[morzh]

I never gave it a thought before, as today except for portable and farm sets we power the rest of them off of MAINS, but when in an ACDC set the drop cord gets replaced with a cap, the set stops being an ACDC and becomes AC only.

Though this is not even important anymore, I am writing this for those relatively new to the field so they did not expect it to work off of batteries or any DC source.

It won't blow up or anything, simply the filaments and pilot light won't light up.

[Arran]

Mike;
Another "Curtain Burner" replacement that won't work on DC is the silicon diode one, or the use of a bucking transformer. In fact the capacitive dropper and the silicon diode trick won't work on alternative power line frequencies, if you set it up for a 60 cps power system it will not work on 50 cps, or anything other then 60 cps. Of course there are not any 120 volt 50 cps power grids that I am aware of, and the European systems that are 50 cps are all 240 VAC, but it will cause a problems even if a 240-120 volt dropping transformer is used. This isn't an issue with a proper AC transformer set, most of those were designed to accommodate 40-60 cps or 50-60 cps.
The "Curtain Burner" resistive line cords were a bit of a misnomer anyhow, at least when they were still in good condition. Having a resistive element spread out over 5-6 feet would make it a little warm, not hot like a toaster, and they were reasonably well insulated, with asbestos of course. However they did warn people not to coil the cords up, or to pin them against a wall behind furniture as the heat would build up inside, nor to shorten the cord.
This is one sort of radio that I have never owned, a set with a resistive line cord, all of my early AC/DC sets use either a ballast tube or a candohm riveted to the chassis. Early AC/DC sets are not that common in Canada, prior to 1939-40 they were not allowed to be sold here, so all the ones I have are American imports.
Regards
Arran

[morzh]

Arran

Yes this is the advantage of active resistance (no pun intended)

At least capacitive is safe as it simply won't work.

A diode (using a diode is only possible with resistors) is not safe as plugged into DC it will let though everyhing and your filaments might burn out.


The cap is probably the best as, as you said, no 50Hz exists here plus no DC MAINs left.

[Mondial]

In fact the capacitive dropper and the silicon diode trick won't work on alternative power line frequencies, if you set it up for a 60 cps power system it will not work on 50 cps, or anything other then 60 cps.

The diode and resistor dropper is independent of frequency. It will work exactly the same at 50 Hz or 60 Hz.

The RMS value of the AC voltage does not change with frequency. If you half wave rectify the AC, its heating effect will not change with frequency until you reach such a low freq that the filament temperature follows the individual cycles. This would only occur at a few cycles per second or lower, until you get to zero frequency (DC) where the diode would conduct constantly putting full voltage on the filaments, or not at all depending on polarity.

[morzh]

Mondial

Yes. I did not dispute that. Diode and resistor is the same active dissipation as with just resistor.
Just won't work with DC allowing full voltage through (ok, less 0.7V )

[Arran]

One thing that must be said though is that a wire wound resistor has some safety factor to it, other then getting as hot as a poker. When a wire wound resistor fails it normally goes open, much like a fuse. With the capacitive dropper and the diode dropper there is the possibility that it could short out putting full line voltage across the filament string if you don't take that into account.
Thankfully you can find large value non polarized capacitors that will fail open rather then shorted, I've seen them on the Mouser site, and in the case of the diode you can chain two together in series. In the case of a diode dropper, don't use a 1N4007, those are current rated only for 1 amp, use diodes rated for at least 3 amps. The tube filaments in these early sets are normally 300 ma or 1/3 of an amp which sounds like lots of head room but why cheapen out? 3 amp diodes are still much cheaper then new tubes, even in pairs.
Regards
Arran

[morzh]

Arran

0.3A is a very light load for a diode rated at 1A. At Vf about 0.85V at 0.3A the power dissipated across the diode is 1/4W which is nothing.

And, if you take into account the duty cycle of 2 that cuts the power in half....this diode is mre than adequate.

No need for 3A diode. At all.

[Arran]

Mike;
I'm not saying that you can't use them, or that they won't work, but why be cheaper and use 1 amp diodes when 3 amp ones are still cheap? Don't forget that the heaters will draw more current the .3 amps whilst the set is warming up as well, it still won't hit 1 amp but when you are only talking about pocket change to use heavier duty diodes why not use them? But just to make sure it would not hurt to use two in series, regardless of type. The main advantage in using a 1 amp diode over a 3 amp is if you are producing a product in the 1000s where nickels and pennies add up to many dollars. However a single tube from an early AC/DC set still costs many times more then either type of diode.
I still would prefer a capacitive dropper of course, which costs more then diodes and resistors of any current rating, but the no heat and soft start aspects more then outweigh the costs. As I said, if you get the type that fail as an open circuit, I think Panasonic makes one such type, then that eliminates the possibility of the cap shorting out and blowing up your tubes.
Regards
Arran