Little Jim - a Marshall inspired MOSFET distortion pedal design

[peterc]

Great thread. I would love to see the Trace schematics, but Google searches only come back with Vox Reactor.

Any further ideas?

Many tahnks
Peter

[teemuk]

Try "TVT9 schematics". 

[peterc]

Thanks!

[teemuk]

I'm guessing the diodes in the gate circuit are to simulate grid current when the grid is positive biased.

This. The diodes basically allow more asymmetry to clipping (similarly to generic triode gain stages, and I think due to having lower clipping threshold than just forward biasing the gate-source diode). Peavey "TransTube" patent describes this in better detail and even suggest minimum percentage of asymmetry to emphasize that aforementioned DC bias point shifting in enough of "tube-like" fashion.
One can do that with ordinary BJT's as well, no specific need to use FETs.

[Vivek]

Is this the correct patent ?

https://patents.google.com/patent/US5647004A/en

[teemuk]

That's one of them. There's also a patent where FETs are replaced with discrete bipolar transistors in darlington configuration (results to high-ish gain and input impedance and so the stage behaves more in "voltage drive" -fashion similarly to tubes and FETs). That is also the circuit version Peavey employs in their "TransTube" circuits. AFAIK, Peavey prefers predictability of BJT's over wide variance of FETs and regards overall clipping asymmetry to be more important characteristic in triode emulation than any subtle differences of FET vs. BJT distortion.

BTW, Peavey's "TransTube" guitar preamps are also damn nice examples of straightforward tube-to-SS conversion because they largely replicate the exact circuit architecture of Peavey's typical tube-based guitar amp preamps (excluding 5150). You know, the ones with that "Ultra" gain channel and alike. Some impedance levels, gains and clipping thresholds are just tweaked accordingly to match the solid-state circuitry. It's not paint-by-numbers -conversion but real close, which is impressive feat given how well these things actually mimick the original circuit.

Enough of OT from me, but yes, one could use the very same ideas to convert, say, some classic Marshall tube preamp, like 2204.

[jonny.reckless]

Ranko, here's the link to the LND150 distortion pedal I managed to come up with:
https://www.diystompboxes.com/smfforum/index.php?topic=125775.msg1199458#msg1199458

[cab42]

I finally got the parts I was missing and have had the Little Jim on the breadboard for a couple of days and it sounds great.

I had some problems making it work, though. I do not have a transistor tester so I just plugged in some BS170's, but had very low drain voltages (app. 4.5 v) compared to the schematic voltages and no or bad sound. After fiddling with drain resistors and VBias, I found out that my bag of 10 BS170's varied a lot and I found that four of them gave drain voltages near the ones in the schematic. Using three of those made it sound comparable to Jonny's sound sample.

I posted a vero layout on page 2. Looking a bit closer I found out that I had the pinout of the J112 wrong. I have fixed the layout and reuploaded so the image on page two is correct. Now I just need to build it and verify.

BTW: Is there any simple method to measure the threshold voltage? The ones I found looked pretty complicated.

[jonny.reckless]

I finally got the parts I was missing and have had the Little Jim on the breadboard for a couple of days and it sounds great.
I had some problems making it work, though. I do not have a transistor tester so I just plugged in some BS170's, but had very low drain voltages (app. 4.5 v) compared to the schematic voltages and no or bad sound. After fiddling with drain resistors and VBias, I found out that my bag of 10 BS170's varied a lot and I found that four of them gave drain voltages near the ones in the schematic. Using three of those made it sound comparable to Jonny's sound sample.
I posted a vero layout on page 2. Looking a bit closer I found out that I had the pinout of the J112 wrong. I have fixed the layout and reuploaded so the image on page two is correct. Now I just need to build it and verify.
BTW: Is there any simple method to measure the threshold voltage? The ones I found looked pretty complicated.

