Wampler Plexi-Drive on the Breadboard

[Ben Lyman]

Why no input cap on this? It works and sounds great but I'm reluctant to build it without an input cap. Is there any instance at all where it might be better to have a cap there? Like if certain pedals are before it or something.
Here's the schematic I used:
http://1.bp.blogspot.com/-rAWTuUNK1aM/TxDYkVflb9I/AAAAAAAAAg4/V81eRfQXHMY/s1600/plexi-drive.PNG

[PRR]

> Why no input cap on this?

The input Gate biases at Zero DC.

All your sources output Zero DC.

So no problem.

(If you trust all your pedals, and any friend's gear you might trial, and assume good gear never goes bad.)

Tube amps are similar and "usually" don't use a cap. However I am seeing more newer tube amps which do. Maybe newbie designers. Just as likely, support problems when users plug DC-leaky gear in front.

Tubes are somewhat tough. They will bias-off but rarely melt.

_I_ would not expose a JFET Gate to the big bad world without some series resistance (33K). They are tough, but 11mA can fry them. 33K protects against anything you hope to find on stage.

Having done that, I might use a cap too.

[mcknib]

Not really sure perhaps something to do with biasing the FET?

Anyway here's a couple of vero versions one with input cap

http://tagboardeffects.blogspot.co.uk/2012/02/wampler-plexi-drive.html

[Ben Lyman]

ok thanks. I'm trying to imagine how that goes, maybe a 2M2 pull down resistor and a big 4.7uF cap..
Maybe something like this? R3 is in question.. does it bias the gate or was it meant as a pull down?


Or maybe this.. again with R3.. hmm.. remove it? or move it onto the gate?

oops, just noticed it automatically changed to R6 in this pic.
anyway, i've been moving things around and trying a lot of different ways.
The 1M doesn't seem to do anything and the 47pF seems to have the same affect no matter where I put it. I'll draw up a new schematic tomorrow and see what you all think.
Thanks!

[induction]

According to Brian Wampler himself in a post on (ahem) some other forum, without an input cap it has a tendency to pop when switched:

Well, the schem posted is from an older version.
The popping is most likely from the jfet at the input. stick a cap in front of it (.022 should do) making sure you have a 2.2M before the cap and a 1M after the cap (closest to jfet). This should help quite a bit.

Take out the 33k in your partial schematics, and you've got the newer version of the commercial pedal. Make sure you sort through your J201's to find ones that bias to 4.5-ish V with the stock biasing resistors.

If you find it to be a too dark, increase the tone pot. The original is 25K A. I went up to 500k A, but I tend to ride the lower pot settings. You might find 50k or 100k to be good, or just leave it alone if you like it as is.

[Ben Lyman]

Thanks induction, that's great info. So, I guess the 1M on the gate is important then, eh? I will go back and include it.
As far as last night's fiddling, it ended up like this:

switches 1 & 3 are extra distortion boosters for a little closer to JCM800 sounds, I like it.

I'm surprised BW would say "0.022uF should do" because I noticed the tone of the whole circuit changed when I tried a 1uF, so I went up to 4.7uF (C6) and got it to sound the same with or without.

Edit: gonna go throw in the 1M about like this I guess, maybe try a smaller C6. Any ideas for that (C3) 47pF? It's gotta go somewhere, right?

[Ben Lyman]

New scheme, moved the diodes for a less compressed, crunchier sound, added another 47uF for more distortion.
I think the input plan is working the way it should, it all sounds really good.
Handles a TS like a champ, classic tones abound.

[duck_arse]

in your this version:


Q1 has no gate bias. the 1M needs to go to ground with a DC path through to the gate (no blocking by caps between). and, if you move the 47pf to parallel w/ the 1M, and after the 33k, it forms a low pass filter somewhere around 100kHz+. it should keep some radio signals down.

[Ben Lyman]

Thanks Duck, I was having some difficulty understanding the whole JFET gate and biasing thing but I think I got it now.

The 47pF after the 33k... the thought had crossed my mind but I was afraid to do anything that might change the sound. I will try moving that little cap around some more and see what happens.

In the original pedal, the 47pF is on the input.. is that maybe to react with guitars and/or cables in a way that an old amp might react?

[anotherjim]

If you think of the arrow in the gate of the FET symbol as a diode (it is), to make the FET work for us that diode has to be switched off or reverse biased, so we need a resistor on the gate to a voltage at least the same as the source voltage and preferably lower. Unless we have negative power rail, that has to be 0v. R7 is the source resistor & will drop some voltage keeping the FET channel a little bit more positive than the gate - and the gate diode reverse biased.

[Ben Lyman]

Thanks Jim, it is beginning to make sense to me.
Here's a new scheme, barely any different but it's going on perf now, then into a repurposed 1590BB, all I have to do is drill for the toggles and it'll be ready for assembly.

[Kipper4]

I guess the 15k Rdrain biasing resistors worked out then?
How's the voltages?
Can't wait to hear it Ben.
Rich

[anotherjim]

Got me thinking : if there was a TS clipper stage after Q1 (yes, an op-amp damn it!), then a final op-amp to buffer the output...

[Ben Lyman]

Rich, ya I got lucky on the bias. I got all these J201's from SBE and they came in at +4.1 +4.2 & +4.9 volts. I tried some of the others, which biased at either below 4v or above 5v and i couldn't tell any difference, pedal worked fine and sounded fine to me.

Jim, I was thinking something in front like a mosfet booster SHO clone thing, on a second footswitch of course. i'm sure you have a better idea about how to add a gain/volume/solo booster though

[Kipper4]

A parallel resistor with R7 (latest schematic) on a switch would be a simple boost solution Ben.

[Ben Lyman]

Ya Rich, I was playing around with those resistors and I did notice some change in distortion and volume. To keep it simple I went back to 1K on all.
For a boost I'm thinking more along the lines of that sound you get when you do this:
a) guitar into tubescreamer with all knobs on 10
b) Full Marshall stack with all knobs on 11 

[Ben Lyman]

Got it into a recycled 1590BB very easily.
Four switches and 3 knobs is against everything I believe in 
Next time I would probably just put both gain boosters on a 2PDT.
induction is right about the tone knob, it could use a little more range but it's ok for me.
Another long boring demo:

[anotherjim]

There's "boost" and there's "escape velocity". A single "SHO" stage in front would give the former. For the latter I'd be tempted to reach for something with, potentially, a lot of sustain type gain in front - which is basically a TS, but hopefully you only need the soft clipper part. Maybe a simple low pass would be useful, but maybe that could have a fixed tuning. It would, I suppose, just go in front with it's own true bypass.

[Ben Lyman]

Whoa cool! "Escape Velocity!" that's the coolest thing i've ever heard of... but what is it, you lost me    Is that where you put something in between Q1 and Q2?

I just had a thought for the input booster "pre-stage" idea. The Xotic EP Booster. The whole set up might come close to some classic rock rigs.

[Ben Lyman]

I breadboarded an EP Booster:
http://2.bp.blogspot.com/-7mInY_-yDBA/Txl0I0iyF6I/AAAAAAAAAo4/afNMQkXQloE/s1600/xotic_ep_booster.png

ran it into the PlexiDrive and it sounded great. I'd like to know how to integrate this without the output buffer if possible. I tried taking the output off R7 but engaging the switch caused a dip and gradual rise instead of instant boost. Should it have worked? Did i possibly do something wrong? Or is there a better way to put this in front of the PlexiDrive?