Hi Guys, Can't help myself with this stuff and wished i knew more about it. I was directed to a circuit (thanks T) that i was impressed with to get me closer to "THE SOUND" i was looking for, OVERDRIVE but not over driven and FUZZED but not funky.
I was looking at the TOS FATT feature hoping to do something with the spare triode and at the moment i am thinking of adding a CLIPPING circuit (similar to the silver jubilee) to it and then tweaking the diodes to achieve the desired effect. (still would like to try it)
I constructed the circuit "A" and liked it a lot and was able to dial in a good match for what i was looking for
This then gave me a bit of an insite to the MOSFETS themselves and the different types.
I found this http://www.geofex.com/
( Look down the page to MOSFET BOOSTERS) This gave me a heap of info that made my head spin but some made sense
The data on both BS170 and 2N7000 were similar just were pinned in reverse and are N-Channel - Enhancement
I replaced the 2N7000 with the IRF820 and it sounded no different to the smaller mosfets, this was great.
I tried it (as a pedal)with my TOS and VJ HOTROD, i was able to really get a great smooth overdrive and even when the OD was on it did not over distort it in any way and i could say that it even added a bit more SMOOTHNESS as well.
I know that some of you guys cringe at the thought of adding more SS stuff to tube amps but what the heck i'll put it out there.
In the "B" circuit i have just added it to a normal input circuit and tied the DRAIN resistor to V1a plate ( as we know that the IRF820 will handle 500v).
Reading the article the idea is to maintain the gate threshold ( i think) and if this is done by the correct biasing (the two 10M resistors ) the amount of voltage applied to the DRAIN can be what ever the mosfet max. voltage is (i think).
I ran both the 2N700 (max 60v) and IRF820 (max 500v) with 9V dc and no noticeable different in sound was produced.
Liking this circuit for that added smoothness (and maybe deleting the volume pot ) and using the IRF820 could this circuit be done the way i have shown, is it a case of the same circuit just run at a higher voltage. Thanks