I breadboarded the PET last night and after debugging got it to work this morning. The only problem is that I get a throbbing sound through the amp when I am not strumming. The throb is controlled by the rate pot. Any ideas?

It's LFO feedthrough.  Can you link to the schematic?  Can't guarantee there'll be a fix, but if you link to a schematic some bright spark might think of something.

What's doing the modulation?  If it's a FET sometimes subbing in a different one (type) helps (one with lower gate capacitance).

It could also be layout.

Here's a link to the schematic:

http://www.guitarraonline.com.ar/lapedalera/modulacion/Pet.gif

I got this problem in mine too. And when you change the 1m pot setting, you will get volume changes, then you will need a clean booster after the tremolo.

The audio is riding on the back of the LFO signal and the LFO output is feeding directly out to your amp so I don't think there's a fix for this.

Perhaps making the .022u output cap smaller might help, but I doubt it.


Try the EA?

Thanks for the help Gez/Marcos. I'll try the smaller output cap first. If that doesn't help I'll have to give the EA a try.

Are there any pitfalls I need to watch out for with the EA off the top of your head?

Thanks again!

Quote from: PhiredogAre there any pitfalls I need to watch out for with the EA off the top of your head?

Make sure you use a high/highish gain trannie for the LFO (the transistor connected up to the LED in the runnoff version).  Make sure pinouts are correct.

There are about a million posts in the archives and most of the problems boil down to one or both of the above.

Edit:  and read the original article, it explains how it works and what tweaks are necessary with the resistors connected to the JFET to get the depth right (at least I think it does, it's been a while since I read it).  

LFO Caps, and transistors. Dont cramp the LFO section, leave probe access to connections above for debugging...I found leaving 1/16'' of lead accessible makes it easier...for a while I was going for ultra compactness, with standing resistors...fine if it works first time...I try to leave a little room evenly spaced on the board [Obviously if you're dipping the board, this is Moot].
 I liked using the nonpolar caps, theyr'e much easier to orient correctly.
 Changing the value of the LFO caps you can change the speed range of the speed pot.
 I whipped out the EA today, very nice BOX...sounds great and nice boost toboot.