[Boatanchors] Tube filaments
Jim Haynes
jhhaynes at earthlink.net
Fri Dec 4 20:57:47 EST 2009
On Fri, 4 Dec 2009, Ed Berbari wrote:
> I would like slightly change the direction of this discussion on tube
> filaments. My background is in medical electronics and when I first got
> started in this field tube amplifiers for biophyscial measurements (ECG,
> EEG, etc) were still common. A leading company was Electronics for Medicine
> (E for M, now defunct) and they used DC voltages for filament supplies. It
> was their way to limit 60 Hz hum from interfering with these low level
> signals. I always wondered why this practice was not used with receivers
> and other low level signal circuits.
DC on the heaters was used in sensitive instruments and some applications
such as tape recorders and other audio equipment.
One problem was that suitable rectifiers to turn AC into DC were slow in
coming. For low voltages and high currents there was the copper oxide
rectifier, and then selenium; but it wasn't until germaninum and silicon
rectifiers came along that we had reliable cheap low voltage high
current rectifiers. And you still have to use capacitors to filter
out the 120 Hz ripple.
Another reason is that if signal levels are high enough you don't get
enough hum from the heater supply to matter much. You get more hum from
poorly-filtered high voltage in the plate supply. You can get a lot of
hum reduction just by twisting the heater supply wires and balancing
them to ground.
Probably another reason is that in most consumer electronics the hum
just doesn't matter much. A typical radio receiver doesn't have a
frequency response down to 60 Hz, especially if you include the poor
performance of the speaker and cabinet at that frequency.
Jim W6JVE
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