Why were resistors put in bakelite blocks? - Printable Version

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RE: Why were resistors put in bakelite blocks? - Mondial - 05-15-2016

It is easy to criticize Grimes design today for the difficulty in repairing the RF section, but in the context of the 1930's period it was successful and quite an accomplishment. 

There were two significant problems back then in creating a short wave RF front end. One was microphonics and the other frequency drift caused by heat buildup in the chassis.

Grimes attacked the first problem of microphonics by mounting the entire RF section on a rubber suspension. That way, vibrations from the speaker did not travel to the sensitive tuning cap and tuned circuits, causing audio feedback from the large speaker and high power audio amp in the same cabinet.

By placing the entire RF subassembly in the center of the chassis, and separating it with air space on all sides, heat transfer from the power supply and output amp was minimized. It also provided a sort of chimney effect so cooler air would circulate from underneath and all around the assembly.

Granted, the subassembly method of construction was not designed with serviceability in mind. These radios were never designed to last 80 years or more. It was more in terms of maximum performance, and this is where they excelled.

The fact that we still talk about the high end models of this generation of Philcos as some of the best performers ever made is evidence of the integrity of their design and engineering.

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