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Vintage threads from the first ten years

Post-PI master any good?

4/27/2000 8:25 AM
Carlos	Post-PI master any good? Hi! I've built a plexi clone using a Hoffman board (excellent). Has anybody tried a post-pi master like in: http://www.ampage.org/blueguitar/pi-m-vol.pdf Does it alter the sound? Before I buy a rather expensive dual gang pot with good tracking (<3dB), I thought I'd better ask at Ampage... Regards Carlos

4/27/2000 11:08 AM
Lars	I had one but I took it away after I bought a Marshall Powerbraker, The sound was much better in my taste. Lars

4/27/2000 2:07 PM
Don Symes	It's reported to sound better than a pre-PI MV.

4/27/2000 2:54 PM
Ray Ivers	Mission Amps MV Circuit Carlos, I downloaded the schematic and had a look at it. My only experience with post-PI master volumes is with my Marshall 4001 (Studio 15), which has what I would call a 'dual-choker with low-level diode clipping' master volume. These choker types (variable resistor across the driver tube outputs) sound OK to my ears at high and mid MV settings; at really low settings I hear the effects of driver-tube loading on the Marshall design, which sounds squashed and muddy to me. These are just my preferences. The Mission Amp circuit will add a lot of series resistance to the driver tube/power tube interface. This will decrease the effects of power tube overdrive on the driver tube (good, I think), drop treble response a bit (?), and cut power-amp gain a bit (maybe not so good... see my next). Introducing another capacitor into the power amp feedback loop may well change the operation of the loop, and along with the aforementioned gain drop could affect the operation of the presence control, if any - this may be why Bruce specified such a large range of feedback resistances - 5K to 100K (quite a range!). I shouldn't think that adding one such 60-degree phase shift into a feedback loop would in itself make the amp unstable, but it will decrease the effect the negative feedback has on output stage performance, which could be a good thing. I have somewhat mixed feelings about negative feedback - I like small amounts of it used locally to extend effective tube life, and it can be great for implementing tone controls, but I like the way output stages sound without the higher synthetic damping factor that negative feedback creates. Do I know exactly what this circuit sounds like? No, I don't. But I think you can reasonably expect that it WILL alter the sound somewhat, as stated above. I think that most of the sound changes I noted will become most noticeable when driving the output stage to overdrive, and of course that's what installing a master volume is intended to eliminate, or at least control. Try it and see - you may love it! Ray Ivers

4/27/2000 2:54 PM

Ray Ivers

Mission Amps MV Circuit

Carlos,

I downloaded the schematic and had a look at it. My only experience with post-PI master volumes is with my Marshall 4001 (Studio 15), which has what I would call a 'dual-choker with low-level diode clipping' master volume. These choker types (variable resistor across the driver tube outputs) sound OK to my ears at high and mid MV settings; at really low settings I hear the effects of driver-tube loading on the Marshall design, which sounds squashed and muddy to me. These are just my preferences.

The Mission Amp circuit will add a lot of series resistance to the driver tube/power tube interface. This will decrease the effects of power tube overdrive on the driver tube (good, I think), drop treble response a bit (?), and cut power-amp gain a bit (maybe not so good... see my next). Introducing another capacitor into the power amp feedback loop may well change the operation of the loop, and along with the aforementioned gain drop could affect the operation of the presence control, if any - this may be why Bruce specified such a large range of feedback resistances - 5K to 100K (quite a range!).

I shouldn't think that adding one such 60-degree phase shift into a feedback loop would in itself make the amp unstable, but it will decrease the effect the negative feedback has on output stage performance, which could be a good thing. I have somewhat mixed feelings about negative feedback - I like small amounts of it used locally to extend effective tube life, and it can be great for implementing tone controls, but I like the way output stages sound without the higher synthetic damping factor that negative feedback creates.

Do I know exactly what this circuit sounds like? No, I don't. But I think you can reasonably expect that it WILL alter the sound somewhat, as stated above. I think that most of the sound changes I noted will become most noticeable when driving the output stage to overdrive, and of course that's what installing a master volume is intended to eliminate, or at least control.

Try it and see - you may love it!

Ray Ivers

4/27/2000 4:25 PM
dale	Re: Post-PI master any good? never quite understood the theory that this sounds a lot better than pre pi. i did a couple amps with a post version, but i never found it to sound better than a pre pi master at stage volume. maybe at low levels i can see why it would be a bit better. besides, it's hard as hell to find dual ganged pots these days in the right value, and they are never close in that if you turn the pot part way in thier rotation, then check the resistance of each one, they are usually quite far off. how much that affects thier performance in this application, i don't know, but maybe someone here can jump in and explain. dale

4/27/2000 4:25 PM

dale

Re: Post-PI master any good?

