[OldRestorer]

Heyo,
  Are these tubes just stuck in the chassis for looks or does this 20 have a different chassis?
OR is this the elusive rare model 20 the world has been waiting for?

[mikethedruid]

well, if the 45s are stuck in the 71A sockets, and the filament wiring has not been changed, they will burn out mighty fast since the 71As are wired for 5V and the 45s have a 2.5V filament.

[Arran]

Kirk
  I think that someone just grabbed some random four pin tubes and stuffed them into any empty socket, it would not surprise me if they were duds to boot. The #71As should be side by side near the middle near the rear apron, and the #80 should be in the rear corner next to the block condenser can. Nope, #45s don't belong in there, the filaments may not burn out straight away but that 5 volts will wear them out fast.
Regards
Arran

[Radioroslyn]

Hi Kirk,
The model 21 uses the 45 toobs but it also has a 14 mfd Mershon filter cap and not the rectangular can like yours. As others have said looks like a model 20 w/the wrong toobs in it.

Terry

[mikethedruid]

Looking at the model 20 schematics, it looks like 45s could be used in the push pull output easily enough by changing the filaments from being in parallel to being in series. The 2.5 V filaments would then add up to the 5V of the winding for them in the power transformer. Whether this was ever done or not, I don't know. It certainly COULD have been a WW2 mod if a repair man had 45 tubes in stock but did not have 71As. More likely someone just stuffed in some tubes to make it look "full." Poses an interesting question though.

[TA Forbes]

After you ensure it is electronically sound enough to fire up, measure the fil V for the output tubes with no tubes in the sockets. If it is 5V, it's a 71A output model. If it is 2.5v, it's for 45s. Another possibility is that a model 21 chassis somehow made it into your 20 deluxe cabinet. 

[Ron Ramirez]

Another possibility is that a model 21 chassis somehow made it into your 20 deluxe cabinet. 

'Fraid not, Tom. A 21 chassis would have the single large Mershon in place of the rectangular capacitor can on top of the chassis, as Terry pointed out.

And, yes, early production 21 sets used up the last of the 20 chassis...but those had 71A outputs.

[TA Forbes]

Ya, forgot about that! 

[OldRestorer]

Darn! I thought I was gonna be filthy rich!
Guess i'll have to be happy with filthy

Me

[Arran]

Looking at the model 20 schematics, it looks like 45s could be used in the push pull output easily enough by changing the filaments from being in parallel to being in series. The 2.5 V filaments would then add up to the 5V of the winding for them in the power transformer. Whether this was ever done or not, I don't know. It certainly COULD have been a WW2 mod if a repair man had 45 tubes in stock but did not have 71As. More likely someone just stuffed in some tubes to make it look "full." Poses an interesting question though.

   Whilst at first though it seems possible it's very unlikely that they would have tried that. For one thing type #45s and #71As are directly heated tubes, so in order to bias them correctly involves using either a center tapped filament winding, with a resistor going between that center tap and ground, or lacking a center tapped winding, a center tapped resistor across the filament of each tube. Another issue may be the filament current, #71A filaments run at about .25 amps a piece like an #01A, #45 filaments run at 1.5 amps a piece, so even in series configuration that's three times the current draw that the #71As run at. For another during the war years the earlier AC tubes were among the easiest tubes to come by since they had been in production for more then a decade, the hardest tubes to come by were the 150 ma types that were used in AC/DC sets since some types debut just before the war.
Regards
Arran

[mikethedruid]

Arran, if you look at the schematic, you will see that the tubes are biased by a resistor on the center tap. One could set the circuit up with the filaments in series and it would work OK. Besides the filament voltage, the other tube specs are really fairly close between the 71A and the 45. http://www.nostalgiaair.org/pagesbymodel...029582.pdf. I only suggest it because it is the only plausible reason to have 2 45s in the push pull section. As I said, I think someone just stuffed them in to fill it up.

[Arran]

Arran, if you look at the schematic, you will see that the tubes are biased by a resistor on the center tap. One could set the circuit up with the filaments in series and it would work OK. Besides the filament voltage, the other tube specs are really fairly close between the 71A and the 45. http://www.nostalgiaair.org/pagesbymodel...029582.pdf. I only suggest it because it is the only plausible reason to have 2 45s in the push pull section. As I said, I think someone just stuffed them in to fill it up.

 The simplest explanation is usually the correct one, that this was just someone grabbing some random four pin tube to fill some holes. I don't know where Kirk got this set but this is quite a common practice just to make a set look good in a photo.
  The bias resistor for the two #71As (which is one section of part #25) is in series between the center tap on the 5 volt winding and the center tap of the power transformer. I'm not certain about how well that arrangement would work connected across two #45s is series, one minor point is that it would be the wrong value, but more importantly the reason that they are connected to a center point across a tube filament is that they are also used for hum ballancing the two ends of the tube filament. In some radios they use a center tapped resistor across each tube filament, in others they use a an adjustable wire wound resistor, but I've never seen any radio that used two directly heated AC power output tubes with the filaments connected in series. Now if the total load of the #45s was only .5 amps, I'm sure that one could rig up a way around the existing bias network, but the two #45s will still draw a current load of 1.5 amps across a winding designed to accept 1/3 that much at the same voltage.
Regards
Arran

[codefox1]

Way back when folks used what they had to get the job done. Dunno the particulars, but the venerable 45 could do just about anything in an old AM set provided you tame the filament voltage. If the tubes are still good (have a vacuum and filament) , STOP! Someone want's 'em., not I.