tape recorder to amp conversion

[dmartn149]

I bought an old RCA tape recorder at auction last night. I thought I would rebuild it into a 5f1 champ style circuit. There is a lot of extra circuitry in the chassis so I figured I would take it all out and start from scratch.
I'm not sure what to do with the extra tube. It's a 5879. The other tubes (the ones I'm planning to use) are 6x4, 12ax7, 6aq5. Can I just leave that one socket (for the 5879) disconnected? What effect will that have on the B+ and heaters for the rest of the tubes?
Any advice? warnings?
Thanks

[Justin Thomas]

I'd try it as a preamp tube channel... sounds like it could be an "American" AC4. High gain, low hum, what's not to love? The 5C1 had a pentode preamp... at least worth trying before scrapping it entirely.

Justin

[dmartn149]

oh yeah, I didn't realize it was a preamp tube. That's what I'll do. Thanks.

[dmartn149]

I'm looking over this amp before I take it apart and I notice that all the heaters are wired in parallel but the 12ax7 has resistors between the heater supply and the tube. What would be the purpose of this? They all run on 6.3v, right?

[Justin Thomas]

What's the values? Maybe they lowered them a tiny bit to extend tube life?

Justin

[dmartn149]

Looks like grey, red, gold, silver to me. 8.2ohms

[Jazz P Bass]

I'm looking over this amp before I take it apart and I notice that all the heaters are wired in parallel but the 12ax7 has resistors between the heater supply and the tube. What would be the purpose of this? They all run on 6.3v, right?

If the resistors are 100 ohms (or there abouts) I would say that they are being used as 'hum balance' resistors ala Fender.

[olddawg]

Just for the hell of it... Before you do anything ... Try using it as an overdrive preamp into another amp. It might surprise you.

[Justin Thomas]

A la Ritchie Blackmore? That'd work for me!

Justin

[dmartn149]

Just for the hell of it... Before you do anything ... Try using it as an overdrive preamp into another amp. It might surprise you.

Do you mean sending the output from this amp into the FX loop of another? Before I did that I would at least have to replace the filter caps.

[olddawg]

Nope... Take the line out straight into the front panel input. Tape recorders by nature are pretty low noise and pack some gain. It's like a tube OD pedal.

[dmartn149]

That might be an option to add while I'm converting it into a guitar amp. Would I just tap into the signal between the last coupling cap and the output tube? Or is there more to it than that? I don't think I want to try it right now. It still has all its original caps, and while it does amplify a guitar, it also squeals while doing so.

[olddawg]

Are you sure it doesn't have a like level out?

[dmartn149]

Well, It does, but according to the manual the line in is also the line out depending on weather it is in record, or play mode. Now that the chassis is out of the cabinet it's kind of hard to tell what mode the switch is in. It's a rotary switch that was operated by a rod and lever connected to the tape transport controls.

[dmartn149]

I've stripped all of the components from the chassis that I won't be using and I'm starting to make a list of parts to order. I have a schematic for a "Micro Champ" I have a couple of questions (for now)
I don't know what the output from my PT is. Do I need to? how can I find out?
The schematic shows an extra power filtering stage (compared to gibson GA5) Is that really necessary or desirable?
Also shouldn't R1 and R2 be connected to ground? or to put it another way, shouldn't that lead that goes to C10 and R15 be on the other side of C10 and R15?
ThanksMicro-Champ-R2B.pdf