I just use one of those cheap component testers: https://www.amazon.com/Mega328-Digital-Transistor-Resistance-Capacitance/dp/B07WT9VVZB/ref=sr_1_3?dchild=1&keywords=component+tester&qid=1605596118&sr=8-3

I bought a bag of 50 BS170s and the whole bag had threshold voltages matched to about 100mV or so, I was pleasantly surprised. As long as you get them reasonably matched, the absolute threshold value is not important, you can tweak the value of R8 to get the drains to sit between 6 and 7 volts. The exact drain voltage doesn't appear to be critical to the sound. I played around with a preset in R8 and I couldn't really hear much of a difference until the drains got to 5.5 and 7.5 volts. Of course at 4.5 volts you've got no voltage left across the FET so it won't work.

[Phend]

^ The tester:
Measures Transistors, Diodes, Resistors, Caps, Mosfet, Jfet, everything except for your weight.
Fast and east to use, I am not promoting this device, nor will vouch for its accuracy.
One problem is I had to get 3 of them, the first 2 had a broken corner on the glass screen.
Returned one, unopened. I modified the plastic box so the battery can be easily replaced.
Paid around $14.00 us. Kind of inexpensive.

[Phend]

Took awhile but I finally got Little Jim on the board.  Sounds great, definitely worth boxing it up.
Biggest change is to replace R8 (drain adjust) with a 10K pot. On the board I am using trims, in the box I will use pots.
Think I will call R8 "Vibas", which can be adjusted to eliminate almost any hiss.
(It can be tweaked but not adjusted to far off. Maybe leave it as a trim, need to experiment)
9.27 v Battery
7.98 v at drains
Matched BS170 at 2.5 Vth
R8 pot set 5.9 k
Per rutgerv suggestion will add a (switched) 330 ohm at C3, it does work.
The 4n7 cap at TR2 did not seem to do any thing nor the 100 ohm R15.
(Maybe those work with other setups)

[Phend]

Done !!  See pictures in BYOS pictures folder.

Thanks to all.........

Now to the secret R & D project for the next build (if it works)...no wood ...lol

[deadastronaut]

^ cool, is that text inlay?.

[Phend]

^ I engrave (raster) a pocket into the wood slide cover. Then clean up the edges in vector mode.
The vector trace takes about 8 seconds to do. Then using the same file I cut out the letters. On Little Jim I used .005 inch (.127 mm) thick self adhesive acrylic and cut it out.  I have also used abalone on some other boxes.  If I were using metal box I would probably use Cermark, a substance that is either painted or sprayed on then using the laser fused to the metal. Can do very detailed designs using that and once it on fused it is there for good, least you get the grinder out. I have applied numbers to stainless steel for the Navy, those parts are used under the sea.  Google Cermark if interested.

[rankot]

Did you try to make this with PMOS transistors, BS250 for example? Andy published Zazen Booster (https://www.diystompboxes.com/smfforum/index.php?topic=119198.msg1157967#msg1157967) using that approach, he claims it has less noise than NMOS. I tried to simulate, and this is the working version, but I don't have time to test it in a real world right now:

Code Select Expand


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WINDOW 3 36 40 Left 2
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WINDOW 0 24 56 Left 2
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WINDOW 3 36 40 Left 2
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TEXT -472 -56 Left 2 !.tran 0 .1 .08
TEXT -472 -24 Left 2 !;.ac oct 16 10 50k
TEXT -472 -88 Left 2 !.param dr=500


[Phend]

^ I haven't,  plus keep us posted

[rankot]

You can try it, since you have it on breadboard already! 

[Phend]

cool, is that text inlay?

deadastronaut's website and effects are exceptional.
Even some vintage space puppet clips (I believe that is what they are). 
I remember watching the Thunderbirds and Fireball XL5 back in the 60's.

[Phend]

Looking for BS250P to try as rankot suggests, Mouser will have them in April.
Newark has them in the UK, Amazon has them for $3.80 each, will wait.....

[rankot]

Maybe you can try with ZVP2106A? They're similar and cheaper. There are even more SMD parts, if you are willing to use them.

What puzzles me is the fact why are PMOS devices less noisy than NMOS (if they really are). The most part of the noise shall be generated by R_ds(on), and that one is already low for NMOS parts (approx. 1.2 Ohm), while, one the other hand, for BS250P only maximum value of 14 Ohm is given in datasheet. I have few of AO3407A PMOS transistors with R_ds(on) of only 50mOhm, so they should have unnoticeable noise figure, but I will have to find some time to test them.