Resistor in parallel with volume pot

[thelonious]

My friend knows a guy who has a /\/\organ D A G fifteen combo. It's a good-sounding little beast with nice VVR power scaling that affects PI and output tubes. We offered to build the guy a free fuzz pedal if he'd let me disassemble and reassemble his $1600 amp. He was very cool about it and let us mess with it for a couple days. I'll email the schematic to Doug for the library after checking it for errors.

Anyway, there were a couple interesting things about this amp. For one thing, the first stage coupling cap was 500pF. Talk about reducing bass early on!

For another thing, the Volume pot (which is actually a gain control between the first two stages) is a 1M audio pot with a 1M resistor soldered in parallel across pins 1 + 3 (see attached picture). The pot has a pull SPST switch used as a bright switch, so I'm wondering if he did this because he wanted a 500k pot with SPST but couldn't find it at a reasonable price.

The only other thing a parallel resistor would do (besides adding resistor noise) is to "soften" the audio taper of the 1M pot to something in between audio and linear, right? I read http://gaussmarkov.net/wordpress/parts/resistors/resistors-6-mods/ trying to understand this better, but his graph only shows cases where the parallel resistor is a fraction of the pot value, not 1:1.

Any thoughts? If I used a separate bright switch instead of the SPST pot, couldn't I just use a regular 500k audio in this circuit and reduce some resistor noise?

[PRR]

> "soften" the audio taper of the 1M pot to something in between audio and linear, right?

No.

> I read

That's for 2-terminal variable-resistor connection, not 3-terminal potentiometer connection.

[The_Gaz]

Yes, I think your right about lowering the value because he could not find the right switched pot. I am not sure what that does to the taper. I think an easier method would be just to reduce the coupling cap to 250pf for the same RC filter without having to do anything to the pot. BTW, 500pf is a common value in the Vox AC30 Top Boost and its derivatives. Since these amps have no negative feedback, there is still plenty of bass.

I would be really interested in seeing that schematic you traced.

P.S. Is it just me, or does that middle joint look cold?

[jojokeo]

P.S. Is it just me, or does that middle joint look cold?

LOL - pretty poor for a $1600 amp that the guy can't use a little bit of proper flux. Things like this say a lot. I agree on the pot, just use a standard push-pull type of the correct value for crying out loud or standard pot + switch would be about the same money? For a $1600 amp this is nuts even talking about saving a dollar or two? Designed properly and you don't need a bright switch. Reminds me of these Emery amps for simple Champ style chassis amps charging well over $1000 bucks for.

[kagliostro]

To me seems that the image that applies to that "mod" is this



You can read about also here (I corrected the link)

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



K

[sluckey]

To me seems that the image that applies to that "mod" is this

No, I believe the resistor is connected between terminals 1 and 3 (outer lugs) in that image.

[kagliostro]

My vision gets worse and worse 

K

[thelonious]

> "soften" the audio taper of the 1M pot to something in between audio and linear, right?

No.

> I read

That's for 2-terminal variable-resistor connection, not 3-terminal potentiometer connection.

Ok, I'm afraid to put this out there, because it's probably even more wrong, but I really want to understand this, and I'm having trouble answering the question by googling. Does the 1M resistor affect the taper at all? If the pot were connected as a variable resistor, I could use the formula for parallel resistance, but a potentiometer connection adds another variable. Would the parallel 1M resistor be seen in series with the bottom half of the pot? So could I alter R_total=(R1*R2)/(R1+R2) and try something like R_total=([R1+R3]*R2)/([R1+R3]+R2)? That would mean that a linear pot would respond more like the attached graph. So I guess a log pot would become an even sharper curve? But of course this could all be complete bunk!

I would be really interested in seeing that schematic you traced.

It's live! You can check it out at http://www.el34world.com/charts/Schematics/files/Misc/morgan_dag15c.pdf

[thelonious]

Here are a couple more gutshots, too, for those who are interested!

[PRR]

> Does the 1M resistor affect the taper at all?

It can't. It's not connected to the wiper.

What it does is make the 500pFd coupling cap feel 500K, while the 120pFd brite cap looks into a 1Meg pot.

If you replace with a 500K pot, you probably want to change 120p to like 240p.

And unless you are building a Museum Quality Recreation, you probably want to eXperiment with all these values, not spend time at the keyboard.

[kagliostro]

Many thanks for sharing the schematic Very Good Job!

