• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

RCA Hi Fi Amp Circuits

Thank you Mark for pointing this out to me.


  • RCA RIAA pre follower.jpg
    RCA RIAA pre follower.jpg
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"LOAD = 50000 OHMS MIN"

Also, if you look at the cathode load resistor for the output stage 12AX7 in that circuit, it looks like its value is only 15k ohms. The voltage chart says there should be +19VDC on pin 8 (the cathode of the cathode follower section).

That means the plate current for that stage is 1.2mA, which is plausible. But a 12AX7 running with a 15k load? Yuck. I wonder if that was a mistake...


2018-11-18 9:13 pm
Pentode input is king for phono stages. Apart from that the circuit isn't that interesting and can be "bettered". To each their own but a CCS as anode load and gyrator on the screen grid instead of R4 should make it possible to remove R5 and to some people "sound better".

FWIW: the circuits in the RCA manual are all solid and will give solid results with little issues but they were aimed at industrial customers that buy 30000 tubes from RCA to deliver the crates of radios with the added phono input they are comissioned to deliver to the retail chain. So the focus is on ease of build, budget friendly, no need to fine tune anything, stable and (back then) serviceable.
The issue with pentodes in the 1st gain block of phono preamps is S/N performance. Partition noise is an unpleasant "fact of life". The nice EF86 is a marginal performer, in the 1st gain block role.

High gm is a plus, in the overall S/N performance "picture". At 1 mA./V., the gm of the 5879 is unimpressive. Those individuals accomplished at setting RIAA EQ networks up (I'm not) can consider the 6AC7, whose gm is a substantial 9 mA./V., for 1st gain block duty.
Wavebourn has been using the 6F12P for the front end of his power amplifiers. Says it is the best tube ever. He started the trend. Maybe Anatoly will drop in for a comment.

I've tested some on the curve tracer here, they do look good. Haven't used one in an amplifier yet. It does seem to be catching on lately. They -were- cheap, not sure about that now though.

One example: