End-All Red Internal Trim Pot Biasing?

Caldo71

Caldo71

Active member
Hey @PedalPCB as an early purchaser of the first batch of these PCBs, I’ll likely be starting my build before you get done with the official build docs. I already bought the components using the values on the PCB itself.

That being the case, I was wondering if you or some other forum wizard could jot down here the method for using the internal trim pots to bias the jfets with a multimeter? I’m a newb, but based on my experience with the Son of Ben, I’m assuming that’s what those are for?

Give it to me step by step like you were talking to an encephalitic meat chicken (with reading skills).
 
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I'm not sure if it's a RG100 preamp in a pedal form or not, but..
Probably there's a onboard charge pump, so you can expect 18V or maybe 24V like the original RG100 preamp.
You have to set voltage only on first two transistors (gain stages). The other two - nothing to adjust. Looks like there are two testing points, right above the trimmers. Connect dmm probe (black) to GND, red probe to the testing point. For starters you can set 1/2 of the supply voltage (output voltage of the charge pump, not the DC jack voltage). You can also check voltages on the RG100 schematic but those might not apply here (different supply voltage)
 
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I can't help until I see a schematic of the actual PedalPCB board. I looked at the Randall sch, but we have no way of knowing what's different between that and the the End-All.
 
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Hey dudes, circling back to this topic…my build has been done for a coupla weeks now and it went pretty flawlessly. SO…

@temol you were absolutely right about those test points, and I was indeed able to get readings that changed with the trimmer twisting, by using the probes as-recommended. But as @Chuck D. Bones pointed out, the numbers don’t mean much if one doesn’t know what voltages you’re SUPPOSED to be shooting for.

Anyhow I adjusted em by ear and I’d say it sounds 97% as good as my storebought RRED, but it still seems just a hair more “farty”. Lacking in string-to-string clarity in chords.

So @PedalPCB I still would really love to know (still no build docs) what values to shoot for on those trimmers. Not sure if I should be listening for roughly equal gain on both, or if the method is to starve one and oversaturate the other, or WHAT!
 
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If you have the original pedal you can measure the voltages. Would be nice to compare them with the Randall service schematic. But you also have to measure the Vcc voltage. I mean voltage after the charge pump.
 
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Tpruitt said:
Any updates on voltages?
My pots range is 15 to 20.85 volts.
My pedal sounds best with one pot at 20v and the second at 16v.
Click to expand...
The service manual of the amplifier dictates 15v on the drain of the first jfet and 11.5V on the drain of the second one. I don't know what exactly is the ouput of the charge pump, but you should go for roughly 60% and 50% of VEE.
 
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Tpruitt said:
Any updates on voltages?
My pots range is 15 to 20.85 volts.
My pedal sounds best with one pot at 20v and the second at 16v.
Click to expand...
Sorry for the super late reply on this…my life has been nutso.

@lrgaraujo and @Tpruitt this is from a separate offline convo about the issue between @PedalPCB and myself…

“
The original RRED doesn't have trim pots for setting the bias, it uses fixed 33K resistors and the 2N5484 transistors are hand selected.

The drains are biased at 12.84V on Q1, and 9.94V on Q2.

You can either bias to those voltages, or you can set the trimmers to 33K (or replace them with 33K resistors) and find transistors with similar specs to the original. I used the trim pots for the prototype and it was indistinguishable from the original.

I posted the specs for all four transistors in the original Wish List thread when tracing the pedal (although Q3 and Q4 aren't critical since they are source followers)”.

FTR I have not yet delved into any if that new info myself but hope it helps.
 
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