# Tube amp schematic.. to FET pedal?



## megatrav (Jul 7, 2021)

I recently ordered the Lemon Rockerverb PCB from Guitar PCB Mania and I'm really excited to build it. Recently, I built a Sabbath from Pedalpcb. I was wondering, how does someone take a schematic of a tube amp and then convert the preamp section to a transistor based pedal?

I have wondered this for a while. It also raises other questions as I was looking into building some other higher gain pedals. Is there a particular reason that pedals like the BE-OD and Revv G2/3/4 don't use transistors to replicate their amps preamp? Is it more about the voicing and using standard clipping options that have been used is other pedals?

I hope what I am asking makes sense


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## Big Monk (Jul 7, 2021)

You want to check out Runoffgroove.com


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## Tha’ToneSeaQr (Jul 7, 2021)

Big Monk said:


> You want to check out Runoffgroove.com


^^^This!!!


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## temol (Jul 8, 2021)

megatrav said:


> . Is there a particular reason that pedals like the BE-OD and Revv G2/3/4 don't use transistors to replicate their amps preamp?



You get repeatable results with every pedal when using ICs. It's like plug and play. With JFET transistors (like J201, 2N5457, etc.) you have to bias each stage. Also you should take into account spread of the JFET parameters.

Taking schematic of a tube amp and replacing tubes with transistors while leaving rest of the circuit as is - it works (usually) but it's not a best way to get a good emulation of the original sound.


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## Feral Feline (Jul 8, 2021)

I've got the Lemon Rockverb as well, waiting its turn in the queue. I had it about a year before the build doc, so I got impatient and wound up tracing it and making my own schematic — literally a couple days later the build doc was released. 😸

Here's the links for the ROG:


			Fetzer Valve
		



			A closer look at the Fetzer Valve
		


From what I understand, sometimes some things need to be scaled down accordingly, too. 
A transistor doesn't have the same requirements as a tube, so a direct swap of JFET for tube in a schematic isn't always feasible nor desirable. What components and values need changing? I don't know, it's been a while since I read up on it and even when I did I only had a vague understanding.

Also take a look at Bajaman's circuits. He does a lot of amp emulation but does it mostly with ICs. 


Sometimes it doesn't matter which path you take, so long as you arrive at your destination.
Sometimes you just need to get there, other times it's more fun to take the scenic route.


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## megatrav (Jul 8, 2021)

Thanks for those explanations! I will definitely do some reading.


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## megatrav (Jul 8, 2021)

temol said:


> You get repeatable results with every pedal when using ICs. It's like plug and play. With JFET transistors (like J201, 2N5457, etc.) you have to bias each stage. Also you should take into account spread of the JFET parameters.
> 
> Taking schematic of a tube amp and replacing tubes with transistors while leaving rest of the circuit as is - it works (usually) but it's not a best way to get a good emulation of the original sound.


This makes sense from an R&D perspective. Its easier to take something that already works and just spend time tweaking the voicing. It probably makes more sense when using a high gain circuit like the ones I mentioned as well.

I have always read that FET overdrives will typically have a better response (closer in feel to playing a tube amp) but as time goes on I think that definitely depends on the circuit. I know the BE-OD and Revv pedals sound huge


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## fig (Jul 8, 2021)

I highly recommend the links @Feral Feline posted to ANYONE with a curiosity for these circuits. After you read it, build it! You'll be glad you did.


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## temol (Jul 8, 2021)

There's also a question - how close to the original tube preamp do you want to get with jfets or opamps? What's your goal?  What are the assumptions and criteria? Matched frequency response and signal envelope?
Here's an example - comparison of the AMT P1 with Peavey 5150. 








megatrav said:


> I know the BE-OD and Revv pedals sound huge



Yes, but do they sound identical to the originals? On the other hand - does it matter?  They're so popular anyway


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## megatrav (Jul 8, 2021)

temol said:


> There's also a question - how close to the original tube preamp do you want to get with jfets or opamps? What's your goal?  What are the assumptions and criteria? Matched frequency response and signal envelope?
> Here's an example - comparison of the AMT P1 with Peavey 5150.
> 
> 
> ...


If I were designing a pedal that is suppose to be a preamp section of one of my amplifiers, then I would want it to have a similar frequency response, but I would also want the feel and compression to be similar. 
I guess that doesn't have to be cascading FETs, if using an opamp or whatever will work just as well. I would say it might be better since its shown to work for Revv and Friedman. 
I have no idea if they're close to the amps and yeah they sell a bunch. They're good pedals as well, so no, I guess it doesn't matter


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## temol (Jul 8, 2021)

So what woul it be? What preamp would you like to emulate?


