[S Clark]

Ok, the amp I'm cloning is working, but the 100K Clarostat volume pot isn't going to do the job.  It goes from no volume at 8:00 to maxed out at 9:00.  Although the pot in the prototype amp appears to say 100K, do I need a 50K pot?  What are the options from $5-$35?  Alps?

[Folsom]

Stepped attenuator from eBay, SMD DACT type. Pots sound awful by comparison.

Are using a log or linear pot right now?

[ted_b]

THIS is how you grow pot by the volume.

[G Georgopoulos]

Ok, the amp I'm cloning is working, but the 100K Clarostat volume pot isn't going to do the job.  It goes from no volume at 8:00 to maxed out at 9:00.  Although the pot in the prototype amp appears to say 100K, do I need a 50K pot?  What are the options from $5-$35?  Alps?

Sounds there is a problem,either the pot is faulty or there is a fault in the amp,the easiest way to find out is put a new pot in,then if the problem remains suspect the amp,using 50k or 100k pot is ok.

all the best

[Steve]

Sounds there is a problem,either the pot is faulty or there is a fault in the amp,the easiest way to find out is put a new pot in,then if the problem remains suspect the amp,using 50k or 100k pot is ok.

all the best

I agree with George. Maybe leads reversed on control. Could also be a linear taper pot instead of an audio taper. Performed extensive testing and Noble is  the very best volume control one can purchase, unless artificial flavors are wanted. Too bad they are out of business, what a waste. Michael Percy is a source. Mid resistance is where the high frequency response will be the lowest. As George has mentioned, 50K is good, even 100K ohms would also work, although a little loss in high frequencies. Switch type will not be better, and almost certainly worse than Noble, due to multiple contacts and resistor quality, besides high cost. TKD potentiometer are also excellent.

Cheers
Steve

[richidoo]

I agree with George and Steve.. Sounds like the amp has a problem like a mistake in the feedback loop, or it just has too much gain for your speakers.

I doubt there is anything wrong with the VC pot, but you could disconnect the wires and measure it easy enough.

Goldpoint has a good article on how to add a pre-attenuator L-pad resistor network before the VC if it turns out your amp is OK and it's just too much gain for your speakers.
http://www.goldpt.com/mods.html

[mjosef]

goes from no volume at 8:00 to maxed out at 9:00.

Sounds like too hot an input signal to me, subbing the pot will do naught, you may need to pre-attenuate the signal regardless of pot used..described here..http://www.goldpt.com/mods.html

[mjosef]

 
Rich pipped me to the tape. 

[Steve]

It is possible that the "hot" lead is connected to wiper arm, in otherwards leads crossed, ground wire ok. The result will be nearly instant maximum volume. A linear taper control would also be a cause.

Cheers
Steve

[richidoo]

It is possible that the "hot" lead is connected to wiper arm, in otherwards leads crossed, ground wire ok. The result will be nearly instant maximum volume.

Yeah, if input and wiper terminals are shorted together, then you lose all the attenuation, AND the signal would mute to ground when turned all the way down. 

At first I thought you meant simply reversing the leads between input and wiper, but that would still work just fine.

[richidoo]

Rich pipped me to the tape. 

Sorry Martin! 
 

[S Clark]

Wow. There is just so much I don't know about what I'm trying to do.  Dang do I miss having Gary around, but you guys are helping a bunch.  Although I've seen stepped attenuators, I didn't know there was such a thing as "linear taper" vs. "audio taper".  Got these Clarostat pots after talking to the guy at Apex Jr about what I was trying to do.  The ones in the prototype are Clarostat Hot Potz Extra Life 100K 9637 type EJ.  The ones I've used simply say Clarostat cm 39169 100K 20 9404.  Both made in Mexico.  Both look pretty much alike.
I'll try changing the leads around, and try a second pot.  If that isn't it I'll call Apex Jr. and see exactly what they've sold me. 
This is just about what the prototype pot looks like.

[G Georgopoulos]

Hi Scott

Any pot will do for the time being to test,later you can replace it with any other you like,another member suggested too much of a signal could cause the pot to max at 9 oclock , that could be the case,if your source is anything conventional i dont thing that's the case.Too much signal maxed at 9 o clock would have caused also a lot of clipping and distortion and you could notice all these.

[Steve]

At first I thought you meant simply reversing the leads between input and wiper, but that would still work just fine.

That is what I meant, reversed leads between input and wiper. Let me explain when the leads are reversed, if I may.

When the source output is incorrectly wired to the wiper arm, and the volume control is "turned down" all the way, the source output is grounded, unlike a correctly wired volume control where the source output is not grounded, the wiper arm is. There is a difference.

With the source output incorrectly connected to the wiper arm, as soon as the volume control is slightly rotated, the volume rises very rapidly, it does not follow the typical audio taper. There is also a large resistance in series with the grid of the tube, so some highs may also be lost, depending upon the value of the miller capacitance and volume control resistance.

As an example, let's say the source has an output impedance (Z) of 100 ohms. The volume control is 50k ohms. If the wiper and input are reversed, the wiper only needs to see 100 ohms to see half the maximum source signal value, vs 25k ohms if the control is wired correctly. To see 100 ohms only takes a whisper of a turn, say 1/32nd of the full rotation. However, to see 25k ohms takes, say, 2/3s of the full rotation of the control (audio taper). Even a linear taper control will need half rotation of the control.

As one can see, the volume is much more controlled when the audio taper control is wired correctly.

Scott, would you also double check to make sure the Clarostat is not a linear taper control? Measure the total resistance, then rotate the wiper half rotation and measure again to the wiper arm. It should not measure half the resistance. If it does, then it is a linear taper.

Cheers and hope this helps.
Steve

[richidoo]

As an example, let's say the source has an output impedance (Z) of 100 ohms. The volume control is 50k ohms. If the wiper and input are reversed, the wiper only needs to see 100 ohms to see half the maximum source signal value, vs 25k ohms if the control is wired correctly. To see 100 ohms only takes a whisper of a turn, say 1/32nd of the full rotation. However, to see 25k ohms takes, say, 2/3s of the full rotation of the control (audio taper). Even a linear taper control will need half rotation of the control.
Cheers and hope this helps.

Yes, very helpful, thanks for the example, Steve

[Steve]

Yes, very helpful, thanks for the example, Steve

Your very welcome Rich.

Cheers
Steve

[S Clark]

Everything checked out with the volume pots, and finding another resistor error fixed that particular problem.
Now I just have to find out why this thing is so distorted/grainy in certain ranges.
Thanks for all the help and suggestions along the way.  I report back if and when I make progress.
Scott