Date: Wed, 5 Apr 2023 19:05:18 +0000 (UTC)
From: Bill Smith <ko4nrbs@yahoo.com>
To: "amps@contesting.com" <amps@contesting.com>
Subject: [Amps] Adding 160 Meters to Swan Mark 1
<In the process of adding 160 meters to my Swan Mark 1 linear I found that
the <replacement filament choke I purchased from MFJ doesn't have the third
winding <on it like the original.? ?Looking at the Swan's schematic the
third winding is <connected to the filament transformer center tap and the
other end of it is <connected in the center of the tube's filaments and the
RF Input.? The filament <choke I purchased from MFJ is Part
Number?10-15115-1.
<How should I go about resolving this?
<Thank you,Bill KO4NR
Just looked at the schematic. The cathodes are in SERIES..and run on 10 vac
( Same as the Kenwood TL-922). The problem is with the series config, you
now require a 3rd choke, to pass the cathode current ( sum of plate + grid
current) back to the junction of the series cathodes.
The TL-922 used a small, 80 uh choke, wound with real small ga magnet
wire. The TL-922 does has 160m.
You should not require anywhere near 80 uh for this application. 18uh and
higher, is more than ample. Kenwood used 22 uh for each winding of the
fil choke. Heath used 9 uh for each winding in the SB-220.
9 uh = 197 ohms of XL @ 3.5 mhz.
18 uh = 203 ohms of XL @ 1.8 mhz.
22 uh = 248 ohms of XL @ 1.8 mhz.
Rule of thumb is you want the XL to be 4 X the input Z of the amp.
IMO, 80uh is gross overkill for the 3rd choke.
Jim VE7RF
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