The owner had recently purchased the amp and found that some of the tone controls weren't working. I was shocked when I opened it up and saw the mess inside. The output tube power supply was stiffened to an incredible extent, and that small transformer was installed to provide a very low voltage supply to the preamp.
The preamp circuitry was also hacked to bits (almost every PCB trace had been cut), and that epoxy-block near the inputs encased a solid-state distortion circuit. Sadly, two additional holes had been drilled near the inputs for a "gain" control and a "lead" input, which didn't work. The presence was moved to the back panel where the line out used to be.
Needless to say, I ripped out all these silly modifications and returned the amp to stock (though I did replace the master volume control with a post-phase-inverter circuit) so the owner could hear what the amp is supposed to sound like. The 2204 circuit is a great platform for modification, but there are much better ways to do it.
I also replaced the electrolytic caps and installed some extra protection features to keep new production EL34s happy.
