Here the tuned input is finally installed, and tested. The tuned input
consists of a 'L-PI' network. The 3CX-6000A7
has a 50 ohm input Z, so the L-PI tuned input was designed in software by
myself, using the GM3SEK PI / PI-L
spreadsheet.
The 3x6 triode has 42 pf of internal capacitance between the chassis
grounded control grid...and the directly heated
('instant on') cathode. That 42 pf has to be subtracted from the
calculated C2 value of the tuned input. Problem is, the previously
installed fil choke assy (5 x oval beads slid over a pair of double
insulated 4 ga, fine stranded power cables),
measured 50 pf of stray C, between EACH fil lug..and chassis. With tube
then plugged in, total stray C rises to a whopping 142 pf...which of course
screws things up badly.
The issue is, the fil choke assy was right up against the chassis in it's
previous configuration. My fix was to remove 2 of the 5 x oval beads, and
re-configure the final assy, so it hangs in mid air, between the pair of
100 amp rated Super-con connectors...and the socket. That reduced the stray
C from EACH fil lug to chassis...down to 22 pf. Ok, so 44 pf of stray C
instead of 100 pf. I was expecting less.
With the original fil assy installed, the L-PI would not resonate any
higher than 28 mhz. With it removed, it would resonate higher, like 40
mhz. The 1/8" tubing coil between the pair of air variable caps was
reduced in diameter, in small increments, and re-tested each time.
Finally, Scott got it to resonate at 50 mhz. New fil choke assy
re-installed, and
L-PI tested yet again. Then again with the bottom lid on. Success at
last. Flat SWR from 49-56 mhz. Also installed was the
bypass caps at cold end of new fil choke assy. Also the 6 x .01uf @ 3 kv
rated disc caps across the cathode. 3 of those caps are paralleled..and
ditto for the next 3 x caps. Both those assemblies are wired in
series..and the entire mess is wired across the cathode socket connections.
Drive RF from the C2 air variable, is applied to the junction point of
the 6 x caps. This makes a better use of coupling caps, and divides the
drive RF current equally 6 x ways, with 1/2 the total current being applied
to each end of the cathode. This is the correct method of driving any GG
/ cathode driven tube.
The input RJ1A ceramic vac relay was also installed, and RG-400 teflon
coax was used between vac input relay and the C1 air variable cap. More
RG-400 used from the same vac relay..to the input SO-239 jack. The cone
shaped shroud over the SO-239 input jack eliminates any Z bumps. A 26.5
vdc supply was temp used to power the vac relay
for testing purposes. A temp 50 ohm resistor was wired from one cathode
socket connection..to the chassis...to simulate the tube's input Z.
This entire process took several hours. When you are dealing with
miniscule amounts of uh (like .15 uh) between the 2 x
air variables....and .015 uh between the C2 cap.... and the junction of
the 6 x discs.... it becomes a slow, tedious process. The value of both
those inductances greatly affects the network. If the tuned input is not
done correctly, you are dead in the water. The BW is exactly where I wanted
it...and jives with software. Enjoy the 6 min video.
https://www.youtube.com/watch?v=Hr7BPxz0UBc
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