never quite understood the theory that this sounds a lot better than pre pi. i did a couple amps with a post version, but i never found it to sound better than a pre pi master at stage volume. maybe at low levels i can see why it would be a bit better. besides, it's hard as hell to find dual ganged pots these days in the right value, and they are never close in that if you turn the pot part way in thier rotation, then check the resistance of each one, they are usually quite far off. how much that affects thier performance in this application, i don't know, but maybe someone here can jump in and explain.

dale

4/27/2000 4:53 PM
andyfuchs	I've found that the way you configure the actual circuit, can change the sound alot. There are a few methods of doing post PI masters. I've seen the potentiometer used as a plate load (dual-100K). It doesn't sound bad. The "dennis" method (The Sundown Governor), which is just putting a pot from plate to plate , which I don't care for the sound of. THis can also be done on the output end of the coupling caps too. This would mess with the grid bias feed resistor value the tube actually "sees" though. There can also be an extra set of coupling caps that feeds a dual 500K pot and then the second (wiper) output) set that actually feeds the tubes. Each version has it's own tonality, and end result. The value of the cap going into the driver can change the sound alot, when using a Post-PI too. If theres too much bass, the distortion can be overly "brown". The values of the coupling caps (when adding the extra set) also needs to be addressed too, since the caps are in series when the pot is fully up. I don't presently use this technique in anything I own or make, as It always struck me as a last resort for producing overdrive without adding gain stages. I've seen it added to many amps I serviced in the past.

4/27/2000 4:53 PM

andyfuchs

I've found that the way you configure the actual circuit, can change the sound alot. There are a few methods of doing post PI masters. I've seen the potentiometer used as a plate load (dual-100K). It doesn't sound bad. The "dennis" method (The Sundown Governor), which is just putting a pot from plate to plate , which I don't care for the sound of. THis can also be done on the output end of the coupling caps too. This would mess with the grid bias feed resistor value the tube actually "sees" though. There can also be an extra set of coupling caps that feeds a dual 500K pot and then the second (wiper) output) set that actually feeds the tubes. Each version has it's own tonality, and end result. The value of the cap going into the driver can change the sound alot, when using a Post-PI too. If theres too much bass, the distortion can be overly "brown". The values of the coupling caps (when adding the extra set) also needs to be addressed too, since the caps are in series when the pot is fully up.

I don't presently use this technique in anything I own or make, as It always struck me as a last resort for producing overdrive without adding gain stages. I've seen it added to many amps I serviced in the past.

4/27/2000 5:31 PM

Trace

quote:
"never quite understood the theory that this sounds a lot better than pre pi."

There's no real theory to it to be honest. It's just boils down to a different "sound" tone wise.

JCM 800 style Master Volume:

1.) This type of MV (Master Volume) is very common on most modern amps and is connected off from the Treble pot.

2.) This makes the EQ seem or feel more "active." When you move the knobs you hear a difference being made.

3.) This MV also pretty much excludes the tone or sound of the PI (Phase Inverter tube). Of course when you crank the MV up to higher levels you hear the PI breaking up but most players do not get the amps this loud in live settings as it is typically to loud! (ha, ha)

Post PI Master Volume

1.) A dual ganged pot is used in place of the bias splitter resistors.

2.) This type of MV actually includes the sound of the PI since the MV is placed after the PI tube.

3.) Now because it is placed after the PI the EQ doesn't seem to make much of a difference when you move the knobs around.

4.) When it's on "10" the amp is back to stock.

I should also say that a post PI is a good idea for older non-master volume amps. The reason most people do not like the JCM800 style MV is because it changes the tone of the amp by not including the sound of the PI at lower volumes and the EQ "feels" different.

If someone wants to retain the sound of their favorite 4-input Marshall (or any non-master amp) then the post PI MV is a good way to go because it retains the sound of PI tube. There are a couple other MV's that go after the PI tube but that's another post! (ha, ha)

Some people like it and some people do not like it. It's mostly comes down to personal preference.

Some people say that the post PI MV sounds thin at lower volume levels but to be honest I do not hear it that way. I think it's important to realize that you are hearing the preamp and PI working when the MV is at a lower setting just as the JCM800 MV. You have to drive the amp to a certain degree in order to make the power tubes come into play because that's the missing ingrediant and is usually why people hate MV's in general.

There is the argument that the dual ganged pots do not track perfectly. This is usually true. They can be off by bit. Most of the time when I check the readings on the bias splitter resistors (in older amps) they have drifted off value and typically do not match perfectly so I guess there are arguments for both sides.

Most vintage guys hate the JCM800 MV's because it excludes the sound of the PI at lower volume levels. The post PI MV "helps" solve this problem. No master volume can simulate the sound of the power section being driven hard though.

Mostly it's a sacrifice. We typically can't crank the heck out of our amps at gigs to where we would like to so we are faced with either getting a MV or a power attenuator.

Most good attenuators are between $200-$300. Most musicians do not have that sort of spare cash on hand so a MV (of whatever type) is usually the way they decide to go (because of the budget).

The sound of the PI is a BIG part of any non-master amp..in my humble opinion.

Trace

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