Part substitution and voltage question about fatpants

[HKimball]

I apologize if this is the wrong forum, but I didn't know where else to post this.

I'm interested in running a fatpants pedal on a 12v power supply, on the theory that the result will be closer to a true echoplex (transistors running at ~23v), and additionally I've heard (but can't confirm) that the dirt cheap MPF102 transistor is actually pretty close to the highly sought after TIS58.

My question is this - for sourcing parts, I need to make sure everything is rated for over 23v, correct? I know that C11 needs to be rated for at least 25v on the standard 18v build, so I imagine I need to look for one in the 30v range? Also, and this could be my noobishness showing, but does the 25v requirement mean that either of the transistors are already seeing voltages like that?

I also saw that there's a 9.1v zener diode which I imagine is part of the charge pump, should I substitute a 12v zener diode for this application?

I'm sorry for all the questions - I have only a very basic competency when it comes to pedals. The demos I've heard of the fatpants all sound fantastic anyway, maybe I'm overthinking things with the contemplated 12v conversion. If that's the case, don't be afraid to say so - again I'm very new to this. All help is greatly appreciated.

[midwayfair]

Yes, you need to increase the zener to 12V.

Use the LT1054 for the charge pump, but if you decide not to, check the datasheet for the maximum input voltage of the charge pump you want to use. Most will explode on 12V.

You should estimate a 20% tolerance in capacitor voltages, but the manufacturers already underpromise on their maximum voltages, so I would say that you should be fine with good quality 25V caps. Obviously if you can fit 35V or 50V caps on the board you'll be much safer, but I wouldn't say it's necessary.

The MPF102 is an excellent choice for Q1 -- it's what I use. They can take much larger input voltages. However, you need to be aware that the gain of the MPF102 is only 1/10th or less of the J201, so you will be getting very little boost if you use it in Q2. 24V compared to 18V isn't going to make much of a dent in that deficit. So you'll end up with a preamp that might give you a really, really tiny boost.

And the utility of changing it will be exceptionally minimal. You're only likely to hear a difference in the amount of distortion you hear (which is already very low) when using a very hot guitar signal. In return you'll be sacrificing the ability to use the boost ... as a boost.

[HKimball]

Thank you for the quick response!

So, from the datasheet I found for the J201 it looks like it can handle ~24v.

You said you have a MPF102 in yours, I imagine that means you're using the J201 in Q2?

Also, on an unrelated note I'd like to thank you for making those listening notes on diodes - they've been tremendously helpful!

[midwayfair]

Thank you for the quick response!

So, from the datasheet I found for the J201 it looks like it can handle ~24v.

You said you have a MPF102 in yours, I imagine that means you're using the J201 in Q2?

Also, on an unrelated note I'd like to thank you for making those listening notes on diodes - they've been tremendously helpful!

I think I used a 2N5457 in my Fatpants. I don't need a ton of boost, but it still offers well over 10dB of boost that way.

The input voltage I was talking about in the MPF102 is the |Vp|. I should have specified ... it's the voltage that the FET can take at the gate before distortion is introduced. In the J201, it's 0.8V. In the 2N5457, it's about 1.5V. Think of Vp as the "real" headroom of a FET -- it will save you from scratching your head if you build a FET-based project and try to get more headroom out of it!