Change input buffer on soundtank to jfet?

[Ben N]

I'm mapping out an Ibanez FZ-5, using Jeff Healey's schematic (http://members.aol.com/jeffhailey/fz5.htm), in order to do his Big Muff mods to it:  switching the tone stack caps, changing the .01uf coupling caps for .1uf, cutting out the 100k R to ground from the third stage.

One of the differences from the BMP is that this pedal uses FET switching, and has an input buffer.  The buffer is the same BJT used for the BMP gain stages and output buffer, namely a 2sc1815.  From the schematic, and based on Jack's Simple Buffers article,  it looks like a simple matter to switch that to a JFET (I have some 2N5457s and J201s) just by switching the gate resistor to 1M and the source to 3k3 or 4k7.  That ought to improve the pedal's bypass performance.

Two questions:  What effect will changing to a JFET buffer have on the pedal's sound?  Mainly I'm thinking about the switching FET right after the input buffer--will there be any problem due to the jfet's higher Zout?  And what about the .047uf input cap--any down-side to switching that to .1uf to reclaim a little more low end?

Thanks,
Ben

[TheBigMan]

That's a mod I'm all set to try on my Soundtank TS-5.  Can't really see any problems with it outside of what you mention: I've seen a couple of Boss pedals (like the DD-2) with a double input buffer, FET cascaded into Bipolar.  Presumably this is to avoid any impedance issues.

Watch the pinout of your FETs though.  I've got some 2SK30As that I'm going to use, they'll slot in without any bending of pins IIRC.

[Ben N]

Hey, BigMan!

That's reassuring--it seemed so obvioius to do, I was actually wondering why no one had mentioned it before.  Yeah, I do have to check the pinout.  I thought aboiut experimenting with some MPSA18s for the gain sections, but the pinout doesn't match, and the specs on the data sheet aren't all that different, at least to my non-technical eye, so I let that go.

I also gave some thought to re-housing in a Hammond box, using the Small Bear actuator, but I was surprised to find that the board will just barely fit in a 1590B, meaning that I would have to keep the board-mounted jacks.  So, again, WTH, I'll leave it be.  I've never noticed the soundtanks to have been hum magnets, so I guess the big steel plate on the bottom and the use of every spare mm2 on the PCB as a ground plane are effective enough.

I wouldn't normally be concerned about the impedance issue except for the switching FET--otherwise, the next component is the BMP boost stage, which by itself should be fine.

Ben

[TheBigMan]

You can just squeeze a TS-5 into a BB, but you need to chop off a fair bit of the circuit board at the switch end, most of which is one huge ground trace.  I'd guess that the Fuzz is the same.

[Jered]

Ben N. said,
   "I thought aboiut experimenting with some MPSA18s for the gain sections, but the pinout doesn't match, and the specs on the data sheet aren't all that different, at least to my non-technical eye, so I let that go."

 Hi Ben, if your talking about 2SC1815 compared to MPSA18's they are quite different in just about every category. Change one thing at a time then check to see if you like the sound, then move on to the next mod.
Good luck, Jered