Question about similarity between Bassman-based preamp circuits

spi said:
This pedal is basically a clone of the TC Elec Spark Boost.

From what I can gather, Iommi was apparently a Spark Boost user (according the TC web site), and somehow Laney got Iommi to endorse their clone of it.

It's definitely not an amp simulator, but it is a nice boost pedal.
Click to expand...

Yeah, I know exactly what it is, that's why I suggested it:
Dark Esbat with the Sabbath and the ELS SuperTrouper.​
OP is going for "Iommi" tones, so there's a bunch of stuff out there to help get those tones beyond just aiming strictly for an AIAB pedal.
Maybe the OP can get to the sound in his head via a combination, 'cause let's face it — pedals can never replace an actual amp, but we try to get close to it.



@Diego
Here's a nutter tingetje to thinker 'bout:

Just as ICTRock pointed out in post #17, "you don't bias mu-amps like you would single jfet stages", and accordingly you don't bias JFETs as you would tubes.
I bring that up because I noticed you've got the SuperGroup amp schematic (well, preamp) amidst the other schematics in post #16 — while certainly the amp schematic needs to be referenced, it is but a launching point to get the sound you want.

For instance, many people will favour mu-amps to emulate an amp's sound, however Bajaman favours using op-amps and has many circuits emulating various amps — his emulations have become popular within the DIY community. Several amps he's emulated with op-amps having been also emulated by others using JFETs or Mu-amps — whatever it takes to reach the end goal/many ways to skin a cat.

Back to JFETs as tubes and Mu-amps — I've read some interesting notes about how the Ampeg B15's tonestack is wrong in many AIAB-pedal clones of the circuit because the cloners didn't SCALE the tonestack's values down for the emulation. Check out the Ginger link below.

Think of it like cooking chilli for 500 people or just 5. Do you use the same amount of ground jalapeño for both batches of chilli?
An amp is dealing with mega wattage/volts, so scaling things down for a 9v pedal that isn't using tubes... well, some things have to change.


megatrav

Thread 'Tube amp schematic.. to FET pedal?'

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...
Chuck D. Bones

Thread 'Why are JFETs such a Pain in the Ass?'

How many times do we see post in the Troubleshooting forum where the problem turns out to be a JFET?

Like Germanium transistors, JFETs are finicky beasts and buying the right part number does not guarantee success.

First, a very brief primer on how JFETs work.
JFETs are voltage-controlled devices. The drain current depends on Vgs (the voltage between gate & source). The gate current is so close to zero that we can safely assume it is zero. We'll talk about N-channel JFETs (arrow on the gate pointing in) because 99% of the time that's what are used in pedals. When Vgs is zero...
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Chuck D. Bones

Thread 'JFET Shootout'

Here's a comparison of most of the JFETs we use in pedal circuits.
I pulled 5 samples each of 15 different part numbers. I measured Vgs,off and Idss. From that, I used the JFET formula to calculate the Vgs and gm (transconductance) at Id = 200μA. I chose 200μA because that's a typical drain current for pedals. Some pedal circuits run the JFETs at a much lower or much higher drain current. It's easy enough to recalc the Vgs and gm for any drain current between 0 and Idss.

This is a small sample size and does not represent the full range of Vgs,off & Idss that we might experience for a...
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tagboardeffects.blogspot.com

Runoffgroove Ginger

Collection of vero (stripboard) & tagboard layouts for 100s of popular guitar effects, with over 1000 verified designs. DIY your own boutique effects!
tagboardeffects.blogspot.com tagboardeffects.blogspot.com


TLDR: Don't get too hung up on minutia, trust your ears.
 
Sunn T from GPCB is a mu-amp thanger. Not sure how to mangle it into SupaTroupaGaroupa territory, but ... maybe will prove useful info some how some way...

