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From the Q&A

With TJ Byers


Not Just For Looks

Question:

I have an old AA5 (All American Five) radio that keeps burning out the 35W4. Each replacement lasts for three months or less, and they aren't cheap. I don't have this problem with the other tubes in the set, so I assume there is something in the circuit causing this. Any ideas?

Jerry
via Internet


Answer:

It's not something in the circuit causing the problem, but the lack of something. Specifically, the pilot lamp used to illuminate the tuning dial. To eliminate the power transformer previously used in tube radios, the filaments of the AC/DC sets are wired in series where the voltages of the filaments add up to about 112 volts. In the circuit shown in Figure 4, one side of the filament string connects to the circuit common which is the line running across the bottom edge of the diagram.

The other side of the heater string (the 35W4 filament) goes through a switch to the high side of the AC line. The #47 dial light also comes from this point to a tap on the 35W4's filament. By itself, the tap won't provide the 150 mA required to light the lamp, so the plate connects to the tap and adds its current to the mix. If the dial lamp burns out, though, the plate current now flows through the filament (about double its rated current), causing it to overheat and quickly burn out. The same arrangement is used with 35Z5 rectifier tubes found in older versions of the AA5. The solution to your dilemma? Immediately replace the $0.50 lamp if it burns out, or be prepared to keep shelling out $6.50 for rectifier tubes.


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