On Tue, 14 Apr 1998 20:30:25 Scott Townley <nx7u@primenet.com> writes:
>Greetings all--
>I have a couple of installed B&W 850A turret coil assy's in some
>homebrew
>amps I'm resurrecting. One in use with 2x3-500Z, one with a 4CX1000A.
>
>I have limited (in fact, nearly none) docs on the B&W 850A. The only
>documentation I have, an old Radiokit catalog, states it so:
>
>2000-4000 VDC
>2500-5000 ohms (plate impedance)
>
>80m L=13.6uH C=150pF to resonate
>40m L=6.5uH C=80pF to resonate
>20m L=1.75uH C=70pF to resonate
>15m L=1.0uH C=55pF to resonate
>10m L=0.8uH C=50pF to resonate
>
>THE QUESTION:
>80m and 40m look about right from the perspective of: (1) calculating
>values for a plate resistance of around 2500 ohms; which is close to
>both
>the 2x3-500Z and 4CX1000A plate impedances at my operating conditions;
>and
>(2) relative scaling (i.e., 40m is half the value of 80m). 10m kinda
>looks
>ok from just a raw calculation, but doesn't seem to scale well.
>BUT 20m and 15m just look whacked. The 20m and 15m calculations puts
>loaded Q around 16 or so, which seems unreasonably high.
>
>Can someone supply either or both of the following:
>1. B&W 850A specs, in the event they were misprinted in my Radiokit
>catalog.
>2. Any explaination of where I turned stupid, or perhaps where it's
>generally known to the community that B&W turned stupid.
>
>Many thanks in advance for eliminating at least my own ignorance!
>
>
>
>------------
>Scott Townley
>Gilbert, AZ
>nx7u@primenet.com
Well Scott, I have to 'fess up since I wrote the Radiokit catalogs from
1987 to 1992. What you read was simply an exact copy of the B&W catalog.
In looking over an even earlier B&W sheet that was included with the 850A
I notice the Load Z is rated at 2000-4000 Ohms and a maximum of 4000V.
That voltage rating goes back to AM days and I know guys using the 850A
at 6-7KV with never a problem.
Where it gets confusing is that they rate the 852 at 1500-1700 Ohms but
list many of the same or similar tubes for the 850A. I also note that
the 852 has a rather linear progression in the capacitor values. The 850A
is not rated for a pair of 3-500Z's or a single 4CX1000A...they require
the 852 if you believe the published data. The 850A is commonly used
with single tube 4-1000A but if you look closely there is a lot of
overlap with many tube(s) dependent on your plate voltage.
The 850A uses that monster coil on 20 and 15 so a Q of 16 is no big deal.
Perhaps they were thinking more in terms of harmonic suppression and
overbuilt the coil. Anyway there is nothing cast in concrete that says a
Q of 12 is the only way to go...that is only in ham lore and perpetuated
by the ARRL Handbook. If you look closely at commercial ham amps that
include WARC band coverage on one of the traditional bandswitch positions
you will see some pretty high Q's...works fine as long as you have a
"real" switch and coils.
Bottom line...I have no idea what B&W used for design criteria but I've
used scores of their networks in all kinds of amps back when I was
actively building "to order". Sometimes I had to move a coil tap or
unwind a 10M turn or two but otherwise never a problem.....even with a
pair of 4-1000A's.
73 Carl KM1H
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