BUILT IN FUZZ

[TIMBO]

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  

[sluckey]

I think you should power the MOSFET from a voltage source (like node D) rather than the plate of V1. And since you like the sound when powered by 9 volts, maybe build a simple 9 volt source. If this will be in a cathode biased amp, just use the output tube cathodes as a low voltage source. Connect a 100-470Ω resistor to the output tubes' cathodes and put a 100µF cap to ground on the other side of the resistor. This becomes the filtered, isolated voltage source for the MOSFET. If you want a regulated 9 volt source, just add a 9v zener across the 100µF. Doing this allows you to use your cheap small MOSFETs.

[PRR]

You only need 1V-2V to blast the heck out of the tube grid input. Using a 300V supply sounds like using dynamite to kill a gnat. Not just overkill, but dangerous to bystanders.

For one small point: the input protection diodes work if the supply is less than 20V. If Gate is allowed to swing 300V, the 20V breakdown is exceeded.

I'm with Steve. Find a source of 8V-20V. Assuming 47K Source to Ground, the current is extremely small (~~0.1mA) so no big headache.

I'm not sure how you arrived at 47K in bottom and 5K in top. Is that from R.G.'s pages?

[tubenit]

Can this be done and simply use a relay to cut the fuzz in and out?

Tubenit

[sluckey]

yes

[TIMBO]

OK Tubenit,i see i got you thinking  . Well i was doing some thinking myself  .It appears that these mosfets can be made to mimic tubes, but can tubes be made to mimic mosfets???? As i do not fully understand the workings of the mosfet or tubes completely i tend to find bits and pieces and try to work it out from the info i can find. This is what i found on the net and is what i was thinking  http://www.diystompboxes.com/smfforum/index.php?topic=92201.0

If the mosfet can increase the gain by a very small amount and if i am correct the SWING by varying the SOURCE can this be applied to a tube to get it to mimic mosfet  Thanks
 

[jojokeo]

I decided to check this SS board out for the first time as here is where I've been spending some of my time over the last several months in going back to my old effect pedal building roots. :)
Jfets can be used very similarly to tubes just not with the high voltage. The triode is replaced by a JFET with the following correspondence of terminals: plate to drain, grid to gate, and cathode to source. This is supported by the fact that both the vacuum triode and the JFET are voltage controlled devices which change their transconductance according to a (usually) negative control voltage. I've been having fun building, experimenting, and creating more pedals for my board to gig & practice with in keeping things fresh in another area.

[jojokeo]

Check this out... a high voltage LND150 JFET

For info how to use and a sound file of it driving an EL84:
http://www.hawestv.com/amp_projects/amp_solid_tube/fetpreamp_200v1.htm