[Ron Ramirez]

Hello Mark,

I saw your post over at the other place.

Terry (Radioroslyn) has already answered most of your earlier questions, but I'll try to tackle the latest ones:

So the autodyne is the oscillator/mixer of the day that wasn't able to include a pentagrid, because it wasn't available yet?

Right. Philco started using a tetrode autodyne circuit in the 2nd version 70, 3rd version 90, and model 51. This was early 1932, and the mixer-oscillator tube used was the 24 (later replaced by 24A). Philco's new radio line came out in June 1932, including the 71 and 91 which used the 36 tube for this purpose.

Models 19 and 89 came out around January 1933. Philco continued to use a 36 tube as a tetrode autodyne mixer-oscillator. The 6A7 pentagrid converter did not become available until later in 1933. With its shortcomings, the 6A7 was a much more efficient mixer-oscillator than using a tetrode as an autodyne oscillator.

For all of their technological innovations, Philco sometimes pinched pennies when they possibly should not have. For this reason, they never adapted the 6A7 to their model 19 or 89, although newer Philcos such as the 14, 17, 18 and even the lowly model 60 used the 6A7.

So how does the oscillator/mixer work using the 36 tube? How is the design different from one using a pentagrid?

Here's an extremely simplified answer. Anyone else here is welcome to elaborate on the following:

The 6A7's first two grids are dedicated to the oscillator circuit; G1 serving as a pseudo-triode grid and G2 as a pseudo-"plate" of the oscillator circuit. The RF signal is applied to G4. G3 and G5 are connected together and act as the screen grid for the entire tube.

G1 and G2 are part of the radio's oscillator tank circuit. The incoming RF via G4 mixes with the local oscillator signal produced by the oscillation in G1/G2. This mixing action produces numerous signals, but the radio only picks up the difference between the local oscillator and the incoming RF - the set's intermediate frequency (IF), which is 260 kc in the 19.

In the 36, the tube is oscillating 260 kc higher than the incoming RF signal which is applied to G1 of the 36. It looks simpler than the 6A7 circuit, and it is. Being a simpler circuit, it also makes the circuit less efficient and prone to troubles that can make the oscillator stop oscillating. Radios using the autodyne circuit are very sensitive to moisture absorption in their oscillator coils, and this will usually stop oscillation in Philco sets using the autodyne mixer while a mixer using a 6A7 might continue to function.

In other words, the autodyne circuit is more finicky.

Now that's a lot of verbiage not explaining things really well. Like I said, anyone is welcome to jump in and explain it better than I did.

What do you think about following the available tech tip to replace the 36 with a 77 tube?

I would not do it. The 89 and 19 sets can be made to work again using the original 36. You just have to work harder at it. Count on the "tickler" winding being bad, as Terry said earlier. Also count on having to bake the oscillator coil in an oven at 200 degrees (no hotter) for 30 minutes to drive out any moisture. Baking should be done before the tickler winding is rewound. If your resistor (10) is 15000 ohms, plan on replacing it with a resistor of 10000, 8200 or even 7500 ohms so that the circuit will oscillate properly.

If you're going to change the oscillator tube to something else, you might as well go all the way and change it to a 6A7. I would only recommend that to an experienced radio repairman, though.

Edit: I see Terry posted again as I was typing.