DIYstompboxes.com

DIY Stompboxes => Building your own stompbox => Topic started by: chumbox on January 18, 2019, 06:51:54 AM

Title: Built RAT clone on breadboard... oddly splatty signal?
Post by: chumbox on January 18, 2019, 06:51:54 AM
Hi All

Breadboarded a RAT according to the schematic at Electrosmash and it's working 95% good but as you let the guitar ring out and the signal lessens it gets quite splatty. Even more pronounced the lower you set the gain knob. Am I right in thinking the normal RAT pedals aren't like this so I must have incorrectly built something? I'm also a bit unsure of where to start looking, as I've been over the breadboard and it 'appears' to be matching the schematic.

Schematic I used is here:
https://www.electrosmash.com/images/tech/pro-co-rat/pro-co-rat-schematic-parts.jpg (https://www.electrosmash.com/images/tech/pro-co-rat/pro-co-rat-schematic-parts.jpg)

Thanks in advance


Title: Re: Built RAT clone on breadboard... oddly splatty signal?
Post by: italianguy63 on January 18, 2019, 07:20:19 AM
I've built from that schematic.. and it is good.

I'd expect be wary of your LM308N or 2N5458... lots of fakes out there now.

MC
Title: Re: Built RAT clone on breadboard... oddly splatty signal?
Post by: pokus on January 18, 2019, 07:46:01 AM
I think there are way too many options which can cause that. Some measurements can help to identify the exact problem.
As I had some similar problems with few pedals, I assume it can be located in the power supply circuit. What's the voltage across R10? Also make sure the 4.5V bias are correctly.

For a quick try you could also put/hold a 47uF cap from the V+ pin on your Lm308 to the V- pin (+ to 7 & - to 4) and if you're lucky it will work just fine.

A picture of your build will also help to identify. It can also be caused by the fact that there is lots of interacting on breadboard. Maybe boxing it will make your problem disappear.
Title: Re: Built RAT clone on breadboard... oddly splatty signal?
Post by: pinkjimiphoton on January 18, 2019, 05:01:28 PM
we need voltages.

check the resistors around the chip, i suspect one may be mis-valued by accident
Title: Re: Built RAT clone on breadboard... oddly splatty signal?
Post by: GGBB on January 18, 2019, 07:09:19 PM
In a RAT, that symptom, when not a build or component error, is often due to mis-bias of the output JFET. Check the voltages on drain, source and gate. It's the G-S voltage that's important - it needs to be within the Vgs-off specs for a 2N5458 so that the JFET is ON. If you have an audio probe, check the sound before and after the JFET.  With a fake 2N5458, or even a real one that is near the edge of the spec, an otherwise perfect RAT can malfunction like this. A quick and easy fix is to add a 1M resistor between drain and gate (a la Turbo RAT). Or find another JFET that works properly (doesn't have to be 2N5457 - a 2N5457 or MPF102 can work just as well).
Title: Re: Built RAT clone on breadboard... oddly splatty signal?
Post by: thermionix on January 18, 2019, 08:20:39 PM
mis-bias of the output JFET

+1 BTDT
Title: Re: Built RAT clone on breadboard... oddly splatty signal?
Post by: chumbox on January 18, 2019, 09:45:13 PM
Hi All

Thanks for the great advice. As a simple first step I disconnected the buffer and ran the signal straight to the output pot and suddenly it worked fine. So I put the signal back into the JFET and ran a 1M resistor between drain and gate as GGBB noted and it worked great. Didn't think it was worth changing the JFET as the others I had were all from the same cheapo source haha

Thanks everyone! Just changed the diodes to red LEDs and sounds amazing as expected. It's pretty heading into Turbo Rat territory now.

 :)
Title: Re: Built RAT clone on breadboard... oddly splatty signal?
Post by: italianguy63 on January 19, 2019, 02:43:30 AM
The cheap source 2N5458's (China) I have seen lately have been out of spec.  (gain too low).

Glad you found a work-a-round!

MC