BillyTone - Conversion of RCA400 projector to guitar amp.

[jmccanna]

Hi,

About two months ago I was at a garage sale and came across a beat up old RCA 400 projector. It was missing the reels and the arms and the belts to drive them. However, I could see that the tube amp was still in there with tubes! I bought it for ten bucks. There was no schematic available for this particular model but I found one that was close. I took it apart and it was a mess. Spider webs, rat dung, leaves and grime. I took it apart and cleaned it up. It actually turned on with my variac.

So, first thing I cleaned it out and just studied it. Its tube complement was definitely vintage with a 6J7 preamp, some 6v6s and a 6SL7. There was an extra 6V6 that was used to pull audio off of the 16mm tape (exciter circuit!).

I replaced the caps and resistors that were out of whack. I adjusted the input on the 6J7 for a guitar input and it worked. A little odd sounding but clean and loud. I had pulled the extra 6V6 because it was not needed but then after some poking around realized I could rewire the socket and fit in a 6SL7 (precursor to the 12AX7) and insert a simple reverb circuit with a reverb tank. I put in a three-wire grounded wire and a fuse too.

I generally followed the Carolina Blues Special schematic (attached) for the reverb (replacing the 12AX7 with the 6SL7). I figured out where to pull the signal and return it.

Then, I mounted it is a box I made from material on hand. Wood from a defunct speaker cabinet, a red lamp lens from my Hammond project and a steel plate from the bottom of what was the amp chassis from a Baldwin organ. Since the amp was mounted vertically in the projector I had to figure out how to mount it in the box. The chassis metal was not a uniform rectangle. I learned how to rivet legs to the chassis to level it out. It is rough looking so I call it BillyTone. The reverb tank is mounted upside down from the top of the box.

The bottom two knobs are the original gain and tone though I converted the tone pot to a volume pot. The upper two knobs are a dwell circuit and a reverb return. Take a listen at:

https://www.youtube.com/watch?v=jt_7wuWhk_4

Next I need to work on wire routing a bit because it was very cramped in there and I think with some more canoodling I can better the noise floor.

I learned a lot from this forum when I worked on my Hammond AO-63 conversion and it translated well to this.

Thanks, James

[sluckey]

Time to celebrate with a Billy beer!

[jmccanna]

Honestly, that is about the best and most perfect response I have seen! I think I will do that!

[jmccanna]

I thought I might add if it was not obvious - the photos are the before the modification.

[Phil_S]

That's a feel good find!

[Tony Bones]

Very good find! How do you like the tone controls?

As a side note, I'm a little impressed with the original RCA circuit. In particular the ground bus. Some of those old P2P amps were kind of haphazard.