Plexi PPIMV/FX Loop - Worth trying?

[assquatch20]

See post #15 for an update with question


Been thinking a lot about getting a 1987X with a loop and adding a MV but I'm not made of money lately. The idea is something bigger than a combo for special gigs and such. But I need to control the volume to some degree and I like having an FX loop. I think most 1987X models have a loop from what I've seen and a MV shouldn't be hard.

But I already have a '76 Plexi I got cheap that has had a hole added in the back for a 50w switch. I don't really like using it but I thought maybe I could put a post-phase inverter master volume in its place. The only other issue for my purposes is I'd like an effects loop. I see some on the market that may work but I'm not sure. If anyone has any recommendations I'd appreciate it.

I want it to be easily reversible and I'd imagine it can be, using two of the four speaker output jacks that the amp already has. I'm wondering if this is worth the effort. It would be probably be fairly affordable to mod the amp with a tech friend but I would have to reverse the mods if I ever resold.

Another option is a Power Station. They attenuate and have FX loops, but then it gets complicated to set up live. That's two more tubes running (when I could be down to 2 power tubes with a 1987X), and another power cable, speaker cable, etc. and I'm practically ready for a road case that'll house a half stack and a few rack units and that's just too far for me at this stage. A rack case with a Power Station and Synergy system + a Plexi sounds cool though.

The option I'm not as thrilled about is just getting my Plexi on the market after I likely remove the 50w switch. I'm not keen to sell for an amp I've not even tried, though there is a 1987X for $2k with a PPIMV on Reverb right now.

So sorry for going on about that but maybe you can offer some good advice. Should I mod it? Would the PPIMV and FX loop play well together? Would the MV be worth it live? Thanks

 

[tschrama]

Not worth it… go for the power station or attenuator.

 

[Pete Farrington]

Would the PPIMV and FX loop play well together?

That arrangement (of a clipping stage after the loop) is far from ideal.
The Power Station looks to be your best option.

 

[assquatch20]

I guess I'll get a rack case then. Thanks for the input!

 

[Purgasound]

Good advice here. There were days when the Lar/Mar PPIMV was all the rage and it seemed like it was being installed in every amp. While the design and theory are great, the actual result has it's drawbacks.

A PPIMV can sound good but the sound and response changes as the volume goes up and down. It can lack punch and lose the response of how the tonestack and negative feedback work. I've gone down this rabbit hole a couple of times and always wind up back in the same place which is removing the PPIMV.

The biggest downside to an effects loop with a PPIMV is that your output is already fully saturated and completely out of headroom. You wouldn't think you would need that much but delays, boosts, and even a chorus need a little bit of headroom to sound good. When you boost, nothing happens except your sound becomes more squashed. This is the big advantage of using the Power Station. You can get the distorted phase inverter sound and still have a clean effects loop.

Many of the old hot rodded Marshall's wound up with that sound because of how a PPIMV couldn't quite get there. You can use a pre-phase master volume but you will lose phase-inverter distortion at lower volumes so to make up for less distortion in the power amp you start cascading gain stages, then you have to tailor the gain stages to not oscillate or flub out... you wind up going down a rabbit hole. The funny thing is sometimes you wind up pretty far from the cranked Superlead sound but end up with something pretty cool in the process.

 

[assquatch20]

Good advice here. There were days when the Lar/Mar PPIMV was all the rage and it seemed like it was being installed in every amp. While the design and theory are great, the actual result has it's drawbacks.

A PPIMV can sound good but the sound and response changes as the volume goes up and down. It can lack punch and lose the response of how the tonestack and negative feedback work. I've gone down this rabbit hole a couple of times and always wind up back in the same place which is removing the PPIMV.

The biggest downside to an effects loop with a PPIMV is that your output is already fully saturated and completely out of headroom. You wouldn't think you would need that much but delays, boosts, and even a chorus need a little bit of headroom to sound good. When you boost, nothing happens except your sound becomes more squashed. This is the big advantage of using the Power Station. You can get the distorted phase inverter sound and still have a clean effects loop.