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## megatrav (Jul 8, 2021)

temol said:


> So what woul it be? What preamp would you like to emulate?


I'm actually not sure. I was just curious how it's done.
There are a lot of amps that would be fun to emulate, but I also know that some "amp in a box" pedals are trying to recreate the entire amp's character rather than just the preamp.


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## megatrav (Jul 8, 2021)

I was curious how they take tubes running at high voltages and change it to transistors. Also, would one transistor be only one gain stage? 
I know that a standard 12ax7 _can _be set up as 2 gain stages


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## Feral Feline (Jul 9, 2021)

Depends on the circuit design?

A lot of amp-sims use mu-amp configurations of the JFET, for instance a lot of Catalinbread's AIAB circuits, as well as other brands. This deviates from the one-JFET-per-tube concept, being more akin to one mu-amp per tube.

Here's the Cat'bread SFT (posted at Thermionic Studios):







Mu-amp > boost > mu-amp > EQ > mu-amp > boost


Ampeg SVT:





Looking at just Channel One, thinking of the tube stages and splits as "mu-amps" and boosts over-simplified:
"mu-amp" > tuboost > EQ > tuboost > "mu-amp" > tuboost-amp

So not identical topology, but extremely similar.


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## fig (Jul 9, 2021)

To the breadboard!


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## jubal81 (Jul 9, 2021)

Something to consider is the filter values and impedance. Vacuum tubes are high output impedance devices. For solid state pedal circuits, which are lower impedance, you should use bigger caps and smaller resistors. The general rule is to multiply the cap values by 22 and divide the resistance values by 22.

The schematic posted above from Catalinbread just used the same values from the amp. It could be reconfigured, for example, with 50K pots instead of 1M. That would be a better fit for use with JFETs and reduce noise.


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## fig (Jul 9, 2021)

jubal81 said:


> Something to consider is the filter values and impedance. Vacuum tubes are high output impedance devices. For solid state pedal circuits, which are lower impedance, you should use bigger caps and smaller resistors. The general rule is to multiply the cap values by 22 and divide the resistance values by 22.
> 
> The schematic posted above from Catalinbread just used the same values from the amp. It could be reconfigured, for example, with 50K pots instead of 1M. That would be a better fit for use with JFETs and reduce noise.


Thanks for that calc. Do the 2N5457s need to be matched?


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## jubal81 (Jul 9, 2021)

fig said:


> Thanks for that calc. Do the 2N5457s need to be matched?


Nope, not at all.


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## Paradox916 (Jul 9, 2021)

jubal81 said:


> Something to consider is the filter values and impedance. Vacuum tubes are high output impedance devices. For solid state pedal circuits, which are lower impedance, you should use bigger caps and smaller resistors. The general rule is to multiply the cap values by 22 and divide the resistance values by 22.
> 
> The schematic posted above from Catalinbread just used the same values from the amp. It could be reconfigured, for example, with 50K pots instead of 1M. That would be a better fit for use with JFETs and reduce noise.


Gold nuggets right there! 👆 Thanks for the education!


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## Feral Feline (Jul 10, 2021)

Jubal81's got the goods! Thanks!

That's the scaling I was talking about, but that I don't know enough about the "what" and "by how-much" and the "why".


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## jubal81 (Jul 10, 2021)

Feral Feline said:


> Jubal81's got the goods! Thanks!
> 
> That's the scaling I was talking about, but that I don't know enough about the "what" and "by how-much" and the "why".


Here's a way to check it out. 
The SFT above uses a James stack. You go to the James stack at Tone Stack Calculator and test values. Look at the response with the default values, then scale like I suggested above (Cap values X 22 and divide resistor values by 22) and you'll see it comes out the same on the graph.


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## Chuck D. Bones (Jul 10, 2021)

Catalinbread skewed the tone stack parts values, presumably to achieve the desired tone.


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## fig (Jul 10, 2021)

One more question;
Can we prune enough parts to fit it on this?
I'm running low on breadboards atm..







I checked, and I DO have enough 5457s so I will give it a go.....just not with that bb


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## fig (Jul 10, 2021)

I'm guessing C15 remains @ 100uF. If this is incorrect, smack me hard.


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## fig (Jul 10, 2021)

This is how I fared...


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## temol (Jul 10, 2021)

Hm.. did you apply x22 factor for all the schematic components?? Why?


