Sir Byron:

Oh. . . .WOW. . . . I think that you have now given us our dose of the start of an Ellery Queens mystery.

For my part. . . .I’ve got some questions. . .let’s see if you can supply the answers.. . .a la Radio Shaque

I am assuming that you have got a tube present in every tube socket in the set, and that there are no empty sockets. . .and those were the numbers that you have supplied us ?

Are there any shared common brands marked on more than 1 of the tubes. . . if still legible. . .and if it is being of a radio manufacturer label ?

Now if this is a bare bones receiver and if only using a mere 4 tubes, THAT is a bare bones receiver for a Super heterodyne. We can see that some of your given tube type numbers readily tie in to some of the required functions of making up a radio receiver.

Initially, there is the 25Z5 which would be the power tube which creates the DC voltage needed for the set.

Then there is the 25L6 tube which would be the final AF audio output amp which would be bringing up the sets audio to a speaker driving power level.

Now that leaves the 6D6, which would typically be an IF or RF amp function on a set with a few more tubes in its lineup. . . .BUT. . . . in this starved circuitry, it could be its sole RF front end entity, in the respect that it is serving as its mixer.

Should that be the case, with the set only using 4 tubes, then it would be necessary to allay the use of an IF tube entirely and dump that 6D6 mixer directly into a single I.F. transformer with its secondary then being detected with the diode of a tube and fed to a stage of audio amplification to then to drive that 25L6 AF output tube.

Setting up the next question. . .is there only one I.F. transformer can in the set ?

Soooooo. . .it seems to now be the necessity of solving the identity of that fourth tube in the lineup that you have given. . . . .and we certainly don’t see the use of any 6D4 thyratrons being used in an “Un American 4 receiver”, PLUS a 6D4 is in a 7 pin mini envelope !

Now, with your mentioning there being “caps” on both of the last listed tubes, that does tie in with the 6D6’s use, exactly, with it having a top mounted grid cap.

Now what I would really expect for the final tube with the top “cap”, would for it to be a 75, or in that family, such that its diode section performs the RF to audio detection function and then, that developed low level audio is passed on to the triode section of that same tube, for bringing it on up to that 25L6’s required driving level.

Another question from my series, would be for you to find the 25L6 audio output tube and realize that the AF input to that tube is going to be coupled into it from its previous audio stage via an audio coupling capacitor and the other end of that cap should be routed over to the wiring and circuitry associative to our mystery tube.

Just as a circuit aid, I placed a thumbnail at the bottom for clarification, it encompasses the utilization of the two tubes mentioned and you see that the capacitor in query is C1 with it going the red arrow route over to thee left and connecting to the plate of the 1st audio tube for acquiring its audio signal.

Now you want to trace down your “RED ”wiring and see what pin number of the tube socket that it is routed to and makes its termination. Then we figure what tube it might be.

Last consideration will be your forwarding the info on the tube socket ID of the mystery tube, be it in the octal category as the 25Z5-25L6 are, or is it of the large 7 pin sizing, such as the 6D6 is.

Lastly, in case the mystery tube is not the 75 or its equivalency in an octal tube, there is the final consideration of it using a common triode in that position and of the utilization of grid detection for the RF detection aspect and conventionally , that would be notable via the utilization of a higher resistive value used for its cathode resistor.

ZUJ’ing for your feedback. . . ..

Applicable Thumbnail:

Well with your pics referencing, one now sees that this radios Super- Hot –Rodent-Dyne design category, will now have to be downgraded down to a mere TRF unit, with you now mentioning that 6C6 as being the mystery tube.


* * *Fly-boy comment

That wooden cabinetry sort of reminds me of a Sonora. .or Fada . .. excluding the knob set, but others that may have seen these old units will have to confirm that being a possibility,


If one consults a Sonora, incorporating the A11 chassis, it seems like that circuitry design would assimilate close enough to make troubleshooting analysis possible using that info.


Now, considering by the photos that your frontal RF coil / transformer is still intact up at the front chassis corner nearside of the 2 section tuning condenser. . .BUT . . .the gaping hole between the 6D6 and 6C6 implies that the RF / detector transformer is cannibalized / missing from the set.


I also see the sets intact power supply filter condensers as being tubular cardboard cased units, downstairs.


In looking at the chassis hole in the frontal area of the 6C6, I’m wondering if an earlier set was using this same chassis pan and might have used a metal wet electrolytic filter capacitor mounted in that area.


The use of several common Arcturus branded tubes doesn’t seem to help in identifying a set maker tie in.


That’s about all that I can see from the data so far; maybe others can chip in now.