Many of the old hot rodded Marshall's wound up with that sound because of how a PPIMV couldn't quite get there. You can use a pre-phase master volume but you will lose phase-inverter distortion at lower volumes so to make up for less distortion in the power amp you start cascading gain stages, then you have to tailor the gain stages to not oscillate or flub out... you wind up going down a rabbit hole. The funny thing is sometimes you wind up pretty far from the cranked Superlead sound but end up with something pretty cool in the process.

It's definitely a neat idea to make it my own but I'll start with a Fryette. I've considered that Hot Mod V2 but not sure how it would work without a MV. Maybe just fine with an attenuator.

But if I've already gotta get a case for the power station (don't wanna sit a 15lb power amp precariously on a plexi) I may integrate a Synergy rack preamp for the extra gain flavors and whatnot. I'd love to run that through a Marshall power section but I bet it'll do okay with a Power Station.

 

[assquatch20]

Hey while I've got some helpful folks here, how hard should it be to remove this 50w switch on a 70's PCB plexi? Should the board need to come up or anything like that? If think not but I might have to have a look. The switch is on the back just to the right of the impedance selector.

 

[FleshOnGear]

Hey while I've got some helpful folks here, how hard should it be to remove this 50w switch on a 70's PCB plexi? Should the board need to come up or anything like that? If think not but I might have to have a look. The switch is on the back just to the right of the impedance selector.

My guess is that your 50w switch is actually a pentode/triode switch, which is a very simple mod. If that’s the case, I would expect that it’s just a few wires between the tube sockets and the switch, nothing mounted to the board. Without gut shots, it’s only speculation, though.

 

[tschrama]

IMHO the design theory of the PPIMV/Lamer is lacking in significantly. Which make me wonder why clipping diodes at the PhaseInverter is so unpopular.. as that is much more like a clipping PA then the Lamer: before threshold the PA behaves normally.. beyond threshold, the signal clips, the openloopgain goes to zero.

 

[assquatch20]

My guess is that your 50w switch is actually a pentode/triode switch, which is a very simple mod. If that’s the case, I would expect that it’s just a few wires between the tube sockets and the switch, nothing mounted to the board. Without gut shots, it’s only speculation, though.

I'm not positive but I think the switch disconnects two power tubes as I was told you'd have to change impedance selection to use it safely. I'm not fond of messing with that old switch much.

 

[FleshOnGear]

Ah, I see. If you have to alter the impedance, then yeah it’s disconnecting two tubes. I’m not so familiar with how that’s usually, or most safely, accomplished. Still probably not done on the PCB.

 

[Trouble Free]

the Tube Amp Expander will solve all your problems.

 

[LPman]

My '69 JMP 50 is set up as a regular 2204-style master model. While I play exclusively with the preamp vol wide open (no attenuation), there are a few instances when I noodle at bedroom levels. On those occasions this amp sounds just as good as cranked (minus the huge adrenaline rush of experiencing the nuclear explosion that a dimed JMP is capable of producing). It would be good if I could have one like this at home to be able to play it all night long. But there is no other amp like this.

I don't know why my amp sounds so extremely good. I owned many and compared it with a bunch similar amps, new and vintage, clones and boutique, nothing comes close. I'm absolutely convinced that my '69 JMP is the best Marshall in the radius of about 3500 miles. Maybe more but I don't like to brag.

 

[1956tvmodel]

I just posted my experience with the Power Station and my 1987x . Check it out. Best piece I've bought in a long while!

 

[assquatch20]

My 1959 has a PPIMV now so I'm digging this thread up to ask another question about this. What if I install a line in or series loop only to slave the Marshall and its cab? Not actually running wet effects between the Marshall pre and power sections, and not even using the Marshall preamp when slaved. The idea would be to combine with a Synergy system for other preamp flavors into the same power amp and cab. For wet effects I would use a line out box between the amp and cab to go into a rackmount power amp and cab setup.

I know it's a little convoluted but it would be kinda ideal for what I'm wanting to do. No power station or extra power tubes to worry about, just a regular stereo tube power amp, which I already have.

 

[GlideOn]

PPIMV, specifically LarMar is the best, most transparent Master Volume I've tried on Fender, Marshall curcuits, even the 2203/2204.

