I'm currently working on a GE model GD63. I need a 49A ballast tube and not only are they hard to find, they are not cheap. I've read that these tubes can be replaced by using a resistor. If this is true, it would be a big help. Not only do I have one of these radios on the bench, I've got another coming in this week... and I bet it needs the same tube...
Any help is greatly appreciated.. and please remember who you are talking to here.. (translated - speak to me like you are teaching a 2nd grader.. ) .. I'm not the greatest at chassis work.
The really cheap way is to use a silicon rectifier diode like a 1N4004. Anode goes toward the AC line the cathode to the filament string. Runs nice and cool too.
Terry
Non polarized caps, wire wound resistors, and a diode combined with a wire wound resistor are all ways to go. You will have to allow for the pilot lamp with any design you chose, most ballasts have a tap for a pilot lamp or two pilot lamps.
Regards
Arran
He didn't need a tap so that's why I mentioned the diode. The downside with resistors is they're big and they tend to get really hot!! He would need a 30watter for this job.
Terry
Thanks everyone. I checked Playthings of the Past and he didn't have a K49A listed. I've called several tube suppliers and they are looking to see if they have one. After learning it would take a 30w resistor and create a lot of heat, I've decided not to go that route. There isn't much room in this cabinet for ventilation, nor under the chassis. How would I use a diode or a cap?
The artist formerly known as Puhpow! 8)
(This post was last modified: 10-18-2013, 08:19 AM by Jamie.)
Simple diode in series with the filaments should give you ~.707 of your line voltage. Not sure if that's enough drop. I'm sure there are folks on here that could give you the math for using a non-polarized series cap.
A resistor will dissipate the same amount of heat as the original ballast. No more and no less. The 49A ballast drops the line voltage by 49 V at a current of 0.3 A. The power dissipated is 49 x 0.3 = 14.7 Watts, so for safety you would want to use a 25 or 30 W resistor. By Ohms law, the resistance needed is 49 V / 0.3A = 163 Ohms. You could mount the resistor inside the original ballast housing.
A capacitor has a reactance which will allow the same voltage drop, but since the current and voltage are out of phase no power is dissipated. To drop the equivalent 49V at 60 Hz, you need a capacitor of about 10 uF. It must be non polarized as there is AC across the cap, so a film cap would be best, although a nonpolarized electrolytic might work if it had a tight enough tolerance.
The diode basically acts as a light dimmer, by cutting off half of the ac waveform, which reduces the effective voltage across the filaments. You may have to connect a resistor in series with the diode to provide the proper voltage. The one problem with using a diode alone is that there is no current limiting, so at initial turnon there is a current surge which can eventually blow out one of the filaments. The resistance of the filaments is very low when cold and increases as they heat up, so the turn on current can be very high and overstress the heater which lights up first. You may have noticed this surge in the heater of a 50L6 or 35Z5 when an AA5 AC-DC set is first turned on.
(This post was last modified: 10-18-2013, 05:54 PM by Mondial.)
Thanks to everyone, very much, for trying to help me. To be very honest, I don't trust myself to attempt the conversion. The two sets that I have, that require this tube, they are very valuable and, sort of rare, radios. I would feel terrible if I messed them up. Plus, I may have to sell one of them, and I want to make sure that any radio I sell is as perfect as possible. If I lived near one of you, I'd mow your lawn or something if you'd show me how to do it.
If it were as simple as soldering a resistor between two pins, I would do it.. but it sounds a little more complicated. I went ahead and spent the money on the tubes. They should be here by Monday. You guys amaze me with the amount of knowledge you have. BTW- The Tube Center is where I ordered from. They seem to have everything.
Considering that these are valuable radios and you want to keep them as original as possible, I think you made the right decision.
The glass enclosed ballast tubes also have another advantage. They are designed to regulate the voltage going to the filaments so that regardless of the line voltage (within reasonable limits ) the tubes are always given the proper filament voltage. An additional advantage is that the ballast tube limits the filament surge current at turn on so that the tubes last longer.
(This post was last modified: 10-18-2013, 06:05 PM by Mondial.)
One thing that many use with the diode dropping trick is to add a current limiting thermistor of some sort in series, like they used in series string TV sets. As the thermistor heats up the resistance drops cushioning the tube heaters since their resistance, or voltage drop, increases as the tubes heat up.
The dropping capacitor is the best way in my opinion, if you can do it, it acts as a soft start for the tube heaters. The problem being is that you need to find the right value of capacitor and you need also need room for it since a non polarized cap takes up a lot more room then an electrolytic of the same size and voltage. It's also hard sometimes to get a cap of exactly the right value, so you may still need to add a resistor in series to get the exact voltage.
Those metal can ballasts are really no more then a big wire would resistor, and they also burn out as you well know, I don't think there is much regulation with those types but still better then a diode. The ballasts with a glass envelope are better, they usually filled with some sort of gas to prevent the element from oxidizing and to disperse the heat, there may also be more voltage regulation due to this.
If you are interested in preserving an older AC/DC radio, removing as much heat as possible from inside the cabinet would be a good idea. Heat and electronics are a bad mix in general, and sets with ballasts get hotter then just about anything else.
Regards
Arran
Yes, the glass enclosed ballasts really are much better. Here is a data sheet from Amperite, which explains how they work and shows a graph illustrating the regulating characteristic.
) .. I'm not the greatest at chassis work. 
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