some pot questions

[12AX7]

No, this isn't about getting high.  But i have a couple questions regarding the 1M master pot in a 2204 style amp. My master is very touchy and jumps from nearly no volume to much too loud for home playing right after i've turned it up about a 1/8". Two questions about this, one, is that jump due to the chnge at the wiper to ground side or the input to output side? Seems it would be the resistance to ground because in metering the pots i've tried the in to out logs hardly change  till it's 1/4 or more thru it's rotation, well more than where it jumps up in volume. I have tried audio and linear taper pots. So the next question is wht's out there that may work? I seem to recall hearing mention of reverse audio taper, but i haven't found those.

[stratele52]

There are no problem with 2204 style amp and 1 meg audio pot. No problem with the pot but maybe you have problem with the amp ? Too much signal to the pot ?

If you write " 2204 style amp " without name , is a amp you built ? You have the schematic ?

I built myself a from a Marshall 2204 schematic with Fender reverb circuit . This amp is amazing . 

[Colas LeGrippa]

coupling cap from MV to the PI leaking DC, I suspect. Or pot defective. Try ''hasch'' .

[12AX7]

Tried a new cap, but no difference. As to stratele52's mention of too much signal, I know the amp has too much at that point but i didn't think that would cause that because even with the preamp gain pot way down it still does it. And i've tried many pots so it's not a bad pot . By the way, what is "hasch"?

[sluckey]

Show us the exact schematic for your "2204 style amp".

[12AX7]

I don't have one, but it's exactly the same as a 2204 from 3rd stage the to the speaker aside from a few changes none of which matter because it did this before those were implemented. The pre is different and does have more gain because for one it doesn't have the cold biased stage. But it's not a hell of a lot at all. Typical classic rock drive, nothing heavy.  And the design is still relatively similar, at least as far as the amount of stages.

[12AX7]

Wouldn't ya know it. I tried 2 types of alpha, bourns, and a couple others and the one that finally solved it was a old pot of unknown make out of a parts bin from probably 25-30 years ago. Maybe there IS something to this vintage thing after all. 

[The_Gaz]

It may have more tracks to achieve a smoother taper, or have a 30% rather than a 15% taper. Modern Alphas, for example, have only two tracks and a 15%, which is fine for most. Maybe you're just a little paranoid :)

[stratele52]

I don't have one, but it's exactly the same as a 2204 from 3rd stage the to the speaker aside from a few changes none of which matter because it did this before those were implemented. The pre is different and does have more gain because for one it doesn't have the cold biased stage. But it's not a hell of a lot at all. Typical classic rock drive, nothing heavy.  And the design is still relatively similar, at least as far as the amount of stages.

Unless you do a sketch from the input to the 3rd stage and show us the complete as build amp's schematic  , you and us  are like a blind .

 

[stratele52]

Wouldn't ya know it. I tried 2 types of alpha, bourns, and a couple others and the one that finally solved it was a old pot of unknown make out of a parts bin from probably 25-30 years ago. Maybe there IS something to this vintage thing after all. 

Do I understand that you find a good pot and no more problem now ?

I do not understand your procedure. We do not find a pot that will work in the circuit by chance.

And potentiometre you have found does not work by magic, not because it is vintage. Have to understand why it works or not. Its characteristics are measured.

More explanations are needed,
 

[12AX7]

We do not find a pot that will work in the circuit by chance

I sure did ! What do you want me to tell you? It works, probably because of what the gaz said. I have no way of knowing. As to the vintage thing, I wasn't serious about that. It was a joke, hence the smilie. In any case, the other pots would go from nothing to loud within a range so small i could not get levels in between loud and nearly off. Now i can. Maybe an unusual taper, don't know.
But I also need to address the signal level issue as stratele52 suggested because i know that too is part of it and probably what it took a certain taper or whatever to control it. Problem is, anything i do in the gain stage affects the tone. I spent a very long time achieving the tone the pre now has, and I don't want anything to change that. I have tried literally most everything imaginable from the input to the master and it's easy to lower gain a million ways, but they all affect the tone. So i think i need to try AFTER the MV. It will not affect the way the MV works, but the PI and PA need to be hit with a lower level.  I will 1st try a PPI master to set it range of the main MV then use a set resistor of whatever value it's set at if that works. It may or may not because it leaves the PI still hit hard. But i'm not sure how i can lower the PI gain tho unless i put a second pot after the main MV then measure it and put a voltage divider of that value there. Not sure how orthodox that is but those are the only 2 ideas i have come up with that may work after the MV.

