Capehart-Farnsworth 3004MPA

[dobulee]

So this turns out to be quite the interesting little set! Thanks for everybody's help in finding some restoration aids.

This 1949 set has an integral AM/FM tuner, and a pretty hefty 12W 6V6 Push pull audio. IT has a 6AV6 phase inverter circuit set up in a configuration that I'm not sure I've ever encountered before (althouth the vast array of positive and negative voltage rails makes reading the schematic a bit cumbersome!)

Taking a page out of the RCA playbook, this is a split sound chassis, with the sound take off right at the converter oscillator! Interesting.

It's got a DC coupled 2 stage video amp, using both halves of a 12AU7, just like the RCA KCS Chassis of the era, except that it's DC coupled. If RCA's late 40's sets are any indication, that circuit leaves a little to be desired for video bandwidth, so I'm interested to see what the Capehart is like. The synch separataion and amps are pretty involved for the era...

It's likely to be some number of weeks before I get into this. But looking forward to it!

[old_coot88]

Originally Posted by dobulee

...IT has a 6AV6 phase inverter circuit set up in a configuration that I'm not sure I've ever encountered before...

That's a weird one. They coulda did it like this. Works great, albeit with slightly reduced gain due to no cathode bypass cap.

[dobulee]

A friend of mine has that turnover record changer! It's a mechanical wonderment to say the least!

Originally Posted by old_coot88

That's a weird one. They coulda did it like this. Works great, albeit with slightly reduced gain due to no cathode bypass cap.

Definitely! Spending some time looking at it in detail. It's sort of a quasi paraphase phase inverter? One of the 6V6 grids driven right after the audio amp/tone stack, the other being driven from the inverted signal of the 2nd 6AV6. What makes it a little bit more interesting is that capehart introduces some negative feedback around the 2nd 'phase inverter' 6AV6. I guess that makes the circuit a little more immune to gain variations in 6AV6's in order to keep the push-pull grid drives fairly balanced. A Cathodyne phase inverter would have used the same tube line-up, but been easier, better matched and fewer parts - maybe that one hadn't yet made it's way into hifi!

[old_coot88]

Originally Posted by dobulee

Definitely! Spending some time looking at it in detail. It's sort of a quasi paraphase phase inverter?

Back in the day it was called a 'grounded grid, common cathode' phase inversion scheme.
During half a cycle, say the grid of the 1st tube swings positive> cathodes go more positive, making the grounded grid of the 2nd tube negative relative to cathode.
Truth be known, the balance wasn't perfectly symmetrical but consumer grade with minimum parts count, and still be "push-pull". The balance was probably on a par with the scheme using the 6AV6 inverter. To tailor the gain, looks like they chose 220K for the plate load resistor and 220K for that series resistor going to the grid.