advantages of post-phase inverter master volume?

[lukeswall]

ok, so, I read that if I want to tame my plexi-clone I need an attenuator or a master volume. someone mentioned on a lost thread of yore this post-phase inverter master volume thing. that person also said that it doesn't affect the tone of the amp when you dime it. is that accurate? it sure seems so.

 

the problem is that I found out that most marshall amps use Pre-phase inverter master volumes. why would they use an inferior solution?

 

that's what the inquiring mind would like to know. thank you.

[lukeswall]

hey, maaaaan.

[Rich]

I've never heard a 50 or 100 watter NMV amp that sounds decent with a PPMV. Usually most of them sound mettallic or plinky to me. I also don't like what it does to the cleans when you roll off your volume. Your mileage may vary. But then again what everyone else thinks sounds good usually sounds a little overcompressed to me.

[lukeswall]

I see.

would it be possible to go back to the original tone in case I try that and decide against keeping it?

[Nik]

It is certainly a reversible mod.

 

The idea is that by puting the master volume after the PI you can reduce the volume and still have the PI distort.

 

It was introduced into the Marshall Club & Country amp range in the late 70s, I think. The designer was Steve Grindrod. Steve, along with many others, believes that when we hear nice power amp distortion, most of it is the PI distorting and interacting with the output tranny, speaker(s) and feedback network, not the power tubes.

 

AFAIK Marshall have not used it in any other amps - strange.

 

Steve Grindrod is now the main man at Vox and is responsible for the Vox Valvetronix and Tonelab series, which essentially are digital modellers with a real, distorting PI in the power amp or output stage.

 

Steve is also responsible for all the recent Vox valve amps.

[gtrbarbarian]

I have put ptp metroamp boards in two Marshall reissues and also built their 100w kit. All of them had this mod, known on the net and in many Marshall circles as the "Rich" mod. I also own a Weber Mass... to my ears, the PPIMV does not effect the tone at all....you do get rich open distortion at bedroom volumes...what you don't get at that volume is the sound of your speakers working at the volumes they were meant to work at, and that is part of the "sound" you are looking for. However, great mod, I highly recommend it, I wouldn't have a non-master Marshall without one. It's not too difficult to do, or have a tech do it. It sounds way better IMHO than the mass at the same volume... if you go to metroamp.com they sell a PPIMV kit for about $8

[BluesMan60]

You get more overdrive from a Post PI MV because it allows the PI to overdrive (which is most of the overdrive sound you get from non master volume amps that are cranked).

[JohnH]

For Marshalls, the humble DSL401 has a PPIMV.

 

Lots of people like to trash this amp, but if you know how to set it right it sounds great.

 

I use it to get overdrive from the PI, keeping the preamp gain low but OD channel volume high, to give that push to the PI tube and reducing buzzy preamp distortion. IMO this is the the key to getting a rich smooth overdrive from this amp, at any volume.

 

I also believe the JVM and VMs have them

 

John

[lukeswall]

great contributions! I learned some very useful stuff with this thread. thanks A LOT!

[Deuterium]

Yeah...I thought the new Marshall JVM series has a PPIV Master Volume. And by all accounts, sounds GREAT at low to even bedroom level playing noise levels.

 

And I agree with some here...I would take a PPIV Master volume amp over a NMV amp using an attenuator, every single time.

[Nik]

 

For Marshalls, the humble DSL401 has a PPIMV. Lots of people like to trash this amp, but if you know how to set it right it sounds great. I use it to get overdrive from the PI, keeping the preamp gain low but OD channel volume high, to give that push to the PI tube and reducing buzzy preamp distortion. IMO this is the the key to getting a rich smooth overdrive from this amp, at any volume. I also believe the JVM and VMs have them John

 

 

Oooooh!

 

That's good new, but why Marshall and others don't employ a PPI MV more oten I don't know.

[JamesPeters]

Because post phase inverter master volumes may help the DSL401 with its buzzy preamp distortion, but other amps may not have buzzy preamp distortion; their designs might work perfectly fine without requiring a bandaid like a PPIMV. Seriously though, if the PPIMV works with an amp that's great, but a lot of amps just don't require them. Don't presume it's what every amp should have.

[Weathered]

If an amp is properly designed around their gain stages, and the designers take the master volume into account, the MV should be relatively good. I've talked with race from Voodoo about his master volume, and for the most part, it isn't tremendously different from the Marshall JMP/JCM800 MV, but he structures his gain so that the PI doesn't need to be slammed to smooth out the amp.

 

In a Plexi, I think that you are well served to try the PPIMV, you may like it, you may not. Another option (albiet quite a bit more expensive) would be the London Power Scaling. By all accounts, this is supposed to be superior in sound to a MV, and it's more expensive, but only you can decide what low volume performance is worth to you.

[lukeswall]

mmm

I'll sure look into the London Power Scaling.

[JamesPeters]

London Power Scaling. By all accounts, this is supposed to be superior in sound to a MV,

 

By some accounts. Some amps do not need special master volume circuits, attenuators, or power scaling. However an amp designed without a MV in mind at all, that's definitely a good opportunity to try different master volume circuits, power scaling, attenuators and whatnot.

[LaXu]

I used to own an amp with Power Scaling and it worked great. I've also tried attenuators and running a dummy load with line-out into a separate poweramp, the latter worked better while just the attenuator makes the tone unnaturally compressed.

 

I currently just use the master volume in my Diezel Einstein. I think it works really well, I don't feel that even at low volumes you're missing out on tone like you do on most Marshalls.

[grumphh]

the advantage?

The sound...

 

This is a '71 Superbass at bedroom volume...

[lukeswall]

the advantage? The sound... This is a '71 Superbass at bedroom volume...

 

slammin'

[Nik]

 

If an amp is properly designed around their gain stages, and the designers take the master volume into account, the MV should be relatively good. I've talked with race from Voodoo about his master volume, and for the most part, it isn't tremendously different from the Marshall JMP/JCM800 MV, but he structures his gain so that the PI doesn't need to be slammed to smooth out the amp.

 

 

Sure.

Some amps get their characteristic sound just from the preamp. Some need the power amp to be driven hard to give their best. With a PPIMV you can drive at least part of the power amp hard at low volume.

[grumphh]

slammin'

Thx

 

Although i have to make a disclaimer - it records much better than it actually sounds

 

When standing in the room the bass is actually quite mushy, and i would not recommend this for anything harder than late 70's hard rock without pedals.

 

But if you are into "classic" marshall tone, the PPIMV is definitely a nice solution

[DeanFreiwald]

I started modifying amps with post phase inverter master volumes back in 1970. If you want a great sound at low volume, it’s the only way to go. But it needs to be done right or it will sound like garbage.  If you want it to sound good you need to use a 1 meg duel potentiometer, not 250k, not 500k and not 2.2 meg which I see many kits using.  Both resistive elements of the duel potentiometer need to be closely matched. If the potentiometer is set half way or a third of the way the resistive levels need to match.  If you have a post master volume that doesn’t sound great, it was probably not installed with the correct parts.