Finally snagged a Zenith tube radio

[mhardy6647]

Note that it's a series string (AC-DC) set and almost certainly not isolated from the AC mains by a power transformer.

[gearhead]

Originally Posted by mhardy6647

Note that it's a series string (AC-DC) set and almost certainly not isolated from the AC mains by a power transformer.

Please translate!
I have several tube radios, & I'm about 1/2 scared of plugging them in.
I'd like to update them so I can use them.
Nice looking radio, Snade! Got any nudie shots, or are you keeping those under your mattress?

[mhardy6647]

Originally Posted by gearhead

Please translate!
I have several tube radios, & I'm about 1/2 scared of plugging them in.
I'd like to update them so I can use them.
Nice looking radio, Snade! Got any nudie shots, or are you keeping those under your mattress?

No power transformer; all voltages are taken directly from the AC line. Since there's no transformer in the power supply, such equipment could, in theory, be operated from a DC power source. Thus the "AC-DC" moniker.

The filaments of all the tubes are in series and add up to 120 V (more or less). Note the 35 and 19 V filament tubes in the schematic. The 7 tubes' filament voltages sum to 108V (if I am reading the top left tube as a 6V).

I have one of these radios at home, too. It's possible that there's an isolating 1:1 transformer in this set between you and the electric grid, but I don't think so...

[Jeffhs]

Originally Posted by mhardy6647

It's possible that there's an isolating 1:1 transformer in this set between you and the electric grid, but I don't think so...

My Zenith MJ-1035 has eleven tubes in two series strings, wired in parallel, with a filament transformer (if a tube burns out in one string it will darken half the tubes in the set, leaving the other string working as normal), so some of these radios were in fact designed with series-parallel filament strings. The 730's filaments may be wired series-parallel and powered through a transformer as well; this type of wiring would make sense in any radio with more than eight tubes, as the combined filament voltage of, say, an eleven-tube set would total well over 117 volts if there are two or more 50-volt tubes in the string. The filament circuit of the MJ-1035, for example, puts the radio's two 50EH5 audio output tubes on two separate strings.

As to a 1:1 transformer offering any kind of isolation from the power line, I agree with you; I don't think so. If one side of the power line is connected directly to the chassis, as in all AC/DC radios, the latter is definitely not isolated from the line, even by the transformer, unless the AC plug is inserted in the wall outlet such that the chassis is grounded. The problem with that is that there is a 50-50 chance of plugging the cord in so that the chassis is hot, rather than grounded; it is almost impossible to know for sure, as I am about to explain. The only way to know for certain is to use a common light bulb, with one side of the socket connected to the center screw of the outlet plate which is grounded. The other terminal is temporarily connected to each side of the wall socket in turn (by inserting the lead--carefully, of course--into either slot of the socket); the connection which causes the lamp to light to normal brightness indicates the hot side of the circuit. This test is almost infallible, unless the outlet was wired incorrectly. Today's strict and by-the-book electrical codes won't allow such a mistake to go unnoticed or uncorrected, but homes built in the '70s and earlier sometimes had cross-wired outlets; often, the problem was never corrected or even noticed until the wall socket had to be replaced, or other work was done on that circuit. As long as the outlets worked, they were left alone.