best post PI master volume

[EETStudent]

Any opinions on what the best post phase inverter master volume is? Right now Im using one seen in matchless chieftan amps, with one PI output connected to an outside lug, and the other output connected to the wiper. Pretty cool stuff, just wondering what others think...

[Bruce / Mission Amps]

Originally Posted by EETStudent

Any opinions on what the best post phase inverter master volume is? Right now Im using one seen in matchless chieftan amps, with one PI output connected to an outside lug, and the other output connected to the wiper. Pretty cool stuff, just wondering what others think...

I don't like it at all... that's what I think. The best one to me is the dual gang 250K to 500K MV pot... wired either backwards or like a volume control and DC blocked to keep any DC off the pots and DC -V Bias where it should be.. on the power tube grids.

[Tubis]

If you have FDB from the OPT to the PI think what happens when the signal going back gets lower but the signal in to the PI stays the same. When you turn down the post PI volume the the PI will get less feedback and will amplify and distort more.

[EETStudent]

Well, I dont have any negative feedback. I had tried a couple things before this post, and Im with you Bruce I dont much care for it I guess.

[jrfrond]

Originally Posted by Bruce / Mission Amps

I don't like it at all... that's what I think. The best one to me is the dual gang 250K to 500K MV pot... wired either backwards or like a volume control and DC blocked to keep any DC off the pots and DC -V Bias where it should be.. on the power tube grids.

I agree with this design being the best. The best way to implement that I've found is to use a dual 250K pot, and use the resistive element as you bias divider, in place of the ubiquitous 2 x 220K resistors usually used, OR run them to ground if using cathode bias. Then the wipers are connected to the PI coupling caps. In this manner, you maintain bass response when turning down the MV. With the single-pot setup, you load the caps too much.

[hasserl]

Originally Posted by Tubis

If you have FDB from the OPT to the PI think what happens when the signal going back gets lower but the signal in to the PI stays the same. When you turn down the post PI volume the the PI will get less feedback and will amplify and distort more.

ISn't that why most people use a MV? To get more distortion at reduced volume levels? What's the problem? I see people post on this all the time and I just don't get what the problem is. I've installed dozens of post PI MV's in amps with nfb, and it's never been a problem, not once.

Well, other than the reduction in Presence or other controls in the nfb circuit. But that I can live with as a trade off for the sonic benefits the post PI MV gives.

As for the single pot "crossline" style MV, I've used it on several amps with good results; I've used it on others with horrible results. My conclusions are it depends on the particular amp you're using it on and it shouldn't be written off universally. I've also used the type Bruce mentioned above, with the dual pot isolated from all dc with four caps, and the "Ken Fisher" style which replaces the two 220k bias mixing resistors. I think the KF style has the best sonics, the isolated four cap version is very close sonically but it is safer in that you don't rely on the pot wiper to carry the bias voltage. The crossline, when it works on an amp, works best at just scrubbing a few db's off the volume, not so good at bringing the volume all the way down to bedroom levels.

[Russ]

Originally Posted by jrfrond

Then the wipers are connected to the PI coupling caps. In this manner, you maintain bass response when turning down the MV.

If the wipers are connected to the caps, then as volume is turned down, the load (and bass response) would go down also.
I think the wipers are supposed to be connected to the output tube grids, not to the coupling caps. The connections on the pot opposite the common connection to ground or bias go to the coupling caps.

[jrfrond]

Originally Posted by Russ

If the wipers are connected to the caps, then as volume is turned down, the load (and bass response) would go down also.
I think the wipers are supposed to be connected to the output tube grids, not to the coupling caps. The connections on the pot opposite the common connection to ground or bias go to the coupling caps.

Yes, you are correct sir, as I look at the schematics for one of my own amps. That's what I get for doing late gigs on weeknights!

[CitizenCain]

There's a varient of the KF PPIMV popular over at the Metro Amp forum. It's basically the KF design with safety resistors added between the wiper and output lug on the dual pots. If the wiper should give out, the resistors see to it that bias voltage is maintained to the tubes.

