Gary,
That is great information - I thought the 8BN8 might have been a misprint
WoW
Is that tube hard to locate?
Inquiring minds,
Bill Maddock N4ZI
Garey Barrell <[EMAIL PROTECTED]> wrote:
Garey Barrell made an utterance to the drakelist gang
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Jim -
Nope, there IS a reason for it to be an 8BN8.
There is a phenomenon whereby the heater (filament) and cathode of the
tube form a "parasitic diode". One side of the filament is grounded, and so
when the filament AC goes negative, 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 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, but Drake had the option of an 8 volt tube used in
series string TVs and could eliminate the resistor.
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 4-B & C-Line Service Supplement CDs
Jim Shorney wrote:
> "Jim Shorney" made an utterance to the drakelist gang
> ----------------------------------------------------------------------
>
> Been playing with my "new" 2A this week. Quick alignment, set the AVC bias
> where it's supposed to be (rather than wide open like it came to me) ... This
> little jewel HEARS stuff. I might take it to FD this weekend and put it in a
> corner with a wire hanging on it for a "vintage" display.
>
> Anyway, the manual and the lettering on the chassis call for a 8BN8 for V8.
> This unit has a 6BN8. What, other than the obvious, is the diff? What's the
> reasoning behind using a higher heater voltage tube in this spot? Does it
> work better? Sound better? Lower carbon footprint?
>
> I await the wisdom of the gurus.
>
> 73
>
> -Jim
>
>
>
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