Wampler Plexi-Drive on the Breadboard

[anotherjim]

I just thought the general idea is to have a "lead" channel without creating a whole separate path (with all it's own eq needs).
Now, a lead channel to me must run to hypersensitive. You can pluck the "dead" string outside the nut & saddles & still knock the house down.
I think I'd also want the output to drive a low impedance line input if need be, as low as 10k, so you can use any power amp, such as a  PA job, but still most preferably into a guitar speaker.

So, I'm imagining 3 stomp switches now. A TS soft clipper after Q1 that can be bypassed. The TS has 2 switchable gain ranges. Overdrive and, well, Escape Velocity. Then a final op-amp buffer to drive whatevers next. The way a TS stage works, and the losses in the network after Q3, should mean you never hear any op-amp clipping. Layout would be to critical stop squeal maybe, although the Wampler is inverting,TS & Buffer are not, so overall inverting will help.

[Ben Lyman]

Sounds good Jim, draw it up and I'll build it! 
Seriously though, I think the ultimate pedal would be something like that, 3 footswitches for crunch, saturation and "clear" boost.
I guess I picture a Fulldrive II with a 3rd switch at the end for a separate master volume.
3 switches is a bit much for me though, I would probably be happy with the 2 switches (boost and bypass) and then run my solo booster after that in a different pedal.

I think you and I (and many others around here) understand this use of a clean boost or TS in front of an overdriven amp but certainly there must be some people that don't quite get it.
To put these pedals in front of a Marshall Stack (etc.) is not always to create a "lead boost" or "volume boost" as sometimes the amp is already maxed out and can't get any louder.

My reason to put an EP Boost in front of (pedal or amp) is to drive the (already compressed and distorted) sound into fuzzy infinite sustain type sound.
I am guessing maybe that is "Escape Velocity"? The volume does not change when I step on my pedal, only the sound of the distortion and super sweet infinitely sustaining feedback.

After having achieved that, the "volume boost" would be from a final stage with a very transparent volume control, or just two volume knobs on a switch along the lines of the "High-Gain Dual Master" if anyone can remember that amp 

[robthequiet]

Hey Ben, 2￠ from the peanut gallery -- from the video I think that a switchable preamp stage would work really well. Sidenote, I think the EP needs the whole circuit to work properly. It's kinda warped a bit from the actual EP input circuit but that's another thread, though worth building by its own right. If you look at the Wampler Pinnacle circuit you might get some inspiration... I'm with Jim on the TS op-amp but maybe you would end up replacing Q2 in the process bcs it seems a bit redundant with gain switch on TS stage?

You're getting a pretty good sound already, maybe just need an output stage to kick the gain up and make the amp work a little harder?

[Ben Lyman]

cool thanks Jim and Rob. I have found either my SHO clone or the Breadboard EP Boost sound good with it. There is a certain sweet spot on the "boost" pot where it makes the Plexi sound great, too much boost and it goes downhill. I also think I might need to adjust the tone to cut some bass while boosting, it gets just slightly muddy. Maybe that's why I always liked my Boss SD-1 in front of my JCM800, a little extra treble and a lot extra gain.

Check out Brett's vid of the Plexi-Drive Deluxe, he gives away the secret 

[anotherjim]

An entertaining demo of the Plexi.
So, it may be the TS is right in front? That would be logical of course for a TS into the amp effect.

Too much boost causes -  constipation. Funny how that happens, could it be positive feedback causing lockup?

That's what's good about the TS thing, it doesn't make the signal get much bigger but improves apparent sustain. So would a boost with clipping diodes, but that is fuzzy or worse - splatty.

If you were looking to steer clear of op-amps, a couple of BMP clipper stages would be kind of like TS sustain?

[Ben Lyman]

I just jammed with my brothers today and used the Plexi Drive with my Boss SD-1 Super Overdrive clone. Results were extremely pleasing to say the least. I ended up leaving the Plexi Drive on the entire time and just used my volume knob and the SD-1 for all the sounds I needed.

Just to be clear, the SD-1 is the yellow one NOT the orange one  and it is pretty much a Boss Tubescreamer.

I really wanted the EP Booster or SHO clone to work for this because of their simplicity but they just don't, the SD-1 with all knobs around noonish was kinda magical.

I'm not opposed to attempting to squeeze the op-amp into a double pedal but now I'm really curious about your partial BMP circuit idea.
There's so many different BMP schematics out there but I guess something like the blue part of this would do, eh?

[anotherjim]

Yes, the blue bit. I'm not an expert on BMP variants, but maybe one that's known for the smoothest sustain? If it went before the Plexi, then probably the input stage & sustain pot should be there too, and all in a bypass. Putting the clippers after the Plexi Q1 might mean no need for the BMP input.

[Ben Lyman]

I haven't gotten to the BMP idea yet. I just figured I'd try the simplest booster I know, I think it is the first thing I ever learned from the Small Bear website tutorials. Actually, this is totally going from memory so it may be completely wrong but it sounds great.

