06-03-2010, 02:09 PM
I just finished restoring a 41-81. This is a DC-only portable, so I built a battery with two D-cells in parallel for the A supply and seven 9 volts in series for the B supply. The radio came with an original battery, so I put my new battery inside the old case. I don't know how to make a new authentic-looking cardboard wrapper for the battery like Batterymaker does.
I notice that the radio comes on instantly like a transistor radio when I turn it on. I know there is no rectifier, but I expected the other tubes to need some warm-up time. Evidently they don't. The question is, why not? I have some AC/DC portables with solid-state rectifiers (selenium or silicon), and they also warm up quickly. On the other hand, I have an early 1960s AC-only table radio with a silicon rectifier, which takes about 15 seconds to warm up in spite of the solid state rectifier.
I thought all tubes would need a few seconds to warm up. Is it just that some tubes need warm-up time and some don't?
Edited later to add: I may have answered my own question. The radios that warm up instantly have direct-heated cathodes (filament and cathode are the same) with parallel filaments, while the the table radio that takes time to warm up uses tubes with separate cathodes and filaments, with the filaments in series. I guess it makes sense that directly-heated cathodes would warm up more quickly.
I notice that the radio comes on instantly like a transistor radio when I turn it on. I know there is no rectifier, but I expected the other tubes to need some warm-up time. Evidently they don't. The question is, why not? I have some AC/DC portables with solid-state rectifiers (selenium or silicon), and they also warm up quickly. On the other hand, I have an early 1960s AC-only table radio with a silicon rectifier, which takes about 15 seconds to warm up in spite of the solid state rectifier.
I thought all tubes would need a few seconds to warm up. Is it just that some tubes need warm-up time and some don't?
Edited later to add: I may have answered my own question. The radios that warm up instantly have direct-heated cathodes (filament and cathode are the same) with parallel filaments, while the the table radio that takes time to warm up uses tubes with separate cathodes and filaments, with the filaments in series. I guess it makes sense that directly-heated cathodes would warm up more quickly.
John Honeycutt
