matamp gt120 preamp

[rocket88]

so as may of you know i'm a massive fan of all things thick and heavy. for those that are huge fans of thick, sludge doom metal like SLEEP you should know the name MATAMP. without going into the history for those that don't know, MATAMP built and designed the classic OR120, but had their own design of a similar amp they dubbed the GT120. it's absolutely legendary, and stupid rare. it's heavier and clearer then the OR120. it took a long time for me to find the schematic, but it's hard to read, so i'm posting an easier to read schematic for the GT100 (100watt version) that really only differs with a few component values.

posting this here before i post it to the main page. right i'm not 100% sure how it will sound and work, because i haven't had time to breadboard it the circuit design and i'm working on theoretical concepts on this one. i figure for the brave souls that want to give this a shot now, and if there are issues work to get it right, it should be done here not on the main page. since it's a FET conversion of the tube preamp section, and since the same thing was done with OR120 preamp (graphic citrus), it should be fine. i added 220pF caps from Gate to ground to  increases gain, and help produce 2nd order harmonics, theoretically help it sound more tube like. it may sound better without them, not sure.

in any event here's the schematic and layout.

[traktop]

Yes!!!
Time ago I made a decal artwork for a green citrus build, but finally didn´t use it, so this looks like a perfect candidate.
The schematic you posted is barely readable due size and low resolution, but I managed to find it with a google search.
Well, what´s happening around v1 preamp tube is a mistery to me...
So there´s a rotary switch gain control which makes a feedback loop around the volume/gain pot, and the result of that slams v2 right? That sounds like this is gonna be loud...
I´ve been taking a look to the presence network, and the feedback point is v4 cathode to ground by a 5k pot, some caps and a 47mh inductor. I was thinking that maybe it could be a cool implementation, (I speak from complete ignorance...).
Cheers, Gilberto.

[rocket88]

hey gilberto. honestly i don't think the presence control is a worthwhile inclusion, as presence controls don't really do much. while it may be possible to add it, i'd wait to mess with it once the layout is verified. unfortunately i don't have time to build it as of now, but going through the layout and schematic i don't see any errors, granted i spent way to much time to get it where i was happy with the size and layout of everything. maybe since i know you'd be into this if you have time give it a shot. i would suggest socketing the pF caps i mentioned up top.

[traktop]

As soon as I finish a couple of builds I´m doing I will give it a shot for sure

[Stymie13]

Got a strip board cut to take a run at this one but realized I have a 4P3T switch rather than a 3P4T. Been looking at the layout and switch pins and can't quite figure out how I can make it work with one less gain boost position. Any suggestions? Thanks!

[Crusty_Fuzz]

Awesome!
Glad that someone also likes these old heavy smokin' things :)
Here original GT120 schematic from Hilbish Design if necessary https://i.imgur.com/JnsJCNS.jpg

[StephanCOH]

Thanks for uploading this. I'll definitely give it a try as soon as I find the time.

Built a Green Citrus about a year ago and it has a fixed spot on my board.
Really curious to see if and how the sound is going to differ between the two builds.

[Stymie13]

Finished this one up and have no signal passing. I don't see where Q3 is connected to 9V from the trimmer unless its supposed to go through the 680p cap connected to the drain. Voltage readings beginning with the tranny in the top left of the layout and clockwise to the one on the bottom right:

          D     S      G
Q1    4.5   1.27   0.0
Q2    4.5   1.62   0.0
Q3    0.0   0.0     0.0
Q4    9.2    1.5    0.0

I'll take another look at the Hillbish schematic that was referenced to try and translate the dual triode conversion. Any thoughts would be greatly appreciated!

[Stymie13]

Rocket, any chance you could post a higher rez photo or link to source of the schematic you used? I can't seem to blow it up large enough to see.

Thanks,

John

[rocket88]

Looks like I forgot to remove a cut that’s not supposed to be there. Put a link over the cut on row L column 15, cut 3 rows over to the left from there drain of Q3. That should fix the issue. Unfortunately I can’t make it higher resolution, I tried when I made the initial post, and I don’t remember the source, as I’ve had the schematic for a few years now.

[Stymie13]

Cool, I'll make the changes and post the results. Thanks!

[Stymie13]

I jumped the cut and was able to bias Q3 to 4.5. I changed Q4 trimmer to 100K to get it down to about 4.8. Still no signal. I would love to report that I found an error in my build which is what I usually find but I've checked all the solder joints, component locations, cut the tracks, etc. No luck. Tried J201s, 5457s, and MPF4393s. No more time to tinker tonight but will post any results when I get back to it. Anyone else?

[rocket88]

Take an audio probe and see where you’re losing signal. That will help get this sorted, cause then we’ll know where the issue is with the layout and I can correct it. . I don’t think anyone else had built it yet, well at least no one else has reported building it.

[Stymie13]

Found a couple switch wiring mistakes before I even plugged in the probe. Corrected them and got signal through the circuit but it's very gated with some octave ghosting. Spent some time probing but was getting lost and confused. I should have some time to spend trouble shooting this weekend.

[Stymie13]

I traced the signal through the circuit and it sounds OK until it hits Q4. I triple checked the layout and its consistent with the Hillbish schematic I'm working off. I don't know enough about audio circuits to diagnose the problem. Q4 voltages are: 4.5 on the gate, 4.82 on the source, and 4.85 on the drain.

Edit: Here's the schematic which can be blown up nicely:
https://i.imgur.com/JnsJCNS.jpg

[Stymie13]

I've looked at a few other JFET amp simulation layouts and this one seems unique. Any ideas on how to properly bias Q4 in this layout would be greatly appreciated!

[Stymie13]

After doing a lot of reading about JFET biasing and looking at other circuits I decided to just omit Q4 and see what happened. Circuit works great but definitely benefits from a master volume as it is LOUD. Gain switch positions 1 and 3 sound best, 2 and 4 are a bit bright. Bass switch works fine but cap values could be altered. Seems to be one of those circuits that you want to socket almost every component to dial in. I may reinstall Q4 with a 1K5 source resistor in place of the 100K to see what happens.

[Crusty_Fuzz]

Good job dude! Any demos planned?
Perhaps it makes sense to use SMD J201... A lot of through-hole J201 is full bullshit.

[Stymie13]

Thanks Crusty but I probably won't be doing any demo clips any time soon. Here's an update though... I reinstalled Q4 with a 1K2 source resistor which allowed me to bias it properly. Unfortunately, it sounded awful so I pulled it out again. The circuit works fine with 3 trannies, just install a 100K Master volume after the output cap.

[rocket88]

hey man. great job on the legwork so far on this beast. sorry for not saying much for a bit, i've been crazy busy and really still am, but here's a few things i'm thinking based on what you said happened, and comparing it to the graphic citrus, which is basically an OR120 preamp, which is similar in structure to the GT120. first off, that 100k off the source of Q4 should be more like 1.5k like you tried.

as far as i'm concerned without Q4 it's not going to sound correct, Q4 needs to be hit so to speak. plus i've been looking to see if anyone has tried the same conversion with this amp and there's a few similar pedals i've found all running 4 jfets. when you said it sounds awful, what do you mean? did you try adjusting the bias to different voltages to see if it problem improved or not? use your audio probe to see where the sound is getting messed up.

try changing Q1-2: 2n5457 and keep Q3-4: j201