Wow! I have to try your suggestions. This project has been on hold through the pandemic but I’m dusting it off now. I was getting my PCBs done by a Chinese company and shipping was a nightmare.

The gain reduction would actually be useful, these things are really hot. I have a few prototype boards to test on, I will report back.

I’d be happy to share the eagle files if anyone is interested.

By adding a 33k resistor between Q1 collector and Q2 base the FFT harmonics improve to 2h=-56.4dB, 3h=-54.8dB, 4h=-69.9dB, 5h=-81.3dB. The cost in gain is about 5.6dB.

The distortion characteristics can be altered and further decreased by increasing the value of R2 (at the expense of more gain), but not as effectively as adding the 33k resistor.

Such a simple fix to isolate the non-linear loading of Q2. Seems even RCA didn’t understand GOOD solid-state design in 1968. No wonder solid-state has a bad rap today.

Simulation notes: SK3020 transistors are small signal Silicon NPN transistors with hfe of 150 max (reasonably close to a 2N3904). Vcc = 19 volts, Input = 1 kHz, 1 mV peak sinewave with a 600 ohm series resistance, Added Rload of 100k at the output.

wow that is super cool. I’d love to buy some PCBs from u if u have extras you would sell.

Hey, yours is the only mention of this stuff I have found.
I’m putting together a console using the “High Dynamic Range Microphone Preamplifier” is this manual.

Check it out here:

https://imgur.com/a/nxigtky

hi steve. no, i have not, but thanks for reminding me. the 1/2 -completed-project-pile is pretty deadly round here…

http://www.amazon.com/Robert-M.-Bourque/e/B0027N9K2C

The theory and servicing of AM, FM, and FM stereo receivers

Don’t be scared off by the insane price listed in Amazon: I see them on ebay in the $15 range somewhat commonly.

ARRL manuals from the late sixties on also have a lot as do GE and RCA transistor manuals.

I always do don. But hum can also get induced into input transformers due to those big plate/filament transfos. I agree that dc heaters are the way to go, i swear by it for tube pro audion builds.