While digging around, I came across a really simple circuit in the RCA Receiving Tube Manual describing a 1WPC stereo amplifier using only two tubes, the 60FX5 power pentodes, and a single silicon diode. The 60FX5s have high enough gain they don’t need a driver stage, and were designed to use a crystal or ceramic phonograph pickup which is compatible with the output voltages of most modern electronics, too.
Looks pretty easy. This is something like you might find in a portable suitcase record player or similar, on the lower end of the cost spectrum.
If I were going to build this – and I might if I ever get a spare minute not working on repair projects – I’d probably start with an isolation transformer like the Triad N68X for safety, and eliminate the 0.22M resistor and 0.1 uF capacitor in the chassis network. The diode would be a 1N4007, naturally. The output transformers might be a tougher, though. I’m not sure of the specs on the Triad S-16X they specified, but with a low-power, economy amp like this one would probably not have met the day’s hi-fi spec, 40 Hz – 15 kHz. Small output transformers just don’t have enough iron to really couple bass well, among other things. There’s the Edcor XSE10-8-3K, offering 70 Hz – 18 kHz +/- 1 dB at $19 a piece. A transformer that’s flat 20 Hz – 20 kHz would be massively oversized and cost considerably more, like the Edcor CXSE25-8-3K coming in at over $90 each unit.
The 60FX5 tubes themselves are about $8 a piece on eBay. Ceramic 7-pin sockets are only around $2 each, too. The controls are probably about $10 – I’d just use a dual 1 Meg audio pot, and find a 2 Meg trimmer for the balance control and pre-set it during construction. The rest of passives would cost about another $10, a power cord, a cake pan from the grocery store for a chassis and you’re at $100 in parts to build the amp from the ground up. You’d need to use very efficient speakers, though!
If anyone builds one of these, and you send it to me, I’ll measure it’s specs with the Audio Precision analyzer!
I just bought an Emerson model 937 suitcase record changer from Ebay. I was going to repair it and give it to my sister as a birthday present. Her rec room has a lot of 60’s era stuff and this would fit right in. The record changer has two 4″ speakers in detachable wing enclosures on either side of the center case. I sketched out the schematic of the amp and with the exception of a couple resistor/capacitor values, it is nearly identical to the amplifier circuit diagram shown here. It has separate volume control for each channel so no balance pot. I am going to change out the 60 year old capacitors, re-tube it and see how it works. I am anxious to see it play.
Webcor made a stereo phono – The “Studio” in the 1950’s with a very similar design.
I just happened to find one today for free on Craig’s List!
Will be a fun project…
I spotted this too in the RCA Receiving Tube manual. This is a cool little design. I wonder why they spec’d an audio taper pot for the balance control?
Too bad someone doesn’the make this amp as a kit. Would make a nice project for a first time builder.