• PA conversion amp

This amp lived the early part of its life as a Stromberg-Carlson PA amp. The power tubes were cathode biased and were drawing about 55ma apiece with 365v on the plates, so I thought it might be class A. It was wired point-to-point between components, hardly using terminal strips, so the wiring inside looked messy.It's gone through 2 incarnations as a guitar amp. The first was based on a Deluxe Reverb preamp, but since I had 2 12AX7 tube stages left over, I used one for a cathode follower to drive the tone stack and the other for an added gain stage in front of the the Deluxe-style preamp. I got this added stage's circuit out of Dan Torres book, "Inside Tube Amps."

The cathode follower offered a little boost in volume and more responsive tone controls. I found I didn't like the sound of the extra tube stage very much, and hardly ever used it.

Although it was originally biased near class A, I didn't like the sound and lowered the idle current to about 35 mA per tube. This got rid of what I heard as flabby bass, and made it a class AB amp. The power tubes were mismatched, so I tried putting separate cathode resistors on each instead of having them share one. The value of these resistors was calculated to account for the differences in the tubes, and to try to bring them into a more similar range of idle currents. However I found this wasn't necessary, that by giving each a 1K resistor, they acted like a matched set of tubes.

This was a decent amp although it covered the bases much like my other amps. I really didn't need more of the same, so after about 2 years I tore it down and built it into a multi-voiced amp based on Duncan Monro's design.

RG Keen has a good article on converting PA amps to guitar amps.