DEMO ‘JMP Super FET’ - Mofeta (EAE Model feT) w/ Super Lead / Plexi inspired mods

[owlexifry]

Build Rating

4.00 star(s)

initially inspired by the mods described in this thread, i wanted to explore some plexi / super lead inspired values/mods with the Mofeta board.

so i started off by breadboarding the Mofeta as per the schematic in the build doc. sounds pretty good.
then I tried the mods seen in the annotated schematic below, and made some mods/omissions to suit my tastes, as described in this breadboarding thread. another mod was inspired from one of chuck bones' posts. (I didn't notice much difference between 3.3K or 10K for R10, so I went with 10K)

at the right gain / drive levels i reckon this gets pretty close to super lead / plexi inspired tones, but holy crap this has way way more overdrive than that.
more like hot rod plexi / SLO levels of saturation. (but this also depends on how you dial in the trimpots on the 5457s)



ended up putting the bright cap for the bright channel gain pot on a switch, because:
- at low gain levels this bright cap is awful.
- at higher gain levels this bright cap is awesome.



final values:


in this demo ive got it running on 12V supply. (sounds better than 9V)
featuring boosting from the frequently underrated and forgotten Frost Drive (VFE IceCream).
this might be one of the best sounding JFET distortions/preamps I've tried/built, EAE have done a ripper design here.

 

[lowpitch]

Nice work and lots of possibilities for mods for sure. Since it follows such a basic tube amp topology, you could probably look at just about any early Marshall or Marshall adjacent tube amp for tone filtering inspiration and build it to suit your tastes - like you did
What did you end up biasing your JFETs to?

 

[owlexifry]

Nice work and lots of possibilities for mods for sure. Since it follows such a basic tube amp topology, you could probably look at just about any early Marshall or Marshall adjacent tube amp for tone filtering inspiration and build it to suit

yep, that’s right. exactly why i did it this way. already got a lot of options for doom tones, and channel II is still pretty fat, so I don’t really feel like i’ve missed much by deviating into marshall specs.

What did you end up biasing your JFETs to?

FET specs: (MMBF5457)

supply @ 8.98V
voltage from chargepump: 16.73V

	Vp / Vgs	Idss	Drain (V)
Q1	1.453	2.82	7.82
Q2	1.458	2.75	5.64
Q3	1.460	2.82	6.83
Q4	1.484	2.82	16.73

these idle drain voltages dialed in for Q1-Q3 were all done by ear (one channel at a time, gain pot set at about noon-ish)
IMO, the halfway/‘noon’ setting on drain trimpots is almost always too sterile and thin.
i found dialing towards 10’oclock gives a thicker tone, more output, and more drive.

i guess it’s about the finding the balance of a thick crisp tone with the most crunch/drive you can get (but also so it can clean up at low gain settings) before it begins to cutoff, rather than aiming for a number.

 

[lowpitch]

yep, that’s right. exactly why i did it this way. already got a lot of options for doom tones, and channel II is still pretty fat, so I don’t really feel like i’ve missed much by deviating into marshall specs.

FET specs: (MMBF5457)

supply @ 8.98V
voltage from chargepump: 16.73V

	Vp / Vgss	Idss	Drain (V)
Q1	1.453	0.282	7.82
Q2	1.458	0.275	5.64
Q3	1.460	0.282	6.83
Q4	1.484	0.282	16.73

these idle drain voltages dialed in for Q1-Q3 were all done by ear (one channel at a time, gain pot set at about noon-ish)
IMO, the halfway/‘noon’ setting on drain trimpots is almost always too sterile and thin.
i found dialing towards 10’oclock gives a thicker tone, more output, and more drive.

i guess it’s about the finding the balance of a thick crisp tone with the most crunch/drive you can get (but also so it can clean up at low gain settings) before it begins to cutoff, rather than aiming for a number.

Awesome, thanks! Yeah, this will be highly individual and depend on your tastes. Just wanted to get a general idea since I got the board sitting here as well waiting to be populated. I think Robert posted the original Model feT FETs were biased to 4.25V which just seems insane to me at ~17V supply voltage. I think it's a good move to bias Q1 for more headroom like you did and let the following stages handle the heavy lifting, regardless of which specs you're building it to.

 

[Feral Feline]

This, to me, is what DIY is all about. No need/want for bone-stock — tailor to suit!
SWEET BUILD!

I was a little disappointed with the annotated schematic... I was looking forward to downloading something new, but I already have it.

I was going to say "I'll hit you up for details on your selected mods if/when I get to this circ", but then I read the bit in post#3 about finding a balance that entails listening instead of aiming for a number — so yeah, I'll just have to breadboard it myself!

I will steal your Bright-SW, though.


