"Useless" FX loop in a Plexi

ZEN Amps

ZEN Amps

Well-known member
The Friedman Plexi thread got me thinking - essentially I don't fully agree with the 'loop in a plexi' is pointless argument. Although it's well known that the PI/power tubes provides most of the drive in a NMV Marshall, the preamp does contribute - especially if you're slamming the input with a boost.

A while back I built a 1959 clone with loop and PPIMV for a client who's a big FX (verb/delay) user. We ran some controlled tests with and without a boost using three setups:
1. FX upfront
2. FX in the loop (Metro board)
3. FX post amp (similar to a power station setup)

With a good loop setup properly for unity gain, and with outboard FX that don't screw with the return level or have other issues, 2 wasn't amazing but still objectively better than 1. Of course 3 was far superior to both.

Two other factors - with a clean boost upfront, the difference between 1 and 2 widened as expected. Also not surprisingly, dialling back the PPIMV helped (output tubes running cleaner). This, and other tests have led me to believe that a loop in a Plexi style amp with a PPIMV (like Suhr and now Friedman) isn't ideal, but can be of some benefit. If done well of course.

End of the day, an ordinary loop with a dodgy pedal will affect overall tone quite negatively. A good line-level loop with quality rack gear can sound decent. Most guys will live in between, but I suspect the former happens enough to turn many off - including builders.

Experiences / thoughts?
 
I guess it depends on what you want from a plexi or any other amp. It's always going to be some kind of compromise however minimal.

Most of my tube amps are Marshall (1959HW, JVM410, Origin 50) or marshall-like, (Engl E651 Artist Edition) I have a couple Fender combos which aren't used much, and a Mesa JP-2C.

I prefer going guitar direct into my Marshalls, including my 1959HW; occasionally with an EQ pedal in front. Rarely I'll put a wah, fuzz, octave fuzz, univibe in front with a strat, but I enjoy just the Marshall tones without anything else and guitars with two humbuckers.


I only use the effects loop in my Mesa JP-2C, in 4CM.
 
There was a thread on TGP the other day about this subject, and Pete Thorn chimed in when the conversation was really getting going and talked about how Yngwie Malmsteen described the breakup of a Plexi, saying Yngwie said a Plexi's distortion is "balanced across the entire amp."

I didn't know exactly what that meant, so I decided to experiment around, mainly using the Plexi models in the Axe-Fx III. Well, turns out that it's fairly straightforward. As you turn up the volumes on a Plexi, the preamp starts to distort at about the same rate and intensity as the poweramp, evenly through the full sweep of the volume knobs.

For the longest time, I thought the Plexi's preamp was completely clean, and the distortion pretty much comes 100% from the poweramp. HOWEVER, I was mistaken. That's not how a Plexi works. You can verify this by adding a PRE-PHASE inverter Master control in the circuit and turn it way down, at which point you can still get a decent amount of preamp distortion if you turn the stock Volume knobs up. Also, if you check the schematic, you can see that the standard Volume controls on a plexi are actually placed pretty early in the circuit, even earlier than some of the preamp tube gain stages.

So what does all that mean?

It means the volume knobs on a Plexi, along with acting as the amp's overall loudness and poweramp breakup controls, also act as preamp gain controls at the same time.

After finding out that stuff, I have changed my position about effects loops in Plexis. I used to think they were 100% useless. Now, I think they can be useful IF you have a PRE-phase inverter Master volume control on the amp as well, which would allow you to run the poweramp as clean as you wanted while being able to independently overdrive the preamp. Or, an effects loop might help a little bit if you're running mild levels of breakup. I still think effects loops in plexis are entirely redundant if you're cranking the amp enough to achieve Van Halen levels of high poweramp distortion though.

You know what would be cool though? A modernized plexi with an effects loop, with both a pre-phase inverter Master AND a post-phase inverter Master, all bypassable if you want.
 
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fryette was hinting at some new toys on the horizon.

howabout a power station with a built in variac?

i’ve heard good things about the suhr ir load box but i can’t try it due to the fixed 8 ohm impedance and my peacemaker is 16 ohm out.
 
Kelly said:
Get a power station and be done with it.
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If I need a plexi with a loop I’d probably look into the Metro stuff. The Metroplex mkii sounds great and has the master volume and loop.

It’s all a compromise so I’d rather get close with a good master and loop like on the Metro than get close with a power station given the cost and extra gear to deal with.

But if I already had a plexi I’d go power station instead of modding and whatnot.
 
Meanjoegreene said:
If I need a plexi with a loop I’d probably look into the Metro stuff. The Metroplex mkii sounds great and has the master volume and loop.

It’s all a compromise so I’d rather get close with a good master and loop like on the Metro than get close with a power station given the cost and extra gear to deal with.

But if I already had a plexi I’d go power station instead of modding and whatnot.
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Haven't tried a MP II but I imagine the loop would work much better than on most any other plexi-type amp.