[Colas LeGrippa]

Tried a new cap, but no difference. As to stratele52's mention of too much signal, I know the amp has too much at that point but i didn't think that would cause that because even with the preamp gain pot way down it still does it. And i've tried many pots so it's not a bad pot . By the way, what is "hasch"?


disregard, no important.

[stratele52]

12AX7, I would like to know what is the resistance of the potentiometer that you have installed and also if it is audio or linear. Also marked by the numbers perhaps? That's how I learn things.

[12AX7]

It's a 1 meg pot who's actual resistance was around 850k. Thats about all i can tell you without removing it, and for fear of screwing it up and not being able to find another like it. i'm not putting a soldering iron to it again unless absolutely necassary ! I do however have another possibility for finding one and tomorrow i'll have access and if there is another i'll post any numbers and check for taper. I'm pretty amazed at the difference tho. This pot acts almost perfectly linear in it's range relative to actual volume to the ear, so if i can find another you can be sure i'll guard it with my life. Well, maybe thats an exaggeration.......................naw

[stratele52]

Thank's for the replay 12AX7
So it is a 1 meg linear which mesure 850 k . I have many 1 meg news pots here. I took some mesurement they go to some around 1.1 meg, some around 987 k and some around 870 k. They are from CTS and Alpha , all new.

And the other pots you try before does'nt work why ? they are audio or wrong resistance ? Or both ?

[HotBluePlates]

And the other pots you try before does'nt work why ? they are audio or wrong resistance ? Or both ?

It's simply that very few pots go from 0-1 in a very smooth taper. The transition from "zero resistance" to "some resistance" is usually abrupt.

For all: most pots don't have a true audio taper, but fake it with 2-3 "different speed" linear segments. Regardless, if you absolutely HAD TO HAVE a perfect audio taper, with a perfect transition from off to on, you'd probably have to pay for a nice stepped attenuator (which defeats your desire for "no step"). The audiophools spend big bucks for a perfect taper, even if they have to go to discrete steps to get there.

[12AX7]

Do you know of any current production that taper "normally" ? I'd love to find a source for pots that work like the one i found because i've often found abrupt transitions in pots used as masters even on a lot of production amps, and every amp i've built to some degree.

Oh, and stratele52, i didn't say it was linear. At least one of the ones that didn't work right was linear, so i wouldn't assume that alone would do the trick. This one that worked, all i know about it is the actual resistance, not the taper.

And the other pots you try before does'nt work why ? they are audio or wrong resistance ? Or both ?

It's simply that very few pots go from 0-1 in a very smooth taper. The transition from "zero resistance" to "some resistance" is usually abrupt.

For all: most pots don't have a true audio taper, but fake it with 2-3 "different speed" linear segments. Regardless, if you absolutely HAD TO HAVE a perfect audio taper, with a perfect transition from off to on, you'd probably have to pay for a nice stepped attenuator (which defeats your desire for "no step"). The audiophools spend big bucks for a perfect taper, even if they have to go to discrete steps to get there.

[HotBluePlates]

I really don't know, because it has never really bothered me that there is a slight jump from zero to "some volume".

[Rev D]

 I saw a pot my other guitarist in my old band had installed in his tweed deluxe that had a great taper. Only downside was it was a Alejandro and cost like 15 bucks a pop. For one pot it might be worth it. I'm pretty sure they're just re-badged 2w mil spec pots by another company like Clarostat or one of those companys. Scroll down the page a bit and you'll see them. While rotating them I noticed a very almost gear like feel (they really are a nice feeling pot, I just couldn't afford parts like that myself), they did make his tweed deluxe a bit better that way, but his main problem is he didn't understand the twin volume controls on the tweed deluxe. Not many do, there are so many sounds to be had twisting both of them. Anyway, here's the link for Alejandro:

http://www.alessandro-products.com/main.php?p=parts

Regards,

Don

[stratele52]

  This pot acts almost perfectly linear in it's range relative to actual volume to the ear, so if i can find another you can be sure i'll guard it with my life. Well, maybe thats an exaggeration.......................naw

As you read, I thought you said that your potentiometer was linear?
Did I understand also you don't know how to read a linear pot with Ohmmeter ?

Yet it is very easy. If so, without wishing to insult you there are probably a lot of testing with erroneous conclusions you did on the amplifier without actually finding the cause of your problem.

A potentiometer for an amp do not need to be special. As long as it has a value aproximation audio and is, in most cases.
In your case, as far as I understand what you mean, your potentiometer accidentally circumvented or mitigate the problem.

 You may feel hurt by my remark, and if I have misunderstood, I took to forgive me English is not my first language


 

[SoundmasterG]

  This pot acts almost perfectly linear in it's range relative to actual volume to the ear,

As you read, I thought you said that your potentiometer was linear?

