***FET+BJT Booster***

[mac]

Someone pointed to this link sometime ago, RG maybe

http://gabevee.tripod.com/sstubepre.html

I decided to work around the idea. I started with a clean booster... YAB...



Depending on the caps you may have a treble or full boost. I added the 101 cap (100pf) as in the fetzer valve.
Note that if Vsource < Vdiode+Vbe it will not bias. For example, if using a 1N4148, Vd+Vbe=1.2v so Vs should be a little above this value. I found that the BF245A I have are ideal with no diode or a Ge diode. A J201 or 2SK117 may not bias with a diode. Anyway, the diode add little to the sound. I just added it because... when I breadboarded the thing I had no wire to use between the S and the B 
I was about to record a sound clip but you may have it breadboarded in a couple of minutes while the tubes are warming up.

Hope you like it.

mac

[markm]

That's Cool!
Is it as "tube-like" as the article states?

[mac]

That's Cool!
Is it as "tube-like" as the article states?

Good question. Where the booster ends and the real tubes start? Compared with single fets or bjt clean treble or full boosters, imho this kicks both channels of my amp better. So the answer would be yes.
But you are the site official boosters tester, you tell me   

mac

[markm]

That's Cool!
Is it as "tube-like" as the article states?

Good question. Where the booster ends and the real tubes start? Compared with single fets or bjt clean treble or full boosters, imho this kicks both channels of my amp better. So the answer would be yes.
But you are the site official boosters tester, you tell me   

mac

Okay then, I'll build it!
The switch just removes the diode from the circuit?
I'd have to use different transistors though too.

[mac]

Sorry for the late response, I'll be away until next 20/9.

I guess the BJT could be any med-high gain, 2n5088/89, 2n3904, bc107/8/9c, etc. Maybe a med-gain low-leakage Ge ¿?
The fet is tricky. I have 2sk246 that bias at 2v wich may be used. My mpf102 bias at nearly 4.5v, too high to set Vc at 4.5v, unless you want the output at 6.5v - 7.0v, which btw I did not try. I have 2sk246 that bias at 2v wich may be used.

mac

[mac]

BTW, I'm testing a distortion with this, 3 stages separated with a 103 cap and a 470k:

input - 103 - circuit -103 - 1M-A gain pot - 103 - 470k - circuit - 103 - 470k - circuit - 103 - 470k - 100K-A volume pot- ouput.

circuit is the schem but with no input and output cap, no volume pot, and no 68k at the input.

mac

[mac]

v.2, untested.



The fet is now a "buffer" respect to the transistor's collector, not to vcc. Kind of darlington, but with a fet and a bjt.

mac

[mac]

This post gave another idea,

http://www.diystompboxes.com/smfforum/index.php?topic=59911

So this is v.3, a little brighter than the others,



mac

[mac]

Sound sample transfet booster v.3 at:
http://guitarfxs.tripod.com

Breadboard to Laney LCII, mic to flat preamp to computer. I wish I could have cranked the amp but the neighbours...
1. gain 2.5, master 2.5, fx bypassed
2. gain 2.5, master 2.5, fx engaged at unity gain
3. gain 3, master 2, fx at full gas

mac