Triode emulater revisited, again: a mu-amp/minibooster variation

[tca]

Hi all,
after repeating all the calculations in Dimitri's paper I started wandering if a mu-amp/minibooster variation could be built for getting the same 3/2 triode law. Everyone likes Jack's minibooster and it is a wonderful booster!

I confess that I was a bit resilient to post the schematic over the web, but I think one should give back the community what one has learned from the community. The mods are quite simple and I haven't found them on the web and they are original in this sense. So the schematic is this:



The lower fet should have a source resistor given by Rs=0.83|Vp|/Idss (or should I say 0.84 ) the same as the ROG/Dimitri fetzer valve circuit, but in order to preserve the 3/2 law the upper fet should be biased roughly at |Vp| by choosing R1 and R2 appropriately.

My favorite setting is R1=2M, R2=1M, R3=330 for the MPF102 and V=9V and it sounds incredible!

Cheers and be good.

[B Tremblay]

Looks like a very interesting approach!  Just to clear up some confusion on my part, what exactly is your name for the circuit?

[amptramp]

You may benefit from a resistor in series with the source of the upper FET.  The output should still come from the upper source but the capacitor to the upper gate should come from the bottom of this resistor.  The resistor value should be the reciprocal of the upper FET transconductance.  That way, as the lower transistor current varies, the upper transistor current varies by the same amount in the opposite direction.  What you have in your version is a lower FET stage with a constant-current source for a load.  It is also a valid circuit, but it behaves entirely differently from the µ-amp or SRPP (shunt regulated push-pull) stage I have described.  Note that the SRPP stage must have a finite load, but this is provided by the bias resistors for the upper FET.

[Davelectro]

Hi all,
after repeating all the calculations in Dimitri's paper I started wandering if a mu-amp/minibooster variation could be built for getting the same 3/2 triode law. Everyone likes Jack's minibooster and it is a wonderful booster!

I confess that I was a bit resilient to post the schematic over the web, but I think one should give back the community what one has learned from the community. The mods are quite simple and I haven't found them on the web and they are original in this sense. So the schematic is this:



The lower fet should have a source resistor given by Rs=0.83|Vp|/Idss (or should I say 0.84 ) the same as the ROG/Dimitri fetzer valve circuit, but in order to preserve the 3/2 law the upper fet should be biased roughly at |Vp| by choosing R1 and R2 appropriately.

My favorite setting is R1=2M, R2=1M, R3=330 for the MPF102 and V=9V and it sounds incredible!

Cheers and be good.

Thanks for sharing this.

The idea is not new, though. Medved's FET Twin, for example, uses a similar approach on its first stage. I even remember this russian (GTlabs??) paper from several years ago explaining how to get triode-like curves out of mu-amps. Very interesting reading.

[tca]

Looks like a very interesting approach!  Just to clear up some confusion on my part, what exactly is your name for the circuit?

Well I don't have a name for it! You can call it mu-valve Really don't have a clue.
But I get your point! (changed title).

Cheers.

[tca]

The idea is not new, though. Medved's FET Twin, for example, uses a similar approach on its first stage. I even remember this russian (GTlabs??) paper from several years ago explaining how to get triode-like curves out of mu-amps. Very interesting reading.

Is this theMedved's FET Twin?



Nice ideas there! Thanks for the ref.

This is also very interesting, I'm going to repeat the calculations. There are some ideas very similar to the ROG approach for replacing valves with fets in classical head amps. Very nice!

I'm looking for the ref of the mu-amp triode-like curves. Thanks again.

P.S.

There is also this old thread most of the old links to the russian sites are unavailable and most of the images and links don't work. Probably this image is all I can get:



This comment by stm is highly illustrative!

And also this, after some heavy digging!



And found it!

[gregwbush]

Awesome tca

I went ahead and breadboarded the Mu-Valve

Used MPF102 jfets
Measured a few dozen for their Vp and Idss with the ROG jfet tester http://www.runoffgroove.com/fetzervalve.html
For the top one I chose the jfet closest to 3.0v Vp
For the bottom one i chose a jfet with a Vp and Idss that calculated out to a source resistor of 300R

With an OP-AMP buffer and "Breakup" (input level) pot in front, and a "Volume" pot and OP-AMP buffer after, i can confirm that the Mu-Valve sounds fantastic! This really warms up (adds music friendly harmonic distortion to) a clean solid state amplifier, with the breakup pot set just around where distortion becomes audible, and the volume set for ~unity gain with bypassed signal

Thanks!
Of course, when driven hard it stops working like a triode.... but hey... can't complain!

