Power transformer in a 12 volt/25 volt radio? - Printable Version
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+--- Thread: Power transformer in a 12 volt/25 volt radio? (/showthread.php?tid=15753)
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RE: Power transformer in a 12 volt/25 volt radio? - morzh - 11-29-2016
When I am suggested sometimes "make a diode rectifier and replace the tube" I always reply: "why not then just to put a transistor radio inside".
To me it is like this: if a part is made of unobtainium hen's teeth, than yes, some replacements are unavoidable; else try to stay as original as possible.
RE: Power transformer in a 12 volt/25 volt radio? - ccomer1955 - 11-30-2016
Yeah. I understand the distaste for that, but in this case, I was a bit worried about voltages on the HV secondary. Even changing over to 6 volt filaments, the transformers I'm seeing seem to either be way above the 160 per plate I need or just under it.
By the way, am I right in thinking that I will need around 3 amps rating for the filament secondary?
RE: Power transformer in a 12 volt/25 volt radio? - morzh - 11-30-2016
Well, I don't know which tubes you are going to use, but if all of them are 6V then just add the filament currents of all used tubes and use 30-50% margin.
RE: Power transformer in a 12 volt/25 volt radio? - ccomer1955 - 11-30-2016
Six volt equivalents all the way around, so: 6X5G, 6L6G (or a 6W6G), 6SQ7, 6K7GT, 6K7GT, and a 6SA7. So that is .6 amps + .9 amps (or 1.2 amps if I use the 6W6) + .3 amps + .3 amps + .3 amps + .3 amps...so around 5 amps, then.
RE: Power transformer in a 12 volt/25 volt radio? - morzh - 11-30-2016
2.7A + 1.3=4A.
No need for 5A.
RE: Power transformer in a 12 volt/25 volt radio? - Arran - 12-01-2016
I have a better idea, forget about the 25L6 and it's goofy tapped winding, and replace it with a 50L6, then connect the rest of the tube heaters, and it, in series with a dropping resistor or capacitor. Then you can order up a more generic power transformer with a 6.3 volt winding, for the 6X5, and a 160-0-160 volt high voltage winding. I don't know why the power transformer went bad in this radio but the 6X5 was likely not to blame, at 160 volts per plate it wasn't being overvolted enough to arc over, and since the set uses AC/DC tubes it wasn't being overloaded current wise, so it was probably a shorted filter cap to blame.
Regards
Arran
RE: Power transformer in a 12 volt/25 volt radio? - Dennis Craft - 12-19-2016
I agree with Charlie, switch to the 6 volt filament equivalents and use the 6W6 in lieu of the 25L6. The 6W6 output impedance is very close to the 25L6 which would help to preserve audio quality. I've done this in the past, but switching from 2 volt filaments to 6 volt versions. It still preserves the authenticity of the set until a correct transformer can be found. Maybe place a note inside the set as to what you have done and why.
Regards, Dennis
RE: Power transformer in a 12 volt/25 volt radio? - Arran - 12-25-2016
If the original transformer is actually shot just treat it as an AC/DC set but with a power transformer for the B+ using a 50X6 as a full wave rectifier. Or you could forget about the transformer entirely and use the 50X6 rectifier in a full wave voltage doubler circuit from a Philco 46-1201. One thing to keep in mind are the maximum ratings of the 50X6 and the 35L6/5OL6 if you go the transformer route, the typical 350-0-350 style of high voltage winding is way too much, I think that 180 volts is the maximum.
The only logical reasons I can figure for why they designed this set the way they did was because they either got a good deal on some 25L6s and 6X5s or they couldn't get their hands on some 50L6s, other then that it would have been due to the whim of the engineers at Stewart Warner. The only thing that I have seen stranger then this was some of the brand Z and G.E sets that used an autotransformer, most of the AC sets that I have seen with the tubes in a series string arrangement used the transformer for isolation of the B+ side and to give the power output stage a bit of a boost.
Regards
Arran