Also, Chuck had some cool mods for the GPCB Sabotage (Sabbath-like):
Chuck D. Bones

Thread 'The Week on the Breadboard: Sabbra Cadabra'

Holy Crap! This thing ROCKS!
I built it stock except I put a 250K trimpot in place R22 and connected pin 2 to C16. This is where the Treble pot would go in a Marshall tone stack. More than enough gain on tap. There is no clean setting unless you dial your guitar volume way down. Who wants to do that? So far, I've explored RANGE settings between 8:00 & 11:00. The only JFET selecting I did was I made sure that Q5 had a higher Idss than Q4. I used a BC109C for Q1. I'll try some mods and JFET subs tomorrow, but I'm pretty happy with stock circuit. It has a lot more gain than The...
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Hey everyone,

Thank you so much for all the help. I might not be new to building pedals, but I'm definitely new to trying to understand the function of a circuit and modding it to taste. So I'm really glad people are willing to help me and share their knowledge. I realize I'm just at the beginning of this journey.
To give a bit of background on the approach: the reason I'm doing it this way is that I wanted to build a preamp in the first place and I already have a Rangemaster. When I was looking at Super Lead style preamps, I stumbled upon a statement that linked a lot of classic amps back to the Bassman, including the Super Lead and the Laney. That's what started this project of getting the Supergroup and Iommis Tone and go the modding route.
I'm also totally aware that you can't get 100% of the tone with just a preamp pedal. It's the whole chain: guitar, pedals, the full amp (preamp and power amp), the cab, the speakers... and then we listen to recordings of that tone so the recording setup has a huge infuence on what we hear in the end. Still, I think this will get me closer to the sound in my head than I am now, and I think having projects like this is a lot of fun. :)


You were right, I was tracing some of the power flow incorrectly (I haluzinated power on R15) on the Laney schematic. Thank you for pointing that out.
Now I get your point that the Sunn T Channel might be to close to the Laney so I basicly get 1 sound with a switch to choose between it. I might have to reconsider what to do with that second channel to get more variety.

Mu-Amp vs. Cascaded Stages: I've only just started reading about this, so I'm still learning. My understanding is that a mu-amp involves one JFET acting as an active load for another, with a direct connection between them. A cascaded design seems to be a series of Jfets connected by coupling capacitors or resistors. Based on that, the Covert circuit looks like cascaded stages to me. The Sunn T project from GPCB looks like mu-amps, or did I still miss something?
The JFET Shootout spreadsheet is helpful. If I understood correctly, and this circuit is cascaded common-source and not a mu-amp, then using a trimmer to properly bias the drain voltage seems almost mandatory if I don't want to buy loads of JFETs and sort them by hand.

Scaling the Tone Stack: Thanks for the resources, they're great! The stock tone stack in the original pedal seems works well, so I'm guessing this modified version will work too, especially since, as you mentioned, I plan to lower the output impedance by changing R8 to 100k, right?



If all of this still does not work, I will build Chuck's version of the Sabbra ;)

Thanks again for all the input, it's been incredibly helpful.
 
I just looked at the docs for the DLS Mk. I and the DLS Mk. II to try and understand this better. If I'm reading the schematics right, it looks like they both use mu-amp pairs (like Q2/Q3 and Q4/Q5 on both versions), while the Mk. III uses a different design.

This brings up another question about mu-amps. With a single stage, like in the Mk. III, it seems like you can solve the JFET situation by just using a trimmer for the drain resistor to set the bias.

But for a mu-amp, since you have two JFETs working together as a single unit, isn't it more complicated to bias correctly? Would you need to go through the trouble of hand-selecting the JFETs as a matched pair to get the right voltage, or is there a simpler adjustment for them that I'm missing?
 
Happened to me several times now where I read through your replies, try to understand everything, and out of that new questions evolve that I eventually ask here, only to realize afterwards that the questions had already been answered and I just didn't get it yet xD Part of the process, I guess...
 
Creo que todos hemos pasado por esto ;) I have piled up hundreds of bookmarks related to circuit design, reverse engineering, electronic & audio theory and so forth over the years. Every time I revisit once I stumble upon some info that I hadn't understood or that had gone over my head when I first bookmarked it. Most of the fun is in the learning process, at least for me.
 