Have you investigated about OT primary impedance ? Which are the tubes ? (12ax7 + el84 ?)

Are you sure about V4 / V5 plate and grid voltages ?

K

[jojokeo]

> Does the 1M resistor affect the taper at all?

It can't. It's not connected to the wiper.

What it does is make the 500pFd coupling cap feel 500K, while the 120pFd brite cap looks into a 1Meg pot.

If you replace with a 500K pot, you probably want to change 120p to like 240p.

And unless you are building a Museum Quality Recreation, you probably want to eXperiment with all these values, not spend time at the keyboard.

totally agree! What's the big deal with a pot taper really on an amp?! So you have the setting on 4 instead of 5 or whatever, no big deal. Maybe on a guitar it's a different thing for volume swells, etc. but for an amp setting...fahgettaboutit and move on.

[thelonious]

> Does the 1M resistor affect the taper at all?
It can't. It's not connected to the wiper.
What it does is make the 500pFd coupling cap feel 500K, while the 120pFd brite cap looks into a 1Meg pot.

Ah. Okay, I get it. So the taper is created by how the ratio between the halves of the pot changes when it's turned, and the 1M resistor can't change that ratio. Thanks for helping me understand that. Do you (or does anyone reading this) have any recommendations on a good book or two to read that will help me better understand the theory and math behind stuff like this?

And unless you are building a Museum Quality Recreation, you probably want to eXperiment with all these values, not spend time at the keyboard.

Good call. I just get really OCD about understanding How Things Work. It's usually a curse, and occasionally a blessing.

Many thanks for sharing the schematic Very Good Job!
Have you investigated about OT primary impedance ? Which are the tubes ? (12ax7 + el84 ?)
Are you sure about V4 / V5 plate and grid voltages ?

Thank you! I wish I had been able to put a variac on the OT and figure that out. Unfortunately, I only had the amp for a Wednesday night and had to take it back the next morning. :( Time was not my friend.
V1, V2, and V3 are 12AX7, V4 and V5 are EL84, and V6 is EZ81.
I'm fairly confident about the voltages, but can't be 100% certain (it was quite early in the morning by that point - ha!). It would have been helpful if I measured the DC resistance of the OT primary, too, to see why the plate voltage drops to 355, but... too late, so sad. Maybe one day I will get another shot.

totally agree! What's the big deal with a pot taper really on an amp?! So you have the setting on 4 instead of 5 or whatever, no big deal. Maybe on a guitar it's a different thing for volume swells, etc. but for an amp setting...fahgettaboutit and move on.

Aw man. I know, you are so right... just see my OCD comment above

[jojokeo]

I would just simply say that the value of the pot is way more important than the taper. But for OCD taper's also important ;) It is nice to have equal balance through the pot rotation. BTW thelonious everytime I see your name I instantly keep thinking Thelonious Monk. I love his old standards and the name is awesome too! Another guy maybe not many people know of is Ramsey Lewis and that's another great jazz or blues musician's name. Great piano player too!

[kagliostro]

Hi Thelonious

It would have been helpful if I measured the DC resistance of the OT primary, too, to see why the plate voltage drops to 355, but... too late

I was asking you about voltages because in the PS section you labeled the A node at 365v and the B node at 364v, instead the voltage values labeled on the plates is 355v and the voltage values labeled on the grid after the resistor is 365v

So I assumed the voltages on the PS nodes are corrected and I've think about an inversion of the voltages labeled on the tube

K

[thelonious]

Hi Thelonious

It would have been helpful if I measured the DC resistance of the OT primary, too, to see why the plate voltage drops to 355, but... too late

I was asking you about voltages because in the PS section you labeled the A node at 365v and the B node at 364v, instead the voltage values labeled on the plates is 355v and the voltage values labeled on the grid after the resistor is 365v

So I assumed the voltages on the PS nodes are corrected and I've think about an inversion of the voltages labeled on the tube

K

It is possible that I inverted them. But it's not that unusual for screen voltages to be higher than plate voltages when there is a choke between nodes A and B and the screen resistors are that small, is it? Add into that the inaccuracy of measuring with a cheap DMM, or maybe that I didn't wait long enough for the amp to warm up before measuring, and that in itself could explain the 1V difference between node B and the screens. But if I did invert them and the plates were supposed to measure 365v, then there would have to be an explanation for why the plates would drop no voltage at all across the DC resistance of the OT primary... I don't know.

