[Jbaker]

A quick question for the experts. Most of the tube radios I have take 20-30 seconds or more to warm up before you can tune in a station. I have a Philco 53-656 lunch box radio begins to play almost instantly, within 1 second, after I plug it in or turn it on. It is a tube radio that runs on AC or battery. Any idea why this model does not need any warmup time? Just curious.

Thanks - Jim

[Steve Davis]

Jim, your radio uses filament tubes or sometimes called direct heated tubes, as opposed to tubes with a separate heater and cathode or indirect heated. Direct heated tubes heat up very quickly, almost like turning on a light.

Steve

[Arran]

That's because your portable uses battery tubes which are between 1.5 and 3 volts each and as Steve said they are directly heated which means that unlike most AC type tubes there is no cathode over the filament that needs to heat up first. The reason I say most AC tubes and not all is that many rectifiers are directly heated, if you look at a 5Y4, 5Y3 or a type 80 tube they are almost instant on as well. Also your portable likely uses a selenium rectifier which is also instant on, it's an early form of solid state rectifier.
Regards
Arran

[Jbaker]

Thanks Steve and Arran!

Jim