RCA 7-TR-3 conversion??

[Rick-S]

I had aquired many years ago an old RCA reel to reel. Now, at age 45 I'm wanting to learn guitar and being that I've always been an electronics type guy thought that that old tape recorder might make for a good fender champ type conversion.

It has 4 tubes, a 6x4 rectifier, a 5879 that I think is used for bias in recording, a 12AX7, and a 6AQ5 output tube. I have stripped the amp from the recorder and it will function as is if I apply a signal on the wires from the old head. I connected an mp3 player to those and got audio through the speakers.

I figured I'd have to strip all the components and start from scratch. I do have a few questions though.

1) The recorder had an external speaker connection that disconnects the internal speakers when one is plugged in. According to the manual, it states the external speaker impedence should be 3.2 ohms. Would a 4 or 8 ohm speaker be ok with that output transformer??

2) Would the 6x4 rectifier tube be ok or should it be swapped out for diodes or a different rectifier tube?

3) Being that the amp "works" as it is, would I be able to re-use the old capacitors or should I just replace them from the start?

Most of my electronics experiance has been in the digital world and I'm not familiar with any analog circuitry let alone that from 60 years ago Any help or thoughts on this would be greatly appreciated.

Rick

[Jazz P Bass]

1) The recorder had an external speaker connection that disconnects the internal speakers when one is plugged in. According to the manual, it states the external speaker impedence should be 3.2 ohms. Would a 4 or 8 ohm speaker be ok with that output transformer??

2) Would the 6x4 rectifier tube be ok or should it be swapped out for diodes or a different rectifier tube?

3) Being that the amp "works" as it is, would I be able to re-use the old capacitors or should I just replace them from the start?


Rick

Answer to #1: 4ohm speaker should be used. (DC resistance measurements are always lower\ ie: 3.2)
#2: Leave the rectifier in
#3: If the amp hums without any input, the filter caps are bad.

[Rick-S]

The reason I brought up the 3.2 ohm speaker is because that was what the owners manual for the tape deck stated was the impedance for an external speaker. Since when plugged in, it would disconnect the internal I assumed the internal speakers were also 3.2 ohm. I know 3.2 ohm speakers can be purchased, they are just pricey and I'm trying to keep costs as low as possible. That was the reason I was wondering if the 4 or 8 ohm would work. If the 4 will work, that would be good, I'll probably go for a Jensen MOD 4 ohm 8" for the speaker. This is afterall my 1st tube project and there is a good chance something won't work as expected.

Since my last post, I have stripped most all of the old circuit from the housing. I left the transformer wires attached to the rectifier socket, and left the filament wiring in place to all sockets except the 5879 (which I figure will not be used). I know the recorder audio had little noticable hum when it was intact so I am guessing that while old, the caps may still be good. However, I plan to replace most of them anyway. The main one I was considering keeping is the metal canned one that is soldered to the frame. This houses a 50 and a 30 uf cap in the one can. This is the first cap after the rectifier on it's outputs.

I've seen a few champ type circuits on the web, some of which use a 6aq5. Are there any that are better than another that you would maybe recommend?

Rick

[bigtwenty]

RCA 7-tr-3

Hello, I was wondering how the conversion was going? Also, I have acquired one of these recorders for $20 at a local thrift store and actually want to do some recording with it. Alas the reels arent moving and i need to figure out why. Do you still have the manual? I would be interested in getting some service info from you if i could as the one i have has no manual. if maybe you could snap some photos of the service pages well enough for me to read and email them to me i would be forever great-full.

Thanks!