If you will notice, he said it is almost perfectly linear in it's range relative to actual volume to the ear.....I take that to mean that it is logarithmic, since we humans hear that way and not linearly. Volume controls are almost always log rather than linear to avoid the sudden ramping up of the volume.

Greg

[Jennings]

Interesting thread here...just thought I'd mention that i always disconnect the bright cap on the Vol pot of Marshall amps and I find it really pumps too much loud treble and gives and "all on" or "all off" effect on the vol.

[12AX7]

As you read, I thought you said that your potentiometer was linear?

No, i said i had TRIED both audio and linear pots. This one i never tested to see what it was because having already tried both audio and linear i realized thats not what was going to matter because they both worked about the same with a VERY small advantage in the linear.

Did I understand also you don't know how to read a linear pot with Ohmmeter ?

I do know how....turn it exactly 1/2 way thru it's rotation then measure from the wiper to each side and they should read very close to the same. And as i said before. i'm not going to unsolder it to check any of that because i don't want to keep reheating a pot that may be the only one i find like this, at least without wasting a lot of money trying pots.

A potentiometer for an amp do not need to be special. As long as it has a value aproximation audio and is, in most cases

In this case apparently it DOES, and if you look at soundmasterg's post i think he may have figured it out. I tried a dozed 1m pots since i built this amp a few years back and this was the only one that worked.

You may feel hurt by my remark

Not at all. You don't know me or the amp or anything i've done to it, so any conclusions you come to about me would be a wild guess and therefore no, i'm not hurt.

I take that to mean that it is logarithmic

You may be right. I was never really sure about log because i have also heard log is just another name for audio. So i was never really sure whether that was true of not. Assuming it IS different which from your post it apparently is, who has these? Anyone without a $5-6 shipping fee for a small part like that so i can just try it?
 

[stratele52]

Log and audio pots are the same. Many time it have "A" letter printed on the pot . Linear pot have " B" letter.  You can use linear pot for volume control but you'll hear the amp at almost full volume at the beginning of the pot. This is not suitable but I see in some budget amp.

Audio is the best for this use.

[12AX7]

Ok, you wanted to know the taper. I had to unsolder it today doing some troubleshooting so while i die i measured it and it's not linear. Audio or log....whether those are different or not seems to still be opinion. What i think we DO know is that not all audio tapers are the same so it may just be a better taper for my purpose. Anyway, with the pot centered and as used for a MV with full CCW being zero, it reads about 750k from the input lug to wiper and about 100k from wiper to ground lug. so there ya go.

[ThermionicEngine]

Greetings,
Certainly appears to be audio taper.  Something to remember- usually MV and volume controls are voltage dividers vs rheostats.  I have had a couple amps where the ground was bad due to corrosion or bad solder joint and the audio response was erratic.  Posssibly the issue resolved itself with the multiple removal/replacements.  If its working now, you don't have to fix it... Allbest, Larry

[stratele52]

    Posssibly the issue resolved itself with the multiple removal/replacements.    Allbest, Larry

+ 1 ThermionicEngine , I agree with that.

[12AX7]

Trust me, thats 100% not why. I tried MANY pots in that position over the years and this is the only one that worked. A ground wire and 2 wires from the tone stack and 2 the PI input, theres just nothing that could cause what you're suggesting and even if there was the chances it never righted itself till this one pot is just not the case. Theres nothing to go wrong. If you absolutely insist i'll put any of the others back in then put this good on back just to prove prove it. I understand fully what you're assuming and have seen that kind of thing many times, but not this time, no way. I know it's the pot. There ARE custom tapers, and this is obviously one of them.

[stratele52]

From the beginning , I would like to understand why your amp need a special pot . What is special with your amp ?

Another thing ,how are you able to trouble shooting this amp without schematic ?  2204 style is not enough.  For me is like going from Montreal to California without a road map.

You are better to sketch the circuit to have the schematic.   I do that almost all the time when schematic is not avalaible. Yes it is long, but shorter than trouble shouting without schematic. And you have the schematic the rest of you life.

[12AX7]

I don't need a schematic. I built this design about 5 years ago and have made 3 of them and i know the circuit like the back of my hand to where i could draw it blindfolded. If you are that curious about the MV, look at a 2204. From the 3rd stage to the speaker mine is identical aside from a few changes that even when identical to a 2204 still had the MV issue. So look at a 2204 schematic and from the 3rd stage to the speaker, thats my amp.

[John]

Lots of help to be had on this forum, but most of the guys here do indeed like to see the full schematic as built, before offering too much advice.

I update the schematic every time I do a tweak on my little build, just because I have a short memory.

[stratele52]

+100 john,

12AX7 you really don't want to understand.