But it is important to understand what you are getting and what you are not.

It has the desired result of getting high gain sound and attitude at lower volumes.



However you are sacrificing some usability of the amp to get that effect:



- You lose the effectiveness of your tone controls, especially the presence control, making the amp more of a "core tone" sound for better or worse.

- You lose a bit of punch and a small amount of fidelity, however all Master Volumes suffer this save for that a LarMar behaves very much as a non-Master when turned up more than halfway. Keep your boost pedals to help make up for this, so its a non-issue.

- The greatest drwback however us that you loose the effectiveness of any internal preamp FX Loop whether buffered or true bypass as anything at high gain will distort and compress to the point of being unusable - because distorting the phase inverter is precisely integral to how a PPIMV works.



If you want an amp that has the ability to play high gain from bedroom to stage, a LarMar PPIMV will be a game-changer.

If you can live with the side effects.

 

[assquatch20]

PPIMV, specifically LarMar is the best, most transparent Master Volume I've tried on Fender, Marshall curcuits, even the 2203/2204.

But it is important to understand what you are getting and what you are not.

It has the desired result of getting high gain sound and attitude at lower volumes.



However you are sacrificing some usability of the amp to get that effect:



- You lose the effectiveness of your tone controls, especially the presence control, making the amp more of a "core tone" sound for better or worse.

- You lose a bit of punch and a small amount of fidelity, however all Master Volumes suffer this save for that a LarMar behaves very much as a non-Master when turned up more than halfway. Keep your boost pedals to help make up for this, so its a non-issue.

- The greatest drwback however us that you loose the effectiveness of any internal preamp FX Loop whether buffered or true bypass as anything at high gain will distort and compress to the point of being unusable - because distorting the phase inverter is precisely integral to how a PPIMV works.



If you want an amp that has the ability to play high gain from bedroom to stage, a LarMar PPIMV will be a game-changer.

If you can live with the side effects.

So I guess the loop return is right before the PI?

If I ran an fx loop without wet effects but instead using the loop only to swap preamps via slaving the Marshall power section, would this be any better? I can place wet effects further downstream into a second amp, fed from a speaker line out box. This may not help though, not sure.

 

[Purgasound]

So I guess the loop return is right before the PI?

If I ran an fx loop without wet effects but instead using the loop only to swap preamps via slaving the Marshall power section, would this be any better? I can place wet effects further downstream into a second amp, fed from a speaker line out box. This may not help though, not sure.

The FX return on the NMV will be wide open, maximum volume.
The FX send will be underwhelming and not what you want to hear when slaving into another amp. The preamp distortion will not have any PI distortion through the FX send. The PI distortion from a cranked NMV is what separates these amps from MV amp.

It's a trade off. I love NMV amps. I likely won't ever install many more PPIMV's. They're not bad in their own right, it just comes with a sacrifice I don't care for.

 

[assquatch20]

The FX return on the NMV will be wide open, maximum volume.
The FX send will be underwhelming and not what you want to hear when slaving into another amp. The preamp distortion will not have any PI distortion through the FX send. The PI distortion from a cranked NMV is what separates these amps from MV amp.

It's a trade off. I love NMV amps. I likely won't ever install many more PPIMV's. They're not bad in their own right, it just comes with a sacrifice I don't care for.

I have a PPIMV though. So do I have a volume control after the return and do I get phase inverter distortion or any other issues if I'm not using the Marshall preamp?

 

[GlideOn]

So I guess the loop return is right before the PI?

If I ran an fx loop without wet effects but instead using the loop only to swap preamps via slaving the Marshall power section, would this be any better? I can place wet effects further downstream into a second amp, fed from a speaker line out box. This may not help though, not sure.

I'm not sure if I can wrap my head around what you're trying to do, but the FX Loop goes in before the phase invertor, not after. It will be affected by the PPIMV.

The best solution for you may be to use a true bypass FX Loop, then use a volume pedal as part of it. That will essentially act like a pre-phase MV, also limiting raging high gain sounds.

Point is that you can't have both. Marshalls JCM800 and under are single channel, shared EQ amps at heart. You can do one thing at a time or try to get creative with pedals