is it ok to use a pot here?

[daz]

I have a cathode biased el34x2 amp that has seperate cathode resistors and caps for each tube. I also have a switch that lifts the 2 cathode caps out of circuit which drops the amp's volume a lot and makes the tone different. i like both settings but sometimes would prefer a setting somewhere between the 2. So i thought why not use a dual pot to gradually take the caps out of circuit with the resistive strip at the end of the pot's throw stripped away so that at full turn towards the highest resistance it would be completely open like having the caps fully out of circuit. The question is, will a pot be ok there with the 30 volts across it? I would think not with well under 100 ma but i figured i should ask to be safe.

[jrfrond]

You don't have to get fancy here with a "no-load" pot. If you use a 100KA pot an hook it up reverse-log using the wiper and log-side terminals, you will achieve the same effect. Or, use a DPDT push-pull pot if it bothers you.

[daz]

I'm not worried about any of that. I can try different values or use a DPDT as you say. But my only concern is the pot will be ok to use there.

[tubeswell]

I have a cathode biased el34x2 amp that has seperate cathode resistors and caps for each tube. I also have a switch that lifts the 2 cathode caps out of circuit which drops the amp's volume a lot and makes the tone different. i like both settings but sometimes would prefer a setting somewhere between the 2. So i thought why not use a dual pot to gradually take the caps out of circuit with the resistive strip at the end of the pot's throw stripped away so that at full turn towards the highest resistance it would be completely open like having the caps fully out of circuit. The question is, will a pot be ok there with the 30 volts across it? I would think not with well under 100 ma but i figured i should ask to be safe.

I am not sure what you mean without seeing a schematic. If the pot is wired as a variable resistor in series with the bypass cap, it will be a tone control - changing the voltage/current through the cap (but I think it would need to be on the 'ground' side of the cap to avoid scratchiness when you turn the pot)

FWIW one of the articles on the Mr valve wizard site explains shelving frequency response by using various combinations of resistors in series with cathode bypass caps

[daz]

Well, yeah...you could call it a tone conrol if you want. But assuming it works as i think it will and gives me varying degrees of the amp with and w/o the caps there, it will also be a srt of squish/power level control because those are the 2 things i notice with the caps cut out...it gets more squishy feeling and the output gets notably lower.

[cbarrow7625]

Daz,

A pot should be fine there. The cap is blocking the DC from getting on the pot. The DC is what is carrying the 30ma, not the AC coupled pot. Put the pot on the ground side to keep all DC off the pot just in case of an internal short in the pot.

I have had the same though 100 times and find that there never really is an "in between". It sounds great at either extreme but in the middle it never comes to life & gives you that perfect blend you think it will. I have had better luck with removing the cap from only one side at a time. That gives a much better "in between" than any pot (just my preference).

That said, if you really want to find that nirvana, hook up each cap on a separate pot. That way you can have one side full on or full off and dial in the other side until you hit the sweet spot (or anywhere in between).

[daz]

I have had the same though 100 times and find that there never really is an "in between". It sounds great at either extreme but in the middle it never comes to life & gives you that perfect blend you think it will.

Hmmm...ok, well you saved me the trouble of trying it. I will however try what you said and try lifting one cap. If that give a nice alternative to the 2 setting i have now (both or none) I will add that as a 3rd option. Tho i'm not sure i can envision it as a possibility with one switch....so I may need 2 switches.

[cbarrow7625]

Awww, c'mon don't let me kill your enthusiasm......try it anyway. YOU might like it. Just throwing in my $.02 here. Maybe I have a weird ear or something.

[jrfrond]

The pot WILL need to be on the low side of the cap going to ground. That is the only issue to avoid scratchy DC shift.

Though you CAN view this as a tone control, it is really varying the gain of the power amp by changing the amount of degenerative feedback in the cathode. Cathode caps are there to bypass the resistor at signal frequencies to eliminate this. The byproduct is an alteration of frequency response. otherwise the amp will go down to DC (0Hz).

If you WANT the drop in gain, what you are doing is fine. However, if you want to vary the low-frequency response of the stage, consider using a smaller bypass cap (under .5uF) and using a pot to gradually parallel a 22uF cap across the smaller one. I dubbed this a "Tightness" control. In fact, this technique can be used in any stage with a bypassed cathode.

[cbarrow7625]

John,

I like your idea about the "tightness" control. It has been a long time since I put a pot on the cathode of any stage, it sounds like you do it regularly. Since there is no DC flowing through the pot (the cap obviously blocks it) that means that there should be no DC shift through the pot. The cap will always charge to the cathode potential through the pot (even if it is on the "wrong" side) so there will be a net zero voltage across the pot. I would assume that there is therefore no DC scratchiness since there is no DC change. Have you tried it the "wrong" way and had scratchiness?

BTW, I am asking anly for acedemic purposes. I agree that the pot needs to be on the ground side for safety reasons.

[jrfrond]

Because any pot in the audio chain will sound scratchy with DC on it, I just neverplace them in the DC path, so I do not have to think about it. It's safe to assume that it will happen. From a design standpoint, it enables you to ground the element lug you are using to the back of the pot while connecting the cap directly to the wiper lug, making for a clean look and easier assembly.

I have done this before. In fact, a design feature I used in amps I used to sell was that I used a dual-gang 1MA/25KL custom pot for my volume control. As the 1M volume pot was turned UP, the 25KL pot (wired in reverse) would DECREASE the bass response in the preamp by gradually removing the 22uF cathode bypass cap, leave the .68uF permanent bypass cap untouched. It automatically tightened up the low end to avoid excessive intermodulation distortion from the low frequencies at high volumes. If you kill the lows at the front-end, it will yield a higher-quality distortion when the amp is turned up by avoiding "rolling" bass frequencies.