GGG FY-2 Si bias info

[jpiddy118]

Hey everyone, been a while, but i'm back at it in the workshop again...

Loaded up an FY-2 using the GGG pcb and some 3904s I had around... using resistor values from GGG schematic, i got a wicked fuzz that might work well for single-note lines, but not quite what i was expecting based on some youtube vids, etc. My original bias readings (with R2 22k, R4 47k) were very low... Q1C~3.9, Q2C~0.3. These do somewhat correspond with Mark Hammer's readings from an original unit, as i found by searching this forum. To get the unit near GGG's suggested voltages, I had to drop R2 down to 8.2k, and R4 down to 22k. Right now, i'm getting Q1C~5.9, Q2C~0.9. These are as close to GGG as i've gotten so far, and it sounds a bit better than the original values, but i'm still planning to experiment further.  Will try 6.8k in R2 later tonight.

Any tips/suggestions/advice? This is my reference point for the sound i'd like to attain: http://www.youtube.com/watch?v=bqxDEV6edRE

[LucifersTrip]

My original bias readings (with R2 22k, R4 47k) were very low... Q1C~3.9, Q2C~0.3. These do somewhat correspond with Mark Hammer's readings from an original unit

can you post or link to them...

as i found by searching this forum. To get the unit near GGG's suggested voltages

can you post or link to them...

I had to drop R2 down to 8.2k, and R4 down to 22k.

can you post or link to the schematic

Right now, i'm getting Q1C~5.9, Q2C~0.9.

what are the full voltage readings for all lugs?

These are as close to GGG as i've gotten so far, and it sounds a bit better than the original values, but i'm still planning to experiment further.  Will try 6.8k in R2 later tonight.

why not try subbing a pot so you don't have to do it in steps...

Any tips/suggestions/advice?

first thing I'd do is check for mis-wiring/soldering errors/wrong resistor values...but definitely try different hfe's in Q2 to see how it effects the voltage.
generally, the idea is to get as close to "correct" voltages without altering the surrounding resistors...then tweak.  that usually gets you closest to the original sound.

good luck

[jpiddy118]

Mark Hammer's reading from an original unit:

"I have an original. With a fresh battery that gives a V+ of 9.78VDC, I measured the following:

Q1: E - 0v, B - .59v, C - 3.40v

Q2: E - 0v, B - .55v, C - .9v"

GGG's build instructions readings:
Q1 Collector 6.6v
Base 0.6v
Emitter 0v
Q2 Collector 2.0v
Base 0.6v
Emitter 0v
Q3 Collector 6.6v
Base 0.7v
Emitter 0.1v

Schematic: http://generalguitargadgets.com/pdf/ggg_secf_sc_bst.pdf?phpMyAdmin=78482479fd7e7fc3768044a841b3e85a

I can't take full readings right now but I will ASAP. My unit sounded best when i got Q1 C up around 7v. Q2 C wouldn't go higher than 1v no matter what i tried. Base & Emitter voltages were ballpark with GGG from memory, nothing that raised an eyebrow.

[PRR]

> My original bias readings (with R2 22k, R4 47k) were very low... Q1C~3.9, Q2C~0.3.

That's improbable unless there is a mistake. Q2C has to be above ~~0.5V to flow current through R3 to Q2B which needs >0.5V to get happy.

For Q2 hFE of 200, Q2C ought to be 1V; perhaps 2V if Q2 hFE is under 100 (as the build-sheet suggests).

Change R3, not R4. Try 2Meg, try 5Meg.

[jpiddy118]

Finally got a chance to sit down with it again this morning... just for sh*ts & giggles, i reverted back to the original R2 & R4 values, and popped a lower gain 3904 in Q1 (was 250, now 200)... my voltage readings are near identical with MH's original unit readings (Q1 B= .61v, C=3.55v, Q2 B= .55, C= .9v), and most importantly, it sounds good. Can't wait to bring it to rehearsal this week and compare with last week's adjusted bias settings (Q1 C was up to 7v, and it sounded great, but maybe not vintage-accurate). I'm content that my clone is close to original... it sounds good thru my home practice amp, hope it sings thru my GT head as good if not better than previously.

I'm thinking there must've been a mistake with those GGG numbers... not sure how JD got them with the given resistor values and transistor hFEs.