Tube amp schematic.. to FET pedal?

[jubal81]

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.

 

[Chuck D. Bones]

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

 

[fig]

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

 

[fig]

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

 

[fig]

This is how I fared...

 

[temol]

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

 

[jubal81]

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.

 

[fig]

@temol , extra credit?

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

 

[megatrav]

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?

 

[megatrav]

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

 

[fig]

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.

 

[Chuck D. Bones]

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...

 

[brokenwizard]

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.

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

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

 

[temol]

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.

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.

 

[fig]

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?

 

[megatrav]

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.

 

[fig]

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.

 

[fig]

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.

 

[fig]

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.