Proco Rat - oscillation/squealing

[Emil]

Hi!

As my first vero build I'm doing the Proco Rat. After some troubleshooting I've got it to the point were it works great on certain settings, but oscillates and squeals alot in many others. I have noted in the comment section that this seems to be happening from time to time with this pedal.

I'm building the larger layout version here: http://tagboardeffects.blogspot.se/2012/01/proco-rat.html

The squealing occurs in different circumstances and is affected by for example amount of distortion and value of the 560R/47R resistor (i'm using trimmers), pick-up selection and the volume control on the guitar. Depending on the pick-up the squealing will occur on a high or low volume/distortion. On the neck/middle position on my strat the squealing is almost gone and on my bridge pick-up it's there on most settings.

Unplugging the guitar also gives a continuous squeal.

Some notes on my particular build:

J-FET is BF256B. Notes in the comments suggested that any J-Fet would do, and I couldn't get the original so I ended up with this.

Trimmers on board are 500ohm and 200ohm. At the moment they are set to around 47R and 500R (or rather 515R when measured) I chose the 500R based on some notes that suggested I would probably want it lowered anyway.

I have added one column and added one row as you can see in the pictures. Just a precaution as this was my first build. The column ended up being used to accommodate the trimmers, which required the cap to be moved further out. The last row is not used.

My voltages are:

IC (LM308) min, and max distortion:

1 - 9.12   8 - 9.12
2 - 4.58   7 - 9.13 (9.16 on max distortion)
3 - 4.15   6 - 4.58
4 - 0.00   5 - 0.00

J-FET:
D - 9.16
S - 2.78 (3.19 on max distortion)
G - 0.00

I'm thinking that my 8 and 1 pin has too high voltages? Should I check for faults around this place? Or perhaps try replacing the 30pf cap (it's actually 33pf) with something higher? Maybe change the 500R trimmer to something higher? Could the J-Fet affect the sound here? Comments suggested that it wouldn't need biasing, but perhaps it does?

Would be very grateful for advice on this.

(Hope the pictures are clear enough, I should be able to get better ones if necessary).




Best regards
Emil Paulsrud

[Emil]

I just read some more on this site on these pedals sometimes needing buffers in the front. Placing my Boss TU-2 infront of it completely got rid of the squealing!

I guess I'll try building the small J-FET buffer (as shown here http://tagboardeffects.blogspot.se/2014/08/buffers.html) as a daughterboard and see what that does.

[Emil]

I built the smallest J-fet buffer as shown above and installed it before the main circuit. I'm happy to report that it got rid of the squealing, and it sounds great!

However the thing is now very loud! Unity gain is just a touch above the lowest setting on the volume knob.

How do I cure this?

I didn't have a 3,3k resistor for the source of the J-fet and replaced it with a 4,7k. Could changing the value of this resistor have something to do with this. Or do I need to do something else?

Any help would be much appreciated!

[Emil]

Any suggestions? It's incredibly loud.
 
Would it be reasonable to try lowering the value of the pot by adding a resistor to it? If so, to which lug(s) would I solder it in this case?

[Frank_NH]

You need to flip your JFET around.  Here's the pinout for the BF256B:

[Emil]

Hi!

I did do that mistake first and the sound was very faint, just audible at max volume.

It's kinda confusing but I think I got it right. I'm doing the first larger layout, and the pinout in that one is reversed from the second more compact one. And the BF256B pinout is reversed from the layouts. So I should have the drain on pin 7 and so on I think.

/Emil

[Emil]

Went back to this one again and found some badly soldered cables. Corrected and got it to work as intended. So this is solved now. Just needs to be boxed .

[Mr Fox]

how did you install the buffer before the circuit? Where did you connect the input and output?

[lasermonkey]

On a dual build I did (Rat + Formula No. 5) where the circuit order was switchable, I found that the Rat squealed quite badly and needed a buffer adding. In this case, placing the buffer directly before the Rat circuit worked. I would have placed the buffer in front of the pedal order switch, but the F5 *really* didn't like it.

Something that might be worth bearing in mind  is that some circuits are incredibly fussy about where the buffer is placed. I found the the Brown Dog and Tim (with boost engaged and gain on max) needed a buffer placed between the input jack and footswitch. Nothing else would stop the howl. I admit, I don't really understand exactly why the buffer position is so crucial in some cases. It still bothers me!

[Mr Fox]

I want to buffer the RAT to avoid the squeaks. See the image below, makes sense?