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## jubal81 (Jul 10, 2021)

temol said:


> Hm.. did you apply x22 factor for all the schematic components?? Why?


Right, you'd just do those substitutions for the tone stack and the gain control. It's not strictly necessary, though. I have a lot of A100K pots, so scaling lets me use those instead.
Be interesting to build it both ways and see if one is less noisy. Never actually done that.

Another Edit: BTW, the SFT sounds epic when you crush the input with a Klon.


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## fig (Jul 10, 2021)

@temol , extra credit? 

Ah! That's much simpler then. Thanks to you both!


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## megatrav (Jul 10, 2021)

I like that this thread turned into something very interesting.

Side question- 
As players, does the type of clipping (opamp, diodes, transistors) feel and respond differently.. or does it just depend on the circuit?


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## megatrav (Jul 10, 2021)

Also, going back to the tone stack values- I noticed that the Wampler Black 65 has different values than an actual AB763 circuit.


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## fig (Jul 10, 2021)

Hey cool more options!

I've made some headway..looks like just some pots left. I've left it per schematic to start. With the values in Excel, I can easily adjust them all at once.


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## Chuck D. Bones (Jul 11, 2021)

Getting back to what Jubal81 said about scaling...
It makes sense to scale the impedances to suit the devices (FETs vs. tubes) and signal levels because higher impedances invite noise pickup.  It has to be done carefully because a tone stack loads the gain stage that drives it, the next gain stage loads the tone stack and so forth.  I suspect that some pedal builders simply avoid the effort involved in optimizing the design by just copying component values from another circuit whether it's appropriate or not.

Fig, those pot pins are too fat for the protoboard holes and will stretch them out so they are too loose for resistor and cap leads.  There's an analogy somewhere in the back of my mind...


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## brokenwizard (Jul 11, 2021)

temol said:


> There's also a question - how close to the original tube preamp do you want to get with jfets or opamps? What's your goal?  What are the assumptions and criteria? Matched frequency response and signal envelope?
> Here's an example - comparison of the AMT P1 with Peavey 5150.
> 
> 
> ...


Man I really would love to see the P1, R1 and maybe S1 on this site. They're such awesome pedals.


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## temol (Jul 11, 2021)

Also I would not stick to x22 factor. Scale the tone stack values as needed for a particular circuit. x5, x10. As C.D.B said above - it all depend what's in front and after the TS.  Remember, you have to scale all the tone stack components - capacitors, resistors, potentiometers. Imagine you have 10k potentiometer (or lower) and apply x22.



megatrav said:


> Also, going back to the tone stack values- I noticed that the Wampler Black 65 has different values than an actual AB763 circuit.



It does not look like 1:1 tube->jfet conversion.


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## fig (Jul 11, 2021)

I'll use that tone stack calc as suggested, but now I _need_ to hear it as drawn initially. I know a couple folks who might_ even _help get me get it fine-tuned if I ask nicely.

Thanks to everyone for the explanations and insight, and my apologies to @megatrav for hijacking their thread. Sometimes I see a schematic and something snaps....or clicks?.....maybe it's more like showing the queen of diamonds to Raymond?


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## megatrav (Jul 11, 2021)

fig said:


> I'll use that tone stack calc as suggested, but now I _need_ to hear it as drawn initially. I know a couple folks who might_ even _help get me get it fine-tuned if I ask nicely.
> 
> Thanks to everyone for the explanations and insight, and my apologies to @megatrav for hijacking their thread. Sometimes I see a schematic and something snaps....or clicks?.....maybe it's more like showing the queen of diamonds to Raymond?


No apology necessary. I hope one day it all makes more sense to me. I'm grateful for the educational material.


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## fig (Jul 11, 2021)

Chuck D. Bones said:


> Fig, those pot pins are too fat for the protoboard holes and will stretch them out so they are too loose for resistor and cap leads. There's an analogy somewhere in the back of my mind...


Yes sir, it is a piss-poor way to treat a breadboard.


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## fig (Jul 11, 2021)

I think this is finished. I'll go over it with new eyes later. Note: As Chuck rightly pointed out you shouldn't shove pots in BBs, but this one was kinda at the bottom of my bin anyway. I'm hoping to score some not-so-jankys when they are in stock.

I could have saved a little more space I suppose. I did keep all the power/ground on one side. The pots are the wrong taper, but I'll likely change them anyway as suggested.


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## fig (Jul 12, 2021)

Well NOW it's done...and very loud, but I am liking it. I'll work on trying some other values in the tone stack.


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