"Mike Williams" <[EMAIL PROTECTED]> made an utterance to the drakelist gang
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Excellent info! Thanks,
Mike W4DL
----- Original Message -----
From: "Garey Barrell" <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>
Sent: Friday, June 22, 2007 11:25 AM
Subject: Re: [drakelist] 6BN8 vs 8BN8 in a 2A
Garey Barrell <[EMAIL PROTECTED]> made an utterance to the drakelist
gang
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Bill -
The 8BN8 is readily available, and like most "odd" filament voltage tubes
designed for TV "filament string" use, is usually very reasonably priced!
Typically $3-4. If you plan to keep your 2-A/B (who wouldn't!?!?! :-) )
it wouldn't hurt to add one to your next tube order just to be sure.
Thom asked about putting a resistor in series with a 6BN8 filament, and
this would certainly work just fine since we're dealing with a parallel
filament system. I guess you could use a pot or substitution box to
select a resistor, but probably just choose one that drops the filament
voltage a volt or so. Since most tube _failures_ are the result of open
filaments, it's not likely that an 8BN8 being run on 6V would "ever" fail
that way.
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 4-B & C-Line Service Supplement CDs
<www.k4oah.com>
Bill Maddock wrote:
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
>
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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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