On Jan 31, 2006, at 3:15 PM, KA9S - 2 wrote:
> I have an old Hammarlund HXL-1 coming to me and I have down loaded the
> manual for it. I have already decided to make some "improvements" to
> the amp beyond fresh electrolytic capacitors in the power supply.
>
> The HXL-1 uses a pair of grounded grid 572B tubes in parallel. It uses
> capacitors to RF bypass the grids of the two 572B tubes. It then feeds
> a low voltage negative grid bias through 47-Ohm resistors to cut the
> tubes off during key up time. When the amplifier is keyed, these
> resistors are grounded.
>
> The Heathkit SB-200 and the Yaesu FL-2100 do similar things. They use
> 33-Ohm resistors to each grid, and then combine them to a circuit,
> which provides the cutoff voltage during key up time and a low voltage
> negative operating bias during key down time.
>
> Other amplifiers with parallel final tubes do not use this
> configuration. They mandate using "matched" pairs of tubes.
>
> It would seem that the 47-Ohm resistors (or 33 Ohm) form a negative
> feed back function. The tube that is drawing more grid current will
> experience higher negative grid bias compared to the second tube. Does
> this help balance the current load on the tubes and thus elevate the
> need for matched pairs of tubes?
>
> Is there some other reason this grid configuration is so popular?
>
> What would be the draw back to directly grounding the grids of the two
> 572B tubes and provide the cutoff and operating bias through the
> cathode?
Jef -- Direct grounding's value is questionable since there is
virtually no difference in grid resonance frequency between direct
grounding and C-grounding. One advantage of C-grounding is that the XC
tunes out some of the XL in the grid, slightly raising the grid
resonance and slightly improving VHF stability. Another advantage of
C-grounding is that it allows for the use of a grid fusing R --
however, this is of no advantage for those who have a free supply
572Bs.
Editorial -- One of the most chortlesome old wives' tales in Hamdom is
that a heavy-duty Cu strap / "direct-ground" has zero L.
end
>
> Jeff - KA9S
>
>
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>
Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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