Framus Cobra Fet version, more gain possible?

[JebemMajke]

Here is the schematic I've used www.framus.de/media/manuals/Amps/Cobra/Cobra_V1-2.pdf , Ive emulated Lead channel, combination of BF245B's and BF245C's was used, that tonestack is really something, it made me actually use those mids xD. Really nice and crunchy, great lead tones and hard palm muting, I got some megadeth/pantera/lamb god/tryptikon/norvegian black metal tones without even trying. I like it, but since this was made for a friend i think he would like more gain and my question is if I would replace that 250k gain pot with 1M gain pot would I squeeze more gain out of it. Also for now Im running it with 12 v, more v's = more gain? Maybe other FET's?

[Richard6]

Did you replace the 100k resistors from v+ to drain with 100k trimmers and bias to 1/2 v+? I don't know much about the Jfets you are using but I know that J201s have the highest gain that I've seen. Replacing the 250k will give more gain, although I'm not sure how much. If you increase v+, about the only thing you get is more clean headroom.

[JebemMajke]

Trimer's took place of those 100k resistors. J201s' are stupidly pricey in my country ( 1.5 euros per fet ), Bf 245C's have little less gain than J201's and BF245b's is almost the same as 2N5484's, and both cost around 10-15 euro cents. I lltry 500K and 1M pots. And maybe reduce voltage to 9 v ( with re-bias ofc ).

[Richard6]

Sounds  like you are on the right track. I would also take a look at the inter-stage divider networks. Try upping those 220k resistors to 470k or more. Sometimes you can take them completely out.

[JebemMajke]

You mean that 220k that stands between V2b and V3a? How about another follower stage?

[defaced]

Drop the source/cathode resistor in the third stage.  The stock value of 39k is derived from the Soldano SLO, I've played with values as low as 2k2.  This will get you more saturation, but also at the extremely low values you will get less definition/more of a fuzz sound/feel.  Partial bypass of that resistor will get you more highs if you feel you need it. 

[JebemMajke]

Cool advice, thnx. I have another problem, it hasn't enough volume. My fet version goes with that 1 gain stage after the tonestack ( V4a ), but I bypassed it and i have the same volume with and without it? Bad wiring?

[euronymous0001]

V4a is a buffer, so it has unity gain 

[euronymous0001]

btw, did you include v4b

[JebemMajke]

No, wasn't sure about that. I thought to try and give it few different buffers and to keep the one i like the most. U think It would fix volume problem if both V4a and V4b were included? Should I than include that 500ka deep pot?

[euronymous0001]

including V4b would increase your output volume. although i do not know how much the increase would be.

[JebemMajke]

Included V4b and placed trimpot in place of that 39k ohm resistor, after some tweaking I ve got the exact saturation i ve heard on youtube while checking this amp out. Now i have slight noise problem. Tonestack has been already scaled down and that 100uF cap is there as well. Any other advice?