I've installed it in two amps with good results and plan on installing it in the Heathkit rebuild I'm doing.

[Tubis]

Originally Posted by hasserl

ISn't that why most people use a MV? To get more distortion at reduced volume levels? What's the problem? I see people post on this all the time and I just don't get what the problem is. I've installed dozens of post PI MV's in amps with nfb, and it's never been a problem, not once.

Well, other than the reduction in Presence or other controls in the nfb circuit. But that I can live with as a trade off for the sonic benefits the post PI MV gives.

As for the single pot "crossline" style MV, I've used it on several amps with good results; I've used it on others with horrible results. My conclusions are it depends on the particular amp you're using it on and it shouldn't be written off universally. I've also used the type Bruce mentioned above, with the dual pot isolated from all dc with four caps, and the "Ken Fisher" style which replaces the two 220k bias mixing resistors. I think the KF style has the best sonics, the isolated four cap version is very close sonically but it is safer in that you don't rely on the pot wiper to carry the bias voltage. The crossline, when it works on an amp, works best at just scrubbing a few db's off the volume, not so good at bringing the volume all the way down to bedroom levels.

No problem at all i just stating the facts. A lot of tweakers don´t realy know about this. You get more PI distortion and less power tube distortion. And the presence changes by the turn of the master volume. Good or bad, i have no opinion about that.
I rely on preamp design pre PI for all distortion sound when i build stuff. There are ways to get the preamp to sound pretty close to output distortion.

[Don Moose]

Originally Posted by Tubis

If you have FDB ...

Around here we spell that NFB (Negative Feed Back).

Just, y'know, trying to help.

[Tubis]

Originally Posted by Don Moose

Around here we spell that NFB (Negative Feed Back).

Just, y'know, trying to help.

Thanks, i´m from sweden and that´s my excuse

[Steve Conner]

Originally Posted by Tubis

I rely on preamp design pre PI for all distortion sound when i build stuff. There are ways to get the preamp to sound pretty close to output distortion.

I have to agree with that. I don't see what's wrong with the idea of getting distortion from the preamp alone that sounds good at moderate volume. When you need it louder to do a gig, then you can turn the gain down on the preamp and start overdriving the power amp.

I always found that my amps need totally different settings for playing at home as opposed to gigging, anyway.

[hasserl]

Originally Posted by Tubis

No problem at all i just stating the facts. A lot of tweakers don´t realy know about this. You get more PI distortion and less power tube distortion. And the presence changes by the turn of the master volume. Good or bad, i have no opinion about that.
I rely on preamp design pre PI for all distortion sound when i build stuff. There are ways to get the preamp to sound pretty close to output distortion.

ANY master volume, whether pre or post PI, will provide for less power tube distortion. Whether you prefer the distortion to come from the preamp or the phase inverter is subjective and a purely personal preference. If you prefer the distortion from the preamp, that's cool, that's for you. But the thread is about post PI MV's, so I'll going out on a limb to assume the OP has already considered those things.

But if output distortion is what you want, you'll most likely want to put the MV after the PI, because that's where most output distortion comes from.

[Tubis]

Originally Posted by hasserl

ANY master volume, whether pre or post PI, will provide for less power tube distortion. Whether you prefer the distortion to come from the preamp or the phase inverter is subjective and a purely personal preference. If you prefer the distortion from the preamp, that's cool, that's for you. But the thread is about post PI MV's, so I'll going out on a limb to assume the OP has already considered those things.

But if output distortion is what you want, you'll most likely want to put the MV after the PI, because that's where most output distortion comes from.

Well, i understand that. I just wanted to say that its just not that you can feed larger signal into the PI that makes for gretaer distortion but the effect of a NFB plays a part. And the presence changes as well.
But as the threadmaker doesn´t have any NFB it doesn´t apply to him but maybe someone else could use the info. And maybe undertstand more why its sounds the way i does when you have post PIMV and NFB with presence.