[Heathkit] sb200/220 fan

[Jim Shorney]

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On Fri, 29 Nov 2002 20:35:45 -0500, Carl Huether wrote:

>There are all kinds of mods I recommend for the 220 series, I wrote them up
>some years ago and they made the rounds of several reflectors. Unfortunately
>i didnt save a copy; something to do on a cold winter night.
>
>One easy mod is to replace the VHF choke, RFC2, with a 10-12 Ohm 20W
>wirewound resistor; NOT cement.
>This will limit arc current to a safe value. That choke is series resonant
>VERY close to the natural parasitic frequency of the tube and contributes to
>problems.
>It is perfectly natural for those tubes to outgas and arc occassionally; if
>its controlled it wont cause damage.


Rich Measures has an impressive amount of info on his website about the
SB220, and parasitics in amplifiers in general.  For the record, I
don't own an SB220; I've got an L4B and an NCL2000 in the house.  I
read his stuff on the SB220 with great interest, though, for
cross-comparison with the L4B.  I did follow his suggestion of checking
for VHF resonances in the L4B anode circuit with a dip meter, and found
a couple; one was quite strong.

- --
Jim Shorney      -->.<--Put complaints in this box
jshorney@inebraska.com
nu0c@amsat.org
Ham Radio NU0C
Lincoln, NE, USA
EN10PT
http://incolor.inetnebr.com/jshorney/
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