[gregwbush]

Did i say breadboard?

I meant perfboard, sorry (although i did breadboard the tca circuit first)

This is my layout with added input buffer, attenuation pot, volume pot and output buffer



Coupling capacitor values are probably not critical, and some are possibly not even necessary (due to my lack of knowledge). I have a feeling it may have made more sense to omit the 1Meg Mu-Valve Rin resistor and use a 1 Meg pot instead of 100k pot plus the 1 Meg resistor. In parallel, the way i have it, the Mu-Valve input impedance may be affected (??)

This is the schematic... tca, please let me know if this infringes your copyright. I'll take the image(s) down if necessary




Here is my yet to be boxed board...



If you've got a clean solid state amp, give it a go... last FX in the chain, on top of the amp works for me right now. You may (or may not) be quite surprised!

By the way, tca... did i do it right?? as far as selection of jfets? The 3v Vp transistor, with 2M,1M voltage divider and 9 volt supply. The voltage at the junction of the 2M,1M would be 3 volts yeah?

[tca]

Hi gregwbush,
I think you nail it pretty well!

> If you've got a clean solid state amp, give it a go...
> last FX in the chain, on top of the amp works for me right now. You may (or may not) be quite surprised!

You can also use it in a tube amp, you may also be quite surprised! It adds a very beautiful tone to the guitar signal.

As for the extra mods that you've done they are wonderful, although, I prefer no minimize the number of devices on the signal chain as much as possible, but you could add, as you did, what ever you like (hacker mode!).

> This is the schematic... tca, please let me know if this infringes your copyright. I'll take the image(s) down if necessary.

There is no need to. I always share my schematics under a Attribution-ShareAlike 2.5 Generic (CC BY-SA 2.5).

You are free:
   to Share — to copy, distribute and transmit the work
   to Remix — to adapt the work
   to make commercial use of the work
Under the following conditions:
   Attribution — You must attribute the work in the manner specified by the author or licensor (but not in any way that suggests that they endorse you or your use of the work).
   Share Alike — If you alter, transform, or build upon this work, you may distribute the resulting work only under the same or similar license to this one.

I wish some more DIYers would  share their schematics with the same license.

Cheers.

[gregwbush]

That's awesome tca... the copyright thing... (sharing is caring, anybody?)

Hmmm, less is more isn't it...

I took a single gain stage and made it into 3 gain stages. More noise, more parts. To make a short story long.... when i breadboarded the basic Mu-Valve that you provided, it sounded great. But because i built myself a quad AMZ op amp buffer type FX loop A/B box type thing; i experimented and found that it sounded more sparkly and bright with a buffer in front than without a buffer in front. I figured the more bright the input, the clearer the Mu-Valve could "hear", and hence the more sweet sounding harmonic distortion it would create.

At this point, i was pretty happy

Then i switched in a buffer after it... and it sweetend the sound even more. I put this down to the lower output impedance of an op amp driving my 3 meter (~10ft) cable... attenuating less highs than the higher output impedance of the Mu-Valve.

That was the best i could make it sound from my breadboard.... ultra high impedance input, low impedance output.

Naturally, since i was going to put solder to board, i thought well ok i'll replicate this best case situation. And that's what i did.

At first, the schematic was different. It's op-amp input had a gain control in the negative feedback loop... Then i read more and realized that no matter what i did, the bloody thing would have a gain of 1 + whatever resistors i chose. That pissed me off... coz i wanted the pot to go from zero to a gain of x1, not from x1 to more than x1 (trying to attenuate, not boost) So i modified it to be a unity gain buffer with a volume knob after...

The other problem in my first schematic was i originally had the master volume after the output OP AMP. Fortunately before i built it, i realized this might be a problem. I didn't know (and still don't know) the gain of the Mu-Valve. What if it's output was more than say 3 or 4 volts peak to peak? That would drive the OP AMP into distortion. So i put the master volume control before the buffer...

And so then it was born. The very high input impedance Mu_valve triode emulating low output impedance cable driving signal enhancer, that won't give you a 400V DC shock.

