My smalley boost turned into a tremolo...how'd that happen?

[Hiwatt25]

So, here's a problem I've never come across.

I finished an LPB-1 this afternoon (another fine MarkM pcb layout) and plugged it in next to my EH LPB-1 nano to see how they compared.  They sounded pretty close and I was happy that everything had worked out.  I decided to throw my Smalley boost in there as well to see how I liked it compared to the LPB-1 and the strangest thing happened.

When I turned it on, it sounded just like a tremolo.  Granted it was a scratchy, dirty and largely unusable tremolo but it was a tremolo nonethless.  The Smalley was hooked up to a wall wart while both LPB-1's were running off of battery power.  When I took them both out of the signal chain, everything was okay again.  I put them back in and it happened again.

Now, I'm not too concerned about this since I don't gig and don't think I'll be using three boost pedals in series but I sure am curious how that might have happened.  Has anyone encountered such a thing before?  Is it common? 

By the way, I think I like the Smalley boost more.  I added an input cap switch (with the help of some other members) so it's a little more versatile and seems to have a little more gain.  Just FYI.

[Paul Perry (Frostwave)]

When an amplifier - or a bunch of amplifiers - turn into an oscillator, it's because of positive feedback. Just like a 'deliberate' oscillator.
The feedback might be via a small residual resistance in the power supply; it might be ultrasonic, via stray capacitances.
How can ultrasonic oscillaton produce tremolo? The high frequency gets rectified by a transistor junction, changing the bias & hence the gain...then it stops oscillating, restarts later & so on. The very first three transistor amplifier I ever made did that, as soon as I squashed it into a matchbox. (over forty years ago.. OC71s!)