Wampler - Plexi-Drive  [traced]

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Re: Wampler - Plexi-Drive

Postby Mbas974 » 07 Oct 2014, 20:17

It worked !
500K is too big value in my taste... I replaced the tone pot with one having 100K and it sound really good now.

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Re: Wampler - Plexi-Drive jfet biasing

Postby jimiguitar » 14 Dec 2014, 07:55

i had no trouble sorting jfets out of 90 i got 23 that that measure between 4.5. and 5.0and 17 that measure 4.11 and 4.5 in the plexidrive mostly also out of the 90 got 7 that bias in the pinnacle out of the same 90 plus more that i'am sure i will find more to fit in other pedals i measured each one and recorded it using plexidrive as reference logging what D S G resistors so i could kinda base by the circuits D S G resistors if my drain voltage will go up or down on the ones i got logged
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Re: Wampler - Plexi-Drive

Postby Ripdivot » 14 Dec 2014, 17:55

I wish I had your batch of jfets. I got 3 out of 50 when I built mine.
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Re: Wampler - Plexi-Drive

Postby jimiguitar » 14 Dec 2014, 18:53

i got them from small bear
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Re: Wampler - Plexi-Drive

Postby storyboardist » 16 Dec 2014, 16:40


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Re: Wampler - Plexi-Drive

Postby jt » 26 Aug 2016, 08:14

Tone pot in the schematic is wrong.It should be 25k linear.
In some gut shots you can see the letter B at the silkscreening.
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Re: Wampler - Plexi-Drive

Postby zedsnotdead » 13 Sep 2017, 20:27

Sorry for the necro guys, but I think I have some pertinent info on this circuit.

I love Brian's work and tried to replicate this pedal using a PCB from a well known UK seller and populating it with components according to the schematic that was posted here. It happens that it doesn't quite match the same tone I get from the original Plexidrive (I bought one). The "replica" was more mid-rangy and somewhat muffled, it didn't have the same clarity as the original.
Mind that I used J201's SMD - hoping to avoid counterfeit fets - and I biased the drain voltage of those 3 like the original.
So I was wondering what could cause such difference...
Opened the original and started to check the components.

So in short:
jt wrote:Tone pot in the schematic is wrong.It should be 25k linear.
In some gut shots you can see the letter B at the silkscreening.


This is true. It is B25K pot.

It also has a input resistor (1K) and cap (100n). The differences are in red. Check:
plexi-drive.PNG


After I done the caps replacement, it sounded identical. Not "nearly". I really mean 99.9% identical.

Now, what bugs me is that "bass" switch caps. It doesn't have almost any impact on the sound, it's 1n or 2n. It is very, very subtle if you compared with the 2n2/470p sound - which has a really noticeable difference in bass response. I wonder if this was done on purpose, or it was just a builder momentary distraction.... maybe indyguitarist could chime in?
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