Besides it being a good loop design, George has inserted some sort of extra gain stage between the tonestack and loop. Not sure exactly what it is (he calls it the Emulation circuit), but in his words it adds 'gain, clipping and sag'. It's not a tube stage so maybe some sort of diode clipping, unsure. The PI and power amp are designed to run much cleaner than a traditional plexi, so now the loop is essentially post-gain (mostly).

The master is pre-PI, presumably because the PI is now not adding much drive. Cool idea, but not sure if the die-hard Plexi guys want the extra circuitry and a cleanish power section, or prefer just to crank a regular style amp and deal with the SPL in another way.
 
TheGreatGreen said:
For the longest time, I thought the Plexi's preamp was completely clean, and the distortion pretty much comes 100% from the poweramp.
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I actually had started writing a similar comment and I guess forgot to hit the "post" button. Luckily it saved:

I think people really understate how much the preamp on a plexi affects the tone. Some seem to imply that it’s nearly a clean amp and everything worth playing through it is dependent on power tube drive. It’s definitely a part of it of course, but I modded my 1987X by tying a pot across the end of the fx loop - effectively a pre pi mv - and it still sounds damn good to me and gets plenty overdriven even at quiet volumes.

---

I've also had amps with a post pi mv and other circuit tricks (power scales etc), and tried out an attenuator but ultimately nothing really emulates the combination of power tube drive with speaker cone movement - other than a loud amp. My attitude is that I want an fx loop on everything, including plexis, though I'm not particular with loops. I'm not a big effects guy but nothing I own seems to care much about what loop it goes through. The only loop I maybe don't love is the 2555X because of its location (pre EQ, for some reason) - it's fine for normal stuff but there are certain things that behave funny in that location in the circuit. Definitely not useless to me, but I can understand why someone who exclusively does recording/post-poweramp/post-mic effects, they'd rather not bother with a loop that could potentially color the tone (which is also why it's not too much to ask for a loop with a hard bypass, even my pretty normal 1987X has that).

Also, for what it's worth, I think the power tubes being a little bit overdriven adding some grit to delay repeats is a *desirable* sound. Just my opinion of course, does not apply to all music or players. I also have one of those Megabyte delays that lets me add some grit to repeats and I also like that pedal on clean amps (and it gets more distorted as they fade out, which I really like).
 
I think due to reading on multiple forums the last year or so, I just haven’t installed my Metro loop.

In the end, I play in the house. I’m not locked in a bedroom or anything, but inside, loud seems extra loud.

So the PS2 not only gives me a fantastic loop, it also gives me fantastic tone at a lower volume…

I will say though, when it’s warmer, it’s fun to take my gear out to the barn and open it up!
 

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I think the post-PI MV illustrates just how much distortion occurs in the preamp and PI combined. The power tubes are running clean yet it sounds ‘almost’ as distorted as fully cranked.

Then if you experiment with a pre-PI MV like mentioned above, that illustrates just how much distortion happens in the preamp. Now both the PI and power tubes are running clean. And the amp sounds anemic af.

All that gives you an indication of just how much plexi magic occurs in the PI. Which is a lot!

Also mentioned above is the fact that delay-before-distortion isn’t always bad. Ed ran the Echoplex in front, The Edge does the same a lot (way less distortion typically), Adam Jones runs a Boss delay up front and they all sound great. It’s a different sound and more of an effect than just ambience. I run an El Cap up front just for that but the delay in the rack (post speaker Line Out) provides the ambient delay/reverb.

I’m still of the mindset that a loop in a plexi is a hard pass. It makes me NOT want the amp. But that’s just me, I can respect others wanting one with a loop.
 
Some type of amp / load/ fx/ power amp/ cab is best for me. At least on an amp that is using a cranked power section. Something like an Ultralead a loop works great.
 
mentoneman said:
fryette was hinting at some new toys on the horizon.

howabout a power station with a built in variac?

i’ve heard good things about the suhr ir load box but i can’t try it due to the fixed 8 ohm impedance and my peacemaker is 16 ohm out.
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Your peacemaker doesn't have 8 ohm jacks? With those 8 ohm jacks in parallel you should be able to plug in the 8ohm load of the Suhr Load box into either of those 8 ohm jacks for an 8 ohm load.
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TotallyRadGuitars said:
Definitely not useless to me, but I can understand why someone who exclusively does recording/post-poweramp/post-mic effects, they'd rather not bother with a loop that could potentially color the tone (which is also why it's not too much to ask for a loop with a hard bypass, even my pretty normal 1987X has that).
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Yeah this is where I'm at. I rarely use a loop, and understand a post-solution works best for recording or performing. However from talking to players and reading online, there seems to be a decent proportion that get at least some utility from a loop, and often bemoan the absence of one.

However I'm sure Friedman/Suhr/Germino know and listen to their customers, and they've spoken. Personally the addition of a good, fully-bypassable loop is welcome - heck even so you can use the return input for some external preamp fun.
 
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