Modding a console amp for stand alone use

[M Jarve]

In another thread I posted about an RCA console I bought. I've already begun modding it- I removed the amp and will be reconfiguring it as a stand-alone unit. I've already purchased new speaker and input jacks as well as a power switch.

However, the input circuit on the power amp has no coupling caps in series between the input terminals and the G2 pin. I am assuming that this is a necessary component. Right now there is just a 10KΩ resistor between the center conductor of the input jacks and G2, and a 1MΩ resistor between the center conductor and the chassis. Is there any value cap I should be looking to put in there? Or, would it be safe to rely on the coupling cap in the source component (CD player in this case)?

I tried locating the caps on the pre/tuner portion, but I can't actually see where the coupling caps are. It looks like it goes 12AX7-> volume-> bass/treble-> balance-> out to amp.

Any recommendations would be helpful.

Attached is a pic of the resistors I described above.

 

[markus]

if I remember correctly, this is a weird one. . .

I don't think the driver for the 6BQ5 is on the amp chassis :scratch2:

I'm sure one of the more knowledgeable tube junkies will chime in :yes:

 

[M Jarve]

That could be, but the pre/tuner only has a single 12AX7A, which I don't think is being used as a driver. I was also under the impression that many EL84 circuits were designed so as to not need a driver. I must admit, though, that I am hardly an expert (or even a novice really) on the subject.

 

[Urizen]

That could be, but the pre/tuner only has a single 12AX7A, which I don't think is being used as a driver.

It could be in a stereo config., each channel using one side of a 12AX7.

Is there a rectifier tube?

 

[M Jarve]

That's what I was thinking on the 12AX7 as well. But I think it's being used in the tone/pre-amp, and not as a driver as such, as it seems to be inline before the volume control, and there is no apparent feedback loop going back from the power amp module to the pre/tuner.

It uses solid state rectification (two old metal-cased diodes).

 

[Kegger]

Can you find and post the schematic?

 

[gadget73]

The tone control circuit has gain to it, though how much depends exactly on the circuit. If its modeled anything like the one in the back of the RCA tube manual, its got a gain of about 2-3. I actually used that circuit to add a little more gain to my homebrew junkbox amp because it wanted just a little bit more than I was getting out of the pair of 6bq5's and the 12ax7 driver/phase splitter stage.

As for the input, just replace that resistor with a .1 uf or so and you should be fine. The value isn't real critical, so feel free to sub that for whatever you have handy. Just don't go too small or you'll lose some low frequency response. It'll take it from a resistance coupled amp to a capacitor coupled. You can get away with the resistor for a lot of things since many sources have a capacitor coupler, but just in case you happen to be running it on something thats got a little DC on it the cap is a good idea.

 

[M Jarve]

Thank you indeed- that's just what I was looking for.

I could not find a SM, but I do have the original sales brochure and instruction manual. Apparently this little gem output as much as 20 watts IHF or 8 watts EIA- probably 2-3 WPC continuous.

I can hardly wait to finish it up just to see what it can do.