How to fix a classic American AM tabletop tube radio

Step 2: The AA5

Sometime in the 1930's, somebody figured out that you could build a radio without a power transformer. Power transformers are expensive, maybe the most expensive single part in the radio. Do away with it and you save a bit of cash. This design became known as the All American Five. No transformer and five tubes. The voltage of the filaments all added up to about 120v. We'll refer to it as the AA5 from here on out.

The AA5 design was produced by the millions in many different incarnations using a variety of tubes. As new tube designs appeared, they made their way in to the AA5 design. You can find AA5 radios that use both 1930's designed tubes and 50's era miniature tubes on the same chassis! Despite the different tubes they used, they all follow a very similar circuit design and are an easy radio to learn to work on. There are derivatives of the AA5 that used more or less tubes but they all had the same thing in common, no power transformer.

Before cracking into that antique radio, we need to stress a few words of caution. An AA5 radio can kill you if you're not careful.

How can a radio kill you? Back to the history lesson! The AA5 radio and its derivatives use no power transformer. To pull this off one side of the line cord was connected right to the chassis. Keep in mind this was back in the day when natural selection was more the responsibility of the user and not the manufacturer. It's very important to not try to work on this radio while its plugged in. You also need be very careful not to touch any metal parts or exposed screws on the radio when its plugged in as they can be energized at line voltage potential.

Ready to dig in? Let's go!

Comments

[CBTman01]

The lack of transformer was not for savings purposes, but to allow the radio sets to work on DC power lines, like those used in those days in many countries, including the US, in addition to AC power lines. Surprisingly, although there are virtually no DC power lines at the 115 or 208 voltages, many long distance transmission lines now carry DC, not AC, because it is cheaper (and feasible) to do so.

If the voltage of the filaments did not add up to 120 or 220 V (used outside the US), the manufacturers added a resistor for extra voltage drop. In many cases, to save space and reduce internal heat, such resistor was wound inside the power cord, which made it warm to the touch when the radio was on.

The safest way to work with a "Five" is to use an isolation transformer (1:1) of the right volt amp rate (around 200 VA). This way, none of the power cord wires would be related to ground and it would be possible to touch the chassis without a 50/50 chance of getting electrocuted.

[ke4mcl]

i cant argue the dc aspect of your comment however the lack of power transformer is a different issue. it's widespread knowledge in many antique radio resources that the AA5 was born out of the desire to make radios more affordable when the depression hit in the 1930's. one of the most expensive single components in a radio was the power transformer. do away with it and you have a substantial cost savings. atwater kent chose to close his plant down instead of produce "cheap" sets when this issue became a big thing in the radio industry.