- Here all the various tank coil taps are completed. Adjustable SS
spark gap installed across the vac load cap. After this video was done, the
2nd adjustable SS spark gap was installed across the vac tune cap. Spark
gap across the load cap protects the load cap. Spark gap across the vac
tune cap protects the vac tune cap AND the bandswitch.
There is also a.... '.7 uh pre-coil'...... that is wired between output
of plate block cap assy....and the hot side of the C1 vac tune cap. The
anode to grid C + stray C (lower anode fins to chassis) and this .7 uh
pre-coil form the step down L network. This transforms the 1600 ohm plate
load Z (7200 vdc loaded @ 2.5 amps) down to just 361 ohms @ 21.225 mhz.
Then a lower, loaded tank Q can be used for the PI network on 15m. That L
network also has a lesser effect on all lower bands. IE: 939 ohms @ 14.175
mhz. 1419 ohms @ 7.150 mhz etc, etc.
The original target value for the pre-coil was .6 uh. (which would
have transformed the plate load Z down to just 483 ohms @ 21.225 mhz).
Trying to tap a tubing coil is tough enough as it gets.
With the temp 1600 ohm resistor from anode to chassis....and analyzer on
output side (and the pair of RJ2B's relays externally powered on), the swr
was 1:1 after tweaking the tune + load to the calculated values for each
band = jives with the spreadsheet / GM3SEK software. Previously, both the
vac tune + load caps were mapped out, with pf vs turns counters....and a
cheat sheet made up.
On a similar note, once the tune + load caps tweaked for 1:1 swr, the
caps are left alone....and the upper / lower freqs for the 2:1 swr points
are noted down. 160 khz wide on 160m, 310 khz on 80m, 600 khz on 40m, 800
khz on 20m, and 1 mhz on 15m.
So far, so good. All measurements for the adjustment of the tune + load
caps were done with the top lid on.
https://www.youtube.com/watch?v=ewWA-2nA_t0
All playlists. https://www.youtube.com/@amprepairguy/playlists
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