Flabulanche - bias voltage values & JFET question

[88126732014]

I recently assembled a Flabulanche. Here is an image of the guts(component side):



Now I will first say that I assemble this from parts I had laying around, so I ended up substituting a few things.  I'm curious to know if my substitution of certain parts is what is causing my issue.

Instead of 1N5817 diodes I used 1N5818 diodes, but I wouldn't think that would matter so much.  Instead of the 2N5457 I used J201.  My supply voltage after the voltage doubler is about 17.98V, but even with my 10K trimmers dialed all the way down, I get about 12.7V at T1, about 13.5V at T2, and about 14.4V at T3.  I am also getting a gate type of effect with both HI and LOW compression settings.  Basically if I play lightly the sound won't come through, but if I dig in on the guitar it will come through.  My guess is these problems arise from using J201's instead of the 2N5457 and not working with the bias circuit, but I would appreciate someone smarter than me to tell me that is the case.

Thanks for any insight.

[midwayfair]

J201 is very, very different from 2N5457 and inappropriate for this effect as designed. This goes beyond the biasing requirements.

J201 have much, much more gain, at least twice as much and sometimes three times as much as a 2N5457 (~6dB for the 2N5457 vs. 13dB for the J201). They also distort at a proportionately lower input signal size -- a J201 will distort if the input signal is larger than about .7V (regardless of how much gain you're getting out of it) whereas a 2N5457 will accept closer to 1.5V before distorting. Using three of J201 in a row will create way more distortion than the pedal is meant to create. And while you can make adjustments in the circuit to limit the signal so that they don't distort as much, either by increasing the resistance between each stage and/or deliberately misbiasing, you're still losing a huge amount of dynamic range in each stage, which was kind of the whole point of the design. And the reason the trimpots won't get you to the right bias point is that J201 require much larger source and drain resistances than the 2n5457s (see the Fetzer Valve article on Runoffgroove for the right values).

Normally I'd be able to suggest a raft of substitute transistors, but FETs are much harder to substitute. There are a handful of FETs that will work okay without big changes:

2N5952
2N5485
2SK30

None of these are ideal substitutes (no such thing among FETs when used for distortion pedals), but they are much closer than the J201 by a wide margin.

If you absolutely cannot get 2N5457s, give me your address and I can send you some.

You'd have to desolder something to get the pedal working properly - either the trimpots or the transistors -- so you might as well get the right transistors.

EDIT: There may be something else wrong, though. Try lifting the diode from the envelope section and seeing if the pedal works (just really distorted) without the compressor in.

[88126732014]

Thanks for the fast reply.  I will first hunt down some 5457's and replace the 201's and see how the pedal reacts.  If the strange gating issue still occurs afterwards, I will cross that bridge then.

Thanks again.

[88126732014]

I finally recieved the 5457's from Mammoth, and i have them installed.  The biasing is on par now, and the pedal functions.  Is there any harm in tweaking the trimpots to taste? or are there certain voltages (too high or too low) that would shorten the lifespan of any components.  The only reason i ask is because I like the way the pedal sounds with the T1 at around 8V.

In any case, the pedal functions fine and definately has a cool sound. I've been on kind of an overdrive kick lately.

Thanks again.

[midwayfair]

Is there any harm in tweaking the trimpots to taste? or are there certain voltages (too high or too low) that would shorten the lifespan of any components.  The only reason i ask is because I like the way the pedal sounds with the T1 at around 8V.

You will not harm any components regardless of where you set the trimpots (but the extremes will not make any sound). Set them where it sounds best to you!

[HKimball]

Somewhat related to this, I have a question - in the build doc it mentions using a j201 in q2 for more gain, if desired. If using a j201, would you use a larger trim pot? Say 50k? I imagine you'd also use (ideally) a 1.8k source resistor? And if so, would the bias point remain the same for q3?

I also have a question about incorporating a fender-style NFB presence control in the 'power tube' (mosfet) stage but I fear that's a little too far off topic for this thread.

[midwayfair]

Somewhat related to this, I have a question - in the build doc it mentions using a j201 in q2 for more gain, if desired. If using a j201, would you use a larger trim pot? Say 50k? I imagine you'd also use (ideally) a 1.8k source resistor? And if so, would the bias point remain the same for q3?

Yes, see the FETzer valve article.

FRT, this thing is insanely gainy already. I suggest building it stock before you decide to change it.

[HKimball]

Awesome - thank you for the response and sorry for derailing the thread.

I planned on using an mpf102 in q1 to reduce the gain fwiw. But I will for sure socket mine and build it stock first.