[mythtv-users] Hum on analog audio to TV

[Scott Alfter]

Don Lewis wrote:
> Even worse were the old transformerless tube sets.  One side of the
> chassis was connected directly to the side of line cord that was
> hopefully connected to neutral.  The tube filaments were all connected
> in series and across the power line input.  I think a lot of the DC
> supply voltages were just supplied by rectifier connected to the power
> line, and the rest were probably supplied via the horizontal sweep
> transformer.  The cases were plastic, and the volume and tuning knobs
> were insulated via plastic shafts.  The antenna and antenna inputs were
> isolated by connecting them to the tuner input through an RF balun.
> 
> Most tube radios were designed this way as well.

Yes...that configuration for a radio was known as the "All-American Five."  The
rectifier typically had a 35V heater, the beam-power tube (used for the final
stage of audio amplification) typically had a 50V heater, and the other three
tubes (typically a pentagrid converter, a pentode, and a triode/dual-diode
combo) had 12V heaters.  Add them up and you get 121V, close enough to the 120V
you get from a wall outlet.  Here's a fairly good description of the type, if
anyone's interested in more details:

http://www.angelfire.com/electronic/funwithtubes/AA5-1.html

(If you have one (I have a couple), put a polarized plug on it and make sure
the chassis side is connected to neutral (a continuity tester between the
chassis and the wide blade should light up/buzz/whatever).  This should
somewhat mitigate the shock hazard that the AA5 circuit can pose.)

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