Fulltone Plimsoul

PedalPCB used to sell one, but apparently dropped it. There were two versions of the Plimsoul. Version 1 matches the patent. Version 2 has less low freq gain. I have not built mine yet, but I did run simulations on both versions. I think some of the resistor values on the schematics on the 'net are screwy. Either that or the patent contains some misinformation because there is no way that a Plimsoul built per the schematics will perform as described in the patent. I have not built the board I bought here, going to breadboard it first and figure out how to make it work right. Mark Fuller had a good idea about simulating power supply sag in tube amps, but I have doubts that his Plimsoul circuit accomplishes that.

In the meantime, stack a Tube Screamer and a Rat and you're most of the way there.
 
Chuck D. Bones said:
There were two versions of the Plimsoul.
Click to expand...

The Plimsoul is possibly my favorite distortion ever. I've owned three different revisions.

The first one (my favorite) is closest to the patent, but every single build oscillates. Three different PCB layouts (two not my own) and two different vero layouts (neither my own)... This one still haunts me to this day. This one used two 1N916A and one 4.7V zener diode for clipping, and an A100K Drive pot. I've traced this circuit from three different units, and compared against another persons trace... The schematic is correct, but the damned thing just won't stop oscillating. Is it the component placement? Probably... But five completely different layouts all behave exactly the same way.

The second one was similar to the first, but only had a single RC filter on the feedback loop. This one works great, and was the inspiration for the SoulDrive PCB. This one used three 1N916A clipping diodes, and a B1M Drive pot.

The third one also had a single RC filter, but uses 5 1N916A diodes for clipping and a B500K Drive pot.

Each one had scattered component value variances... 1K / 10K series input resistor, 470R / 1K filter in RC network between gain stages, cap (or no cap) across clipping diodes to ground, etc...



Random note, I was always intrigued by the similarities between the Plimsoul and the Peavey Tube Sound Distortion.

Peavey Tube-Sound Distortion.gif
 
Can you confirm the operation of the STAGE 2 control? It looks like the clipping LEDs don't really kick in until you get past 9. Unless the two gangs are not both 100K. I can see how the feedback thru the LEDs to Vref might cause oscillations because it's a positive feedback path. The oscillations are low freq, right?
 
Chuck D. Bones said:
Can you confirm the operation of the STAGE 2 control? It looks like the clipping LEDs don't really kick in until you get past 9. Unless the two gangs are not both 100K. I can see how the feedback thru the LEDs to Vref might cause oscillations because it's a positive feedback path. The oscillations are low freq, right?
Click to expand...

I breadboarded this once and experimented with separate pots for the dual gang stage 2. IIRC, the pot in series with the leds didn’t really bring in the led clipping until pretty far into the travel (maybe 9 like you said, somewhere around there). I thought if I were to build it I’d have to separate the pots and use a different value for that, or just use a switch, but I haven’t circled back to it yet. Again, IIRC. It was a while ago.
 
Chuck D. Bones said:
Can you confirm the operation of the STAGE 2 control?
Click to expand...

Both gangs of the dual pot are the same value and taper.

I'll have to find my original, it's still packed up from the move. I want to say the oscillation is in the 800Hz - 1kHz range, just going by memory.

I've eliminated the Stage 2 clipping path as the source of the oscillation in the past though. It's happening in the first gain stage, and altering (or removing) one of the RC filter pairs stops the oscillation.

I'm fairly certain it's a component placement / routing issue... It's been a while since I fooled with it, but:
1) Original "v1" PCB does not oscillate
2) Original "v2" PCB does not oscillate when modified to "v1' specs
3) Clone "v2" PCB does not oscillate when built to "v2" specs (one pole filter)
4) Clone "v2" PCB oscillates when built to "v1" specs (two pole filter)


I've gone so far as to pull components from the original board and install them in the clone board (yes, this has been an ongoing struggle for me, for longer than I'd like to admit)
 
ssefton said:
People verified the layout at this site: http://tagboardeffects.blogspot.com/2012/06/fulltone-plimsoul.html

Thoughts?
Click to expand...

I've built them all over the years..... it oscillates. The v1 always oscillates unless you modify the circuit. Every year or so this one catches my eye again and I struggle with it for a little bit longer, and I always end up running away defeated. ?

I'm really surprised that it has been discontinued, this was (in my opinion) one of Fulltone's best designs.
 
Robert said:
I contacted Mike Fuller about the possibility of licensing the design. He was surprisingly cool about it, but the Plimsoul actually isn't dead. There's a new version coming.
Click to expand...
That is super cool (about both the licensing and the new version).
 
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