[George Guma]

I am looking for information how to convert a Model 47 DC Philco radio to AC operation. Is there anyone out there who has done this. Would greatly appreciate hearing about it.

[Arran]

Most people don't try to make these DC only sets operate from AC all all, they just sit them on a shelf an look at them, though I am sure that it is feasible to make one functional in it's original state. Since it was designed to operate from 110 volts DC you would need to operate the set from a 110 volt DC power supply, at least for the B+, that is without severely butchering the chassis. This is not to be confused with an AC/DC set, these radios do not have a rectifier tube or power supply built into them like an AC/DC set would have. If I remember correctly the tubes would be connected in series string, and would also use a line dropping resistor, much like an AC/DC set, but the B+ side would run directly off the 110 volt DC power line with maybe a filter cap to get rid of any noise.
I've never seen one of these sets in the flesh but I don't think they are all that common, the main market would have been limited to urban areas with DC power grids. One easily reversible mod that you could make would be to split the input to the 110 volt B+ line from the tube filament string, and bypass the ballast so you can run the tube filaments off of exactly whatever voltage they need.
It looks like the tubes in this set are all AC tunes with indirectly heated cathodes so you may be able to operate them off of AC, once the B+ circuitry is split off. The heater voltages end up totaling 87. 8 volts at 300 ma, you could build a DC power supply, with transformer, to run them but that's a bit on the heavy side, but something like a small isolation transformer may work. But if you are going to operate the tube heaters off of DC you might as well build a DC power supply with enough current capacity to supply the B+ as well.
One idea that may work is the diode trick, which drops the line voltage down to I think around 85 volts, then it would be DC but unfiltered DC, for that you could skip the power transformer. For the B+ you have no choice, you need a 110 volts DC, but for that you could build a transformerless power supply off board. So you have two options, run the set off of a higher current 110 volt DC power supply, with isolation, or go the cheap route, mod the set a little, and build a transformerless power supply for the tube heaters and the B+, with no isolation. I would try going with the former.
Regards
Arran

[Ron Ramirez]

I had a 47B for a while that had been converted to AC operation. Someone added a power transformer in place of the large condenser can, and an 80 tube in the ballast tube socket. It ended up being sort of like a 91 circuit in a 71 chassis.

Why not find and restore a 71 chassis and K-7 speaker (assuming it is a cathedral) instead if you want to be able to plug it in the wall? Restoring a 71 would be less hassle. You could then sell the 47 chassis and speaker to someone who would prefer to keep it original. (You will not be able to use the 47 speaker in an AC chassis as the K-13 field coil is only 70 ohms.)

[George Guma]

Hi,

Thank you for such a detailed and speedy reply to my Email.
I obtained my 47 several years ago and kept it collecting dust on the shelf till now, when I decided to resurrect it to life. Someone has apparently attempted to convert it to AC in the past but has given up on it after tinkering about and doing some rewiring which just made things a bit more complicated . I know that there are several ways to proceed and I have decided to abandon the series filament approach which appears too complex since not all tube filaments have the same current requirement, requiring compensating, heavy resistors. I have thus mounted a separate, small transformer for the filaments of the 6.3 volt tubes and a 25 volt transformer for the two "43" output tubes.
For the high voltage supply, I have introduced a full wave bridge rectifier in a straight forward supply circuit with two electrolytics and a filter choke. There are two chokes in the circuit already and I hope to be able to use one of them for this purpose, but I have not yet figured out a good way to do this. Hope you can suggest something.
My other problem is that the schematic for the set shows the output transformer in a typical set up with a center tap to the B+ supply, however the actual transformer on the speaker has no center tap, thus I don't think I can use it for the push pull set up shown on the schematic. I was thinking of wiring the two "43" output tubes in some sort of a parallel configuration. (hope you have a suggestion there).
Anyway, this seems a cumbersome way to go about it for a set that may not be worth the trouble, however, I am retired with plenty of time on my hands and this keeps me challenged and off the streets.

[Arran]

The tubes should all be 300 ma, at least the 43s are, but with 25 volt filaments. In fact if any of the tubes were more then 300 ma they could not operate them in series, this isn't a matter of opinion, it's a fact, no equalizing resistors used nor needed, just a line dropper. I think that you just made more work for yourself by separating the pair of 43s from the rest of the tubes, no need to re-invent the wheel here. The only difference between this set and a typical AA5, in term of series string, is that it has 8 tubes rather then 5, and six of the tubes have 6.3 volt filaments and two have 25 volt filaments, and the string has a 300 ma current draw rather then 150 ma.
Regards
Arran

[George Guma]

You are right, all the tubes have 300ma heaters, but for whatever reason, I dislike running filaments in series. I also found several heavy resistors wired in the filament wiring by someone in the past, but I removed them all without attempting to figure out why they were there and I just assumed that they were some sort of current compensating attempts. That is, however, not my problem. The difference in the schematic and actual output transformer configuration (no center tap) is something I am trying to figure out now.

[Arran]

Check the resistance of the speaker field coil, maybe someone swapped out the speaker for another out of an AC set like a Philco 60? If so that would explain why the output transformer does not have a center tap. Either that or someone did not understand that the set has a push-pull output stage, or intended to convert it to using a single 42 tube and replaced the original center tapped unit.
If someone added power resistors to the filament string clearly someone was messing around with this set before, and lord only knows what they had in mind. Unfortunately when people do things like this they seldom make a diagram, and we can't read theirto see what they were thinking of at the time. This is why when I run into such things I undo it all and try to return the set to the way it left the factory rather then trying to make it work with someone else's kludging in there.
Series connected filament strings are really quite efficient, the current consumption is considerable lower then if they were connected in parallel. They is one improvement that could be made over the original design though, and that would be to add a current limiting thermistor in series with the tube heaters.
Regards
Arran

[George Guma]

I also thought that the speaker had possibly been changed, but after examining it closely, it is quite obvious that it is original. Especially since the drawing of the speaker assembly on the Philco Service bulletin-No. 131 clearly shows only four wires connecting it to the chassis, while, unless I am mistaken, there ought to be five. It is a bit confusing and I wish there were someone out there who could clear up this somewhat confusing situation.