RCA M1-12295 PA Amplifier.

[twiggybush]

I just picked up this 1950ish amp in excellent condition. I'm wondering if I have the building blocks for a RCA OP6 preamp of sorts.
http://www.theused.com/manuals/rca/r...2288_12299.pdf
Top of page schematic, link (hint, view,rotate 90,enlarge)
If I plug into mic1 then remove the wiper contact of R5 and feed it to J3 (phono1) I'd have 3 successive stages of 6J7.
I'm wondering if I could achive line out by placing a switchable jack at the junction of R10 andC8? This would leave the power amp hanging??? Not sure how to dress that off.
Am I crazy? I'm interested in the huge gain for use with my homemade ribbon mics.

[Steve Conner]

What a cool looking amp!

Now for some electronics theory. A ribbon mic absolutely needs a good quality step-up transformer designed for the purpose. Without it, you won't achieve an acceptable signal-to-noise ratio no matter how many stages of tube gain you use.

[twiggybush]

Thanks Steve, My ribbon mics have Edcor RMX-1 trannies. The amp has RCA input trannies (bottom right of photo). I agree about needing a good beginning. I'm shooting for color here so pristine is not expected. I'm encouraged by the match up of the mic as is. Slight roll off on the bottom but fairly stable. Noisier than I'd like. I was thinking that without hurting the amp I could play around with the 6J7 preamp section. This is my first experience with the 6J7 tubes and I like what I hear so far. Different, tart, kinda jumps up out of the hash. Very clean when at it's peaks. When I plug a guitar in the phono 1 and adjust it with mic 2 it sounds like a Deluxe. Tons of distortion. Not quite Metal but more than Crunch. The 3 spark plug looking thingies are the metal can 6J7 tubes.

[Steve Conner]

Well, that's good that you have the right amount of input transformers.

I guess there is no reason why you shouldn't add an extra gain stage, but if your DIY mic has an adequate output you shouldn't need one. Running the amp with more gain than it's designed for could cause problems with noise and hum pickup.

Adding a line out in the way you suggested will cause trouble because all of the preamp nodes are high impedance. It will struggle to drive cable capacitance and the inputs of solid-state equipment. To get you started, I would add a dummy load to the speaker output and use that. The output stage may seem oversized for a line driver, but it isn't really that much bigger than the line output stage of something like a classic Fairchild compressor. And the output transformer will give you some more "color".

You can break the connection between "0 ohms" and chassis, to get a floating balanced output. A couple of 100 ohm resistors in series would be recommended, to stop you from blowing the inputs of your solid-state gear if something goes horribly wrong.

And again, a dummy load of 8 ohms or 16, or 60, 250 or whatever tap you choose to use, is required to stop you burning up the 6V6s and output transformer.

[twiggybush]

Here's what I'm shooting for. Back in the day, you could hear string bass sweeping the floor with low end. Nat King Cole records come to mind. I believe the formula was a high output ribbon with a high gain 6j7 pre. I found an audio demo of an RCA OP6 clone at RCA OP-6/BA-2 Hybrid Mic Pre Amp: Listening Test/Shootout | Preservation Sound I heard it and the host said it "moved the room" unlike the other pre he'd compared. I'm not sure about my phase invertor or the 6V6's so I'd like to tap the 6j7's for their low end. So I'm a thinking for the same price in dummy load and resistors I could get a transformer output coming off the preamp. Multi stage preamping has always eluded me. I've experimented in the past but just get a lot of noise added. The original OP6 had claims of 95 db gain. I've a hunch that the peculiar response of the tubes allows this with out the noise build up. The difference between pentode and triode. Pentodes stay clean when triodes generate distortion. My mics are about average output for a ribbon so they need more amp when used at great distance to a source. Scott, thanks, you've been helpful in steering me where I'm going.