FET substitution in ProCo RAT

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Lithium_Grim
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Post by Lithium_Grim »

Hi there,

More and more FETs are getting EOL and manufacturers are moving from THT to SMD. Nothing new so far.
Although 2N5458 THT are still available they will slowly disapearing from the market. Now the question is what more future proof replacements will be.
I am making a ProCo Rat and thought of replacing the 2N5458 THT with a J310 SMD (MMBFJ310) in the output buffer. It should be a direct replacement on paper. Or not? Any thoughts on that?

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modman
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Post by modman »

I think the FET is not crucial to the design: just used for switching the led:
soulsonic wrote: 02 Jan 2021, 05:18
andregarcia57 wrote: 01 Jan 2021, 21:37 I am thinking of building RAT2 on top of the TONEPAD layout,
I researched about variations from PROCO RAT to RAT2,
would these be the modifications?
thanks
I think the only real difference between the Rat and Rat 2 is that the Rat 2 has an LED bypass indicator with a JFET circuit to switch it (similar to RG Keen's Millennium Bypass). The 10K resistor to ground added right before the volume control is to lower the output impedance so that the JFET LED switch will function reliably. This is why the output cap was increased to 10uF, to keep the frequency response the same. Point is; the frequency response is the same and the rest of the circuit is the same, so as far as I'm concerned they sound the same. You could argue that increasing the value of the input tie-down resistors to 2M2 gives more low frequencies, but the combination of 22nF with 1M already gives such an extended low end that I doubt you'd hear the difference.
Another thing to note: the majority of Rat 2s made during the 2000's use the OP07 opamp instead of the LM308. But if you want a "classic" Rat 2, like is typical of a 90's era one, then use an LM308.
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Lithium_Grim
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Post by Lithium_Grim »

I might be misunderstanding your answer, but I was referring to the FET in the output stage, not the LED switch. So there it could/might make a difference and I was wondering what your thoughts were on that. For the LED switching it is irrelevant I agree.

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Post by modman »

Lithium_Grim wrote: 07 Oct 2021, 07:37 I might be misunderstanding your answer, but I was referring to the FET in the output stage, not the LED switch. So there it could/might make a difference and I was wondering what your thoughts were on that. For the LED switching it is irrelevant I agree.
I've seen too many schematics omitting switching and am clearly wrong.
I do have some of those Jfets I can send you if you don't live transatlantic...
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Post by Lithium_Grim »

Thank you! I still do have a bunch of them laying around. I was more thinking in the lines of learning how to read datasheets good enough to know if transistor A is a good substitute for transistor B.
Give the man a fish and he has food for a day. Learn a man to fish and he has food for life (given that there are still fish, a fishing rod an maybe even a boat).

I read that J310 was a good substitute for the 2N5458, but as the world changes even more rapidly and transistors going more and more obsolete I thought it was about time to learn how to determine via datasheets if A is a good (tonal-/response wise) substitute for B.

I guess in pedals there are 4 main applications of a transistor:

1. Buffer
2. Gain stage
3. Tone/EQ
4. Switching stuff (duh)

My main interest is to learn how to determine substitutes in the first 3 applications. Switching is not something I am really in to. Any help is deeply appreciated!

PS maybe even interesting to split it in FET and BJT?

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Post by plush »

Output stage is just a voltage follower. Pretty much any jfet with suitable vgss should work.

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EddieTavares
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Post by EddieTavares »

You can use a simple NPN bjt like 2n3904, 2N5088, BC549... It just have to conect the bias resistor to the 1/2 v (4,5 v) or add another resistor conecting base to 9v, there is no noticeable difference to my ears, maybe a little volume drop. I found these in the Proco Brat schematic.

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