[EDIT: not disappointed at all, DL'd the schematic as I did NOT have it]

 

[owlexifry]

This, to me, is what DIY is all about. No need/want for bone-stock — tailor to suit!
SWEET BUILD!

ay cheers

I was a little disappointed with the annotated schematic... I was looking forward to downloading something new, but I already have it.

what i’ve done here is entirely derivative, so i didn’t really feel that annotating another schematic to claim as my own set of mods was that necessary or warranted.
maybe one day when i have an ‘original’ idea lol.

I was going to say "I'll hit you up for details on your selected mods if/when I get to this circ", but then I read the bit in post#3 about finding a balance that entails listening instead of aiming for a number — so yeah, I'll just have to breadboard it myself!

if you’re like me and find yourself doubting and wondering about certain values it’s definitely worth breadboarding just for the improved sleep quality.

I will steal your Bright-SW, though.

hell yeah you could even use an on-off-on and have x2 cap values

 

[owlexifry]

original Model feT FETs were biased to 4.25V which just seems insane to me at ~17V supply voltage.

ahh cool, that's interesting....
4.25V doesn't seem too bad... perhaps on the extreme end, but maybe it just works for the FETs they used.

I think it's a good move to bias Q1 for more headroom like you did and let the following stages handle the heavy lifting, regardless of which specs you're building it to.

perhaps something like that, not entirely sure on the rationale, but Q1 = bright channel, Q2 = normal channel, and Q3 is the common 2nd stage. (Q4 fixed).
i think i just started off with getting Q3 just right (with Q1 + Q2 in the ballpark), and then finished off dialling in Q1 and Q2 individually. mostly a taste thing.

 

[MattG]

This, to me, is what DIY is all about. No need/want for bone-stock — tailor to suit!
SWEET BUILD!

100% this. Awesome badass work and build!

 

[owlexifry]

100% this. Awesome badass work and build!

thank you!
glad i took my time with this one. all hail the mighty breadboard.

 

[Feral Feline]

...

what i’ve done here is entirely derivative, so i didn’t really feel that annotating another schematic to claim as my own set of mods was that necessary or warranted.
maybe one day when i have an ‘original’ idea lol.

...

hell yeah you could even use an on-off-on and have x2 cap values

I humbly rescind my previous statement of disappointment — I didn't realise you'd marked down your chosen mods on the schematic!

I thought it was just a link to the one I'd posted in TucsonSean's thread, which was me simply regurgitating other people's (TucsonSean's & Robert's) hard work. I did think it was helpful to get all those mods down in one place, though.

Saying I already had that schematic... well, humour doesn't always work and that attempt sure fell flatter than gob-splatter from the 50th floor...
I've downloaded your annotated-annotated schematic and will play with those values on the breadboard.


L VE the on-off-on switch idea, even better!

 

[temol]

	Vp / Vgs	Idss	Drain (V)
Q1	1.453	0.282	7.82
Q2	1.458	0.275	5.64
Q3	1.460	0.282	6.83
Q4	1.484	0.282	16.73

Regarding Idss - I think we need to move the decimal point, assuming that the current is in mA. For 5457 the current is more likely 2.8mA and not 0.28mA

 

[lowpitch]

ahh cool, that's interesting....
4.25V doesn't seem too bad... perhaps on the extreme end, but maybe it just works for the FETs they used.

perhaps something like that, not entirely sure on the rationale, but Q1 = bright channel, Q2 = normal channel, and Q3 is the common 2nd stage. (Q4 fixed).
i think i just started off with getting Q3 just right (with Q1 + Q2 in the ballpark), and then finished off dialling in Q1 and Q2 individually. mostly a taste thing.

Yup, you're right. Forgot those are in parallel. I kind of think of the first gain stage in a preamp like an OD pedal for boosting the front of an amp. Dial in too much drive early on and it's going to sound too compressed, lack bass/highs and won't clean up properly because it precedes the gain control. Then again, sometimes that's also what works, so best to do it by ear and taste like you say.

 

[owlexifry]

Regarding Idss - I think we need to move the decimal point, assuming that the current is in mA. For 5457 the current is more likely 2.8mA and not 0.28mA

ah crap, you’re right.
that’s the value off the multimeter. correct - it’s supposed to be multiplied x10. i’ll fix.

 

[owlexifry]

I humbly rescind my previous statement of disappointment — I didn't realise you'd marked down your chosen mods on the schematic!

I thought it was just a link to the one I'd posted in TucsonSean's thread, which was me simply regurgitating other people's (TucsonSean's & Robert's) hard work. I did think it was helpful to get all those mods down in one place, though.

Saying I already had that schematic... well, humour doesn't always work and that attempt sure fell flatter than gob-splatter from the 50th floor...
I've downloaded your annotated-annotated schematic and will play with those values on the breadboard.


L VE the on-off-on switch idea, even better!

hahah you’re all good mate
enjoy the breadboarding.
yeah i probably should have done the on-off-on, but im happy with just 470p.