To make a long story short, disregard the above, measure some jfets and build tca's Mu-Valve

[gregwbush]

Jeez i can talk crap at times, my apologies

I'm back again to say
I'm HOOKED on this circuit!
Really enjoying playing "clean" at the moment

Simpler and (in my personal opinion) better than a mini-booster

tca, have you tried cascading multiple Mu-Valve's, or anything cool / special??

[tca]

tca, have you tried cascading multiple Mu-Valve's, or anything cool / special??

Hi,
I wanted a clean "triode" emulator so I haven't tried that out. You could try something like this (based on Jack's minitubes):



http://www.diystompboxes.com/smfforum/index.php?topic=64618.0
http://www.diystompboxes.com/smfforum/index.php?topic=19200.0

But you can do MUCH MUCH better in simulating a triode see: http://www.diystompboxes.com/smfforum/index.php?topic=93889.0 The details are not all there but you can get a lot of good ideas from reading it!

All the players that use this mu-valve  circuit (build my me) report  exactly what you describe, a clean, smooth sound that one can not stop playing with it, it adds a new extra dimension to your tone! They use a plain version, not even with a volume control and as the last device on the chain.

Keep up the good work!

[gregwbush]

Damn... i was hoping you'd done something even more cool yourself. It was worth asking 

But you can do MUCH MUCH better in simulating a triode see: http://www.diystompboxes.com/smfforum/index.php?topic=93889.0 The details are not all there but you can get a lot of good ideas from reading it!

Interesting you mention that... I read that thread the same night i discovered your Mu-Valve! I also read Dimitri's paper and have looked at the much more complicated triode emulator circuit he published in 2004. Looks interesting but has too many components which i do not have on hand. So i probably won't try it out. Got a bunch of old radio / hifi parts; transformers, valves, valve sockets, chassis, etc. I feel it is my time to start mucking around with high voltage! Buying parts for that has priority over parts for a complicated emulator atm

All the players that use this mu-valve  circuit (build my me) report  exactly what you describe, a clean, smooth sound that one can not stop playing with it, it adds a new extra dimension to your tone!

It really does, people!
For such a small part count. Awesome stuff. Don't be deterred by having to measure some jfets. Build the RunOffGroove jfet tester, read the instructions... and just do it! 

[tca]

Damn... i was hoping you'd done something even more cool yourself. It was worth asking  

I do have some more cool stuff... but it is not time to show it yet.

For such a small part count. Awesome stuff. Don't be deterred by having to measure some jfets. Build the RunOffGroove jfet tester, read the instructions... and just do it!  

It is simpler than that, it is not necessary to match both jfets.

Cheers.

[tca]

Here is the tested vero:



Cheers.

[chptunes]

Nice looking design and layout TCA.. I love Orman's Mini-Booster [and Tweed Stack (dual MB)] very much.  This appears to be a cool variant.. easy to build with just a few common parts.  I may throw this together, though I probably don't have any 300 ohm Resistors.. would I get good results with 330 or 220?

Thanks,

-Corey

[amptramp]

For the price of an extra resistor, you can have the SRPP design shown here:

http://www.tubecad.com/articles_2002/SRPP_Deconstructed/SRPP_Deconstructed.pdf

Ignore the fact that he is using tubes - there is a great deal of information in this article.

[tca]

I may throw this together, though I probably don't have any 300 ohm Resistors.. would I get good results with 330 or 220?

I've build all mu-valve circuits with 10% tolerance resistors, so try the 330.

For the price of an extra resistor, you can have the SRPP design...

Hi, that is the 2nd time your refer the SRPP design... Thank you for your persistence, I'm going to build one just to see how it sounds. Thanks.

P.S.
I've checked my calculations of the mu-valve and I had taken into account a SRPP configuration . Probably putting the gate to  source via a source resister on the top jfet, a resistor with half the value of the bottom jfet source resister, will held similar results. Have to breadboard this.

[tca]

I may throw this together...

You could try  this SRPP version (with or without a output jfet buffer)



or with R=0 and R1=150 (any value from 100 to 300 will work, low gain for R1=100 and higher for R1=300).

Cheers.

[chptunes]

Cool.. so, how does this new alternate schem compare to the vero layout above?  You've added a bypass Cap across R2.  Does one schem sound cleaner than the other, different frequency peaks, etc?  I have an empty AMZ Mini-Booster PCB and an empty AMZ Dual Booster PCB that would work nicely, if one of these circuits warrant a complete pedal..

Thanks,

-Corey