guess what you're saying is the track is in a way shorting to the back of the pot? But I'm still stumped beca use it was several 1M pots that had the less gain.

[John]

http://antiqueradios.com/forums/viewtopic.php?t=54746&highlight=whiskers


Lot of good info on that thread. Basically, there's little tiny metal "whiskers" that spring up, and apparently several things can cause it. Take an ohms reading from the wiper to the pot body. It should be infinite, or O.L., or something like that. Anything measurable just might be your problem. Again, JUST a guess.

[HotBluePlates]

The only thing I could think of would be the PEC pots had little whiskers going from the carbon track to the pot body. ...

... Wondering what you mean by whiskers. ...

Whiskers (Wikipedia)

[shooter]

Nice read!, was that from PRR's upcoming book  

[sluckey]

Tin whiskers are well documented. I don't recall ever hearing about carbon whiskers.

[jojokeo]

I was suspecting that these were composed of stainless steel and sure enough they are...to me stainless steel just doesn't ground as easily or well? The electrical conductivity of stainless is well below silver, copper, & gold, etc. while electrical resistivity is way above - in each direction.

[sluckey]

I've worked with thousands of mil spec RV4 pots in a variety of military and civilian avionics and ground equipment. Never once saw one that the body of the pot was relied upon for a ground.

[jojokeo]

I've worked with thousands of mil spec RV4 pots in a variety of military and civilian avionics and ground equipment. Never once saw one that the body of the pot was relied upon for a ground.

But most of us are not "military avionics and ground equipment personnel". That being said, how do you know exactly how the grounds were done in any of these amps we're discussing? I'm just trying to conjure up ideas like everyone else to try to help to explain what's being discussed as even a remote possiblity... there appears to be people without this problem or phenomenon and yet there are a few that apparently do. So one wonders....what could be the difference(s)? Does it make sense that it's somewhere in their technique? Or an unlikely manufacturing issue (that chooses people indiscriminately)? I think it's the former?

[sluckey]

I wasn't talking about amps in that post. I was speaking specifically about my experiences with mil spec RV4 pots. I suspect that very few are used in guitar amp as compared to military and industrial applications. I certainly would not pay $10 to $20 for a pot to go into a guitar amp when a $2 Alpha will do the same job. OTOH, I would never put a $2 Alpha pot in an ASR-11 terminal radar system either.

There are lot's of amp building practices that work fine but would not be acceptable in mil spec applications. Using the body of a pot to provide a ground for a circuit is one such practice. I doubt the people who wrote the specs for a RV4 pot ever intended the body to be used for a ground connection. Probably a lot more focus on tolerance, reliabability, durability, and good environment seal.

None of this explains the difference in 'gain' that the OP experienced. I don't know what made the difference. I'm surprised that there is a real difference. 

[Champ_49]

Hi all.  Thanks for the input on this.
For my builds I didn't use the pot body as a ground. I used galactic grounding.This method was from the TUT 3 book.  Thanks Kevin he helped me out a lot.  I have to say this worked for me the best. Especially becuase I built a high gain amp and with galactic grounding I get minimal noise. 
As for the pot issue I think there is something wonky going on with the pots. Reason being , I only get this issue when I use the pec.  When I use alphas I get the proper gain. The only variable would be the pot.
Still stumped but it seems like I'm not the only person that had this issue.

[Ken Moon]

Some Alpha and CTS audio taper pots aren't really tapered - the track has 3 or 4 linear sections that are made to approximate a taper.

It would be interesting to measure the resistance of Alpha vs PEC pots at various pot settings. Maybe "lass gain" really means that the voltage divider formed by the two sections of the PEC pot is different than the Alpha at a setting the OP uses a lot.

To have "less gain", the wiper would have to be, electrically speaking, closer to ground on the PEC pot at the position(s) of interest.