Anyone have thoughts about that? I don't have enough experience yet to know which scenario is more likely. Are those voltages unusual enough that I probably messed up the measurements? If so, I'll gladly fix them and send Doug the new schem.

[thelonious]

Now that I think about it, even if I switched the voltages, they wouldn't make sense. If node B were at 364v and the screens were at 355v, each screen would have to be drawing
I=E/R
I=9v/100R
I=0.09A=90mA at idle in order to drop 9 volts across its 100R resistor, which is also bogus. What did I do wrong to get these crazy voltages?? haha

[thelonious]

After doing more reading and seeing threads like http://www.el34world.com/Forum/index.php?topic=13320.0 and http://www.el34world.com/Forum/index.php?topic=11925.0 , I think the voltages are close to correct (say, +/- 1 or 2 volts) on the schematic I posted.

The choke between nodes A & B, the small screen resistors, and the power supply split at the VVR (so the first 2 preamp tubes are not contributing to the current drawn from the screens node), all make this amp likely to have higher screen voltages.

I don't know why the screen voltage measured a volt higher than node B voltage. Obviously that is wrong, but it might have been wrong from something as simple as the B+ changing between measurements because I didn't let the amp warm up long enough before measuring.

[kagliostro]

May be also there was a line fluctuation

K

[super&plexi]

hello all, greetings from sunny so.cal.*. this is something I do frequently in stompbox building. sort of r&d, on a budget. maybe something picked up from geo efx,et al., just easy to start w/1meg, (ha! everytime I think of Family Guy...Peter blowing disdainful Rasberry...MEG! plffffrt!) slap on another, and viola's,or cello's, you got 500k, and didn't have to pull pot, unless other way was better. then you just untie/twist, and yer back. maybe 500k is unavailable/backordered, it's alternate value for 1c. Regarding solder joint, agree. Without going into much detail, we just sold our store in H.B. where I came in contact w/every brand of amp that comes to mind, and that/this one above is pretty clean compared to some of the,(I detest the Boot peep, Boo-Weak beautique) moniker, but you know what I mean. I never, or at least try not to ever disparage others work, but maybe because of labor laws, EPA,...MAYBE SATAN heh!, alcohol/lacquer thinner cleanup of rosin-flux splatter is atrocious on some darn expensive hand made merchandise.*wish I could've sent some EAST. L.T.& J.Fouse

[darryl]

The "resistor in parallel with volume pot" has appeared elsewhere - assuming this schematic is correct. ( It does have at least one error. )

http://www.el34world.com/charts/Schematics/files/Misc/badcat_hotcat_30.pdf

[printer2]

Probably just a bit of tuning going on adding the resistor.

[thelonious]

Thanks again, everyone!

BTW thelonious everytime I see your name I instantly keep thinking Thelonious Monk. I love his old standards and the name is awesome too! Another guy maybe not many people know of is Ramsey Lewis and that's another great jazz or blues musician's name. Great piano player too!

Jojokeo - I love both of those guys! Fantastic players. And lately, I've been digging Brad Mehldau. I am actually a pianist who also messes around with electronics and guitar. ;)

[Willabe]

I'm in with the in crowd.     

Ramsey's a Chicago man.


             Brad     

[super&plexi]

Ramsey Lewis has a radio show, nights on 88.1 Long Beach CA. kkjz I think, used to be kbca, kkgo, whatever, it's on internet also, & I think free. (on internet, I mean), He's even been known to play "Around Midnight" around 9 o'clock.......... too inside?

[alerich]

What's the big deal with a pot taper really on an amp?! So you have the setting on 4 instead of 5 or whatever, no big deal. Maybe on a guitar it's a different thing for volume swells, etc. but for an amp setting...fahgettaboutit and move on.

In only halfway agree with this. If you use all audio taper pots in an amp even where the schematic calls for linear taper it's not a big deal. You just turn the pot up higher and be done with it. Using a linear taper pot where there ought to really be an audio taper pot can be a pain. Then you are left with a touchy control that goes from too little to too much with just a little nudge. Think Fender amp reverb control pots.

[12AX7]

Side note...taper in a amp CAN be important. Sometimes a master volume can be so touchy at low volumes they are near impossible to get the exact volume you need. I had that issue in a 2204 circuit and took me a while to find a pot with a taper that helped. Turned out to be a NOS CTS.

[Tone Junkie]

Anybody notice if i'm not mistaken that this is a rocket with a VVR.
Bill