The first part is on the breadboard (between S5a and S5b) and the second part represents the pedal as it is already built.
So S5a/b would be a 3PDT with it's own LED indicator for "BOOST"
Then I guess, another 3PDT w/LED would bypass the whole pedal, like a Fulldrive... or I suppose it could be separate from the boost section to allow separate use of each.
Undecided on that and not too concerned about it yet.

I don't know how to incorporate this boost section onto the whole pedal, would it be as simple as the way I have drawn it?
I also don't know if that boost transistor is all good, specifically the base bias? If it sounds good, it is good?  or no..?

[robthequiet]

If it pumps up the pedal from sparkles to grind, or from grind to wail, then yes. No pops or sputters?

[anotherjim]

Without a bypass cap on R22, I'm not sure if Q4 is a boost?
Whatever, if Q4 collector is somewhere between 3 & 6 volts the bias is about right.

Incidentally, I was [cough!] playing with a stage on the breadboard yesterday, with a Jfet stage in front just see how it coped with that.

Copied this from the input cap as far as the 1k/220n before the tone - gotta have that RC network. It sounds pretty good just with that.

[Ben Lyman]

Rob- seems good so far but I have only given it a short test run at low volume. Hopefully i'll get a chance to crank it up later.

Jim- I tried it with a bypass cap (and a cap+diodes collector to base) it didn't seem necessary to me because the extra distortion was sort of "over-ridden" by the Plexi distortion.

Is it a boost...  Is it not a boost... hmm...
I guess that all depends on what the definition of is is... 

So it's on your breadboard too? So many questions! Do you have the original or mine with extras?
My observations on my own:
D2, D3 +Red LED extra switch not necessary, forget that stuff, especially with a boost up front.
C5, C14 extra distortion caps, super cool, can't live without them! (but on a switch for sure)

My boost: it's a clean boost and it really slams the front end hard enough to do what it needs to do. It can also be used as a "choke" to cut guitar input level for a clean-up.

I need to try your 1/2 TS op-amp idea, that looks easy. Could maybe even use a LM741 or similar?

[anotherjim]

No I haven't done the whole thing, just want to see how the TS stage acts after one of the FET stages with some gain of its own. I just have a 100k pot after the TS to pad down the output level at the moment, because the level out is very hot.

I used the first "normal" op-amp that came to hand which is a 1458, which is only (I think) a dual 741. Of course, the classic TS chip is the 4558, which is an improved version of the 741 type. I'd probably spec a dual chip even if I was only using one half. The pin layout of the duals is easier to work with.

I am truly puzzled if that input buffer of yours can be any more than a volume control. It has similar input impedance to the Plexi. That doesn't make it useless or irrelevant since, as you say, it can preset the drive level into the Plexi.

[anotherjim]

A little more time with it. I think the TS standard might cut bass too much, so I changed the  input cap - shown as .05 & I used 47n. Started 100n which was better. 220n better still. Maybe due to the little practice amp on my test bench. I also had a 1M gain pot. The Fet stage in front, I tried with & without the 47u bypass cap and boost on or off doesn't cause the TS to "splatt" out.
Next, try the TS in front of the Jfet.

[Ben Lyman]

Is my input booster a buffer? It is a 2N5088 and the out cap comes off the collector so it creates a huge volume boost. This volume boost is clean but it's enough to drive the Plexi wild.

I wonder if using a TS boost for the input would allow using the second half of the dual op-amp as an output boost? So:
into 4558(a) with boost switch, then into plexi, then into 4558(b) with switch for clean boost 

[induction]

Is my input booster a buffer? It is a 2N5088 and the out cap comes off the collector so it creates a huge volume boost. This volume boost is clean but it's enough to drive the Plexi wild.

It's a common emitter amplifier with a gain just below unity and inverted phase. It doesn't have the impedance characteristics of a buffer, but it shouldn't act as a signal boost with the component values you've given.

Are you sure that the emitter resistor and the collector resistor are both 10k?

Have you tried A/B'ing the output of the input stage against the bypass signal?

[Ben Lyman]

Ha! You're right, the emitter resistor is 1K, a lucky accident on my part. Funny how that worked out
Here's what the schematic should have looked like:

I still haven't had the chance to give it a proper test but here's a vid at low volume

[anotherjim]

That is a boost.

[Ben N]

Another 1 tranny solution that would probably sound good, because it has some modest roll-off of the top and bottom built in and was pretty much designed for this, is "Gus' Overdrive".

[Ben Lyman]

Ya, it's boosting alright. I was playing around with it and thinking I was on to something but unfortunately I stopped to get a quick reality check, plugged my SD-1 into the Plexi... dangnabbit, that's the magical sound! If I can get a chance I am going to bread 1/2 a TS and see what it's like.

Ben N- thanks for the Gus link, looks nice and simple, some high/low cut might be just what it needs. I'll give it a shot

[Frank_NH]

I've been following this thread with some interest, as I've found that a Klon(e) set to boost (i.e. tone at 7, gain at 3, volume at 5, 0 - 10 scale) into a tube screamer sounds really good!  So maybe designing an "integrated" two stage overdrive to get that magical sound makes some sense.