[MrFixr55]

Hi Roger,

First off, you're welcome!! Solving problems like these keeps my brain going.

I am stunned that the radio worked as you described with a 6J7 or 6K7, or even a 6A8. In a "Cathodyne" Oscillator Mixer, the oscillator circuit is from plate to cathode, leaving the grid for the RF signal input to be mixed with the oscillator frequency, thereby producing the IF Frequency at the plate. The "pentagrid" mixer used the cathode, first grid and the second grid (the "plate grid") as the oscillator, and the cathode, 4th grid and plate as the Mixer, mixing the RF signal with the oscillator signal. (The 3rd and 5 grids serve to isolate the oscillator from the mixer and are used as screen grids, connected to a positive voltage lower than B+).
The triode-heptode used the triode as the oscillator and the heptode as the mixer. Since the triode grid is connected to a grid in the heptode, I can maybe see the 6A8 working but the 6J7 or 6K7, with no feedback to the cathode? WOW!! Well, most agree that Philco builds a good radio.

Is the tube diagram in your radio a factory diagram or is it missing. If a factory diagram, was 6K7 printed in the same font as the label or handwritten? If no label, were the tube numbers stamped on the sockets? If so, maybe someone grabbed the wrong one during assembly. The folks who built these radios were skilled assemblers who worked from pictorial diagrams, not schematic diagrams. Knowledge of electronics was optional.



In the late 1980s, American companies were getting used to some Japanese practices. One of these was a "JIT" (Just In Time) supply chain. Philco was fairly big into car radios (as was Motorola) and the "Loctal" (Locking Octal) or "Loctail" (Locking tail, another name for Loctal) tube, invented by (I believe) Sylvania for Philco were basically designed for Car radios. Philco was an early proponent of using car radio tubes in their household radio. Philco was one of the first companies to use tubes with 6V filaments and indirectly heated cathodes, such as the 36, 38, 41, 77, 78, etc., with filaments designed for 6VDC operation in household radios, finding that these tubes worked just fine on 6VAC to the filaments. The 7J7 and 6J8 are very similar in function, even in internal connections, the only difference being Loctal vs Octal base and the pinouts. It seems that they would switch from one to the other depending on availability or what deal they could get. Philco advertised and sold their own tubes but they did not manufacture tubes. They were manufactured mostly by Sylvania, who also manufactured many of Zenith's tubes.

BTW, how is the audio when you use the Bluetooth?

Definitely write back to this thread with the outcome when you get the 6J8! I gotta know!

Best regards,

John, MrFixr55