[Ron Ramirez]

Hmmm. All this talk of 6A8 vs. 6J8 tubes has me curious. I have a 39-55 out in the garage; I picked it up really cheap to have an extra stepper unit in case I find the one in my 41-616 is bad once I get started restoring it. I'll have to see what kind of det-osc tube it has in it.

Overly simplified comparison of the 6A8 and 6J8:

The 6A8 does not have a separate oscillator section, but the first two grids in the 6A8 perform the same function as the grid and plate of the triode section of the 6J8. In the 6A8, G1 is the oscillator grid and G2 is the oscillator "plate." The local oscillator signal produced here is mixed with the incoming RF signal from G4 to produce a third signal, or intermediate frequency.

In the 6J8, the (separate) oscillator grid G1 is connected to G3 of the heptode (mixer) section in order to inject the local oscillator signal; this is mixed with the incoming RF from G1 of the heptode section to produce the IF frequency.

In the 6A8, G3 and G5 are dual screen grids; in the heptode section of the 6J8, G2 and G4 are the screens. The heptode section of the 6J8 has an additional suppressor grid (G5), tied to the common cathode.

Compare both and you will see they have essentially the same pinout. Pins 2 and 7 - filament; 3 - plate; 4 - screen; 5 - oscillator grid; 6 - oscillator plate; 8 - cathode.

Can they substitute for one another? The tube substitution guides say "yes." While they might pass in an AM-only set, if the set has SW you should stick with the tube the set was designed for. Some even advocate using a 6K8 (a triode-hexode converter similar to the 6J8 but without the suppressor grid) in place of the 6J8.

Remember, any substitutions will affect alignment.