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Hahaha, thank you, I'm glad it's not just me. I just was feeling a little bit bad about the little circles I created by not understanding as fast a people answered ;) It is fun, especially as I never cared for the internals and function of circuits, I only wanted tu assemble and then play them. I am actually surprised that I did understand a little bit of something xD
 
So can somebody please tell me if I understood right, because this is hard to grasp for me:
Mu-amp in general: JFET 1 acts as an active load for JFET 2 -> source 2 to drain 1

SRPP: JFET 1 gate is directly connected to voltage divider to bias (DLS II)
Mu-amp: JFET 1 gate is directly connected its own source to bias

The DLS I seems to have some kind of Mu but it does not seem to fit those types I described.
I do not see either of those in the DLS III (Covert) or at least they have a resistor in between, so is it a Mu-amp or not? ':)

So IF all three DLS types can be described as Mu-amps, Mu-amp design is a little bit more flexible as I initially thought (?)
 
Diego said:
... Part of the process, I guess...
Click to expand...

You guessed right.

I'm still learning and you're already asking questions beyond my ken.

If you take a look at the build-docs for the Aion Horizon and Aion Tritons (DLS I, DLS II, DLS III) respectively, you might get the answers you seek.
Also check out Aion's tracing journals.


My basic understanding of Mu-Amps is when I see a schematic with a couple of JFETs, one above the other and not in series — it's a Mu-AMP!

😸


Compare Escobedo's SyRuPP (which looks like a Mu-Amp to me 🤪)...

syrupp.gif



to Jack Orman's Mu-Amp booster...

miniboo.gif
 
Thank you for sharing all of this, it has helped me immensely!
The truth is, I did not really understand all this stuff ;) What I did is accept facts I read, for example that mu-amps are vertically stacked and are self-biasing, or that you might have to scale tone stacks according to impedance. I did not understand how or why. But I do not expect that would come in this one project, and I think it's time to build this project now and see how the circuit behaves in real life.

You have selected the perfect example. The Mini-Booster lokks like what I thought was a mu-amp because of the vertically stacked JFETs and the direct connection. But the SyRuPP is where I was not sure, because there is a resistor in between (the trimmer in that case), and because of that we don't have an active load (?), hence no mu-amp (?). And here you can also see that I did not understand the theory behind it, but have only learned how the arrangement looks.

As an update to the project itself: I decided to continue with the Aion Triton because it has the drive and tone stack switchable independently. For the drive section, I decided to commit to the Laney on one side and the Model T on the other, as the shared components are close with these two amps. Another advantage of using the Sunn is that its tone stack is very similar to the Laney's, so I can have a more distinctly different tone stack on the second channel. Here I decided to leave the Super Lead EQ and just see how it fits the two drive sounds. It's also nice with the Sunn that it uses a lot more gain on Q2, so I can see how the JFETs react to being driven differently and have a low and high gain channel. By accident, this has become an absolute Doom project xD
 
spi said:
This pedal is basically a clone of the TC Elec Spark Boost.

From what I can gather, Iommi was apparently a Spark Boost user (according the TC web site), and somehow Laney got Iommi to endorse their clone of it.

It's definitely not an amp simulator, but it is a nice boost pedal.
Click to expand...

I own it, I can confirm it's a very good booster, I use it with the mid boost option on but it's good in clean mode and fat mode too.
 
Feral Feline said:
You guessed right.

I'm still learning and you're already asking questions beyond my ken.

If you take a look at the build-docs for the Aion Horizon and Aion Tritons (DLS I, DLS II, DLS III) respectively, you might get the answers you seek.
Also check out Aion's tracing journals.


My basic understanding of Mu-Amps is when I see a schematic with a couple of JFETs, one above the other and not in series — it's a Mu-AMP!

😸


Compare Escobedo's SyRuPP (which looks like a Mu-Amp to me 🤪)...

to Jack Orman's Mu-Amp booster...

miniboo.gif
Click to expand...


I've never fully understood that 3.3uF cap, its function and how to decide its value, I often saw it in dist circuits with the same value of the cathode cap across the cathode resistor of the equivalent triode stage emulated, but I don't know the relationship
 
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