Here is an interesting essay on pot construction and tapers:

http://www.geofex.com/article_folders/potsecrets/potscret.htm

[Ed_Chambley]

You got me curious as I do have 3 amps with PEC pots.  Soooooooooooooooooooo, I changed 2 of them last night with alpha pots.  Other than the immediate difference in taper at full up I did not notice any difference.  I only changed the Volume pots, but one was a EL34, 1987 Plexi and the other was a AB763 Deluxe.

I have read where CC resistors distort and have tried that as well.  I really do not hear it, but keep in mind.  My problem could be that I am not using the Copper Tone Super Duty AC cord Mercury sells.

Just joking of course.

I do not use the back of the pots for grounding, but can certainly solder to stainless.  I like PEC pots as they have a nice feel, but I also have a slew of NOS pots I mostly use.  Allen Bradleys and Clerostat.  I just bought and used some of the new sealed Honeywell Mexican made pots I have in SE EL34 amp with a TMB stack and has verb.  I really like the feel and taper of the Audio pots.

I am sure there is something to it as we have a couple of folks who have experienced a difference.  Could it be something as simple as different parts?  I mean if I am building an amp and paying PEC prices I am probably using "special" caps and resistors, maybe even HIFI quality stuff.

The 2 Pro amps that were built.  Was everything exactly the same except for the pots?  Most amps have great tolerances and some just don't sound the same as others of the exact type of build.  Too many variables.

The question I guess is how can the exact amount of resistance cause varying results?  Are your Alpha pots distorting like a CC resistor?  Does a open back pot sound different than a sealed one?  Right now I am sticking with resistance is resistance, however I do believe there is a difference.  There are a lot of builders who quickly say that using the best parts do not always make the best sounding amps especially when trying to capture that old Tweed tone.

[jojokeo]

"I have read where CC resistors distort and have tried that as well.  I really do not hear it, but keep in mind. My problem could be that I am not using the Copper Tone Super Duty AC cord Mercury sells."

"I am sure there is something to it as we have a couple of folks who have experienced a difference.  Could it be something as simple as different parts?  I mean if I am building an amp and paying PEC prices I am probably using "special" caps and resistors, maybe even HIFI quality stuff."

"There are a lot of builders who quickly say that using the best parts do not always make the best sounding amps especially when trying to capture that old Tweed tone."

Ed, maybe you just need "good quality" cables and you might hear it then? (just don't tell the wife) 

[Ed_Chambley]

"I have read where CC resistors distort and have tried that as well.  I really do not hear it, but keep in mind. My problem could be that I am not using the Copper Tone Super Duty AC cord Mercury sells."

"I am sure there is something to it as we have a couple of folks who have experienced a difference.  Could it be something as simple as different parts?  I mean if I am building an amp and paying PEC prices I am probably using "special" caps and resistors, maybe even HIFI quality stuff."

"There are a lot of builders who quickly say that using the best parts do not always make the best sounding amps especially when trying to capture that old Tweed tone."

Ed, maybe you just need "good quality" cables and you might hear it then? (just don't tell the wife) 

The majority of us who played with loud stage volume probably spend way more money to hear what we cant hear anymore.

I will say the EP booster and the Xotic RC are much better investments.  I have also come up with a nice buffer circuit that is small enough to fit in my Les Paul.

[punkykatt]

Ed, i did speak with the owner of the Pro head with the PEC pots I built 5yrs ago. He is suppose to drop it off to me when he gets a chance. (hopefully soon) I will try some alpha`s on the volumes and see (hear) if there is any more gain.

[alerich]

The majority of us who played with loud stage volume probably spend way more money to hear what we can't hear anymore.

I have come to the conclusion that guitar related tinnitus impacts the same specific portion of the audio spectrum where the average female voice resides.

That's my story and I'm sticking to it.

[Ken Moon]

+1 for alerich's comment.

I have found that, after 40 years of  marriage (and lots of loud music), my ears can no longer hear the frequencies emitted by my wife's vocal cords 

[punkykatt]

+2  on the last two comments.