Garey Barrell <[EMAIL PROTECTED]> 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
<www.k4oah.com>
Jim Shorney wrote:
"Jim Shorney" <[EMAIL PROTECTED]> made an utterance to the drakelist gang
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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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