Why Is The 8BN8 Tube Is Used In The Drake 2B Receiver
By: Garey Barrell / K4OAH
|Some have wondered why Drake used an 8BN8
tube in the 2-A/B receiver rather than the "normal" (for the 6.3 VAC
filament supply) 6BN8. Speculation that it might have been a typo, a
error, or even a "good buy" of some surplus 8BN8s a la early
Heathkit products has surfaced over the years. Actually the answer
is "none of the above". There IS a reason for it to be an 8BN8.
There is a phenomenon in vacuum tubes whereby the heater (filament) and cathode of the tube form a "parasitic diode". One side of the filament is grounded, and so on negative half-cycles of the filament supply, rectified 60 Hz pulses are superimposed on the cathode, resulting in hum in the audio.
The "cure" is to reduce the filament voltage slightly, since the parasitic diode action falls off much faster than the indirectly heated "real" diode of the tube's cathode and plate. This is not a problem in most conventional uses of tube diodes, since the cathode is usually operated at or near ground and/or at a fairly low impedance. The culprit in the 2-A/B is the NL diode, since it's cathode is connected directly to the Hi-Z audio input.
Hallicrafter's used a series resistor in the filament circuit of the 6H6 that they used in their ANL, but Drake had the option of an 8 volt tube designed for series string TVs and could eliminate the resistor.
73, Garey - K4OAH
Glen Allen, VA