Here the manually tuned, tuned input is installed. Typ a pair of 4
section broadcast caps are used for this application...and has been used
several times in the past. (17-540 pf per section, and all 4 x sections
strapped in parallel.....= 68- 2160 pf ). With a pair of 4 x section caps
used, they only have to be padded on 160M. In this case, the 4 section
broadcast caps are too long.
Plan B. A pair of 2 x section broadcast caps are used (load cap used on a
SB-220).....and padded on 160M and 80M). Each section of the 2 section
caps are 15-450 pf...and again, both sections are strapped in parallel.
The progressively shorting bandswitch is from the kw tank circuit of a
TL-922 amp.
The bandswitch is used to change taps on the (aprx) 3.5 to 4 uh coil seen
in the video, and also add padding to each 2 x section cap on 80m, and 160m.
Each air variable gets it's own 6:1 vernier reduction drive assy. A small
2" diameter turns counter drives each 6:1 vernier. Each cap only rotates
180 degs to get from min to max (1/2 rev). With a 6:1 vernier installed,
it then requires 3 full revolutions to get from min to max C. With the
mini turns counter installed, (0-99 increments per rev) it's then 0-300
tiny increments to get from min to max C. This will provide for the
ultimate repeatable.... 'dial it up by the numbers'. A simple cheat cheat
is then made up for several parts of each band.
In normal operation, it's just change the bandswitch to the correct band,
and dial up both caps to their respective pre-sets...done. Then the input
SWR is always 1:1 across each band. In the past, when using the same
manually tuned, tuned inputs, we also have to factor in the tube's stray C
(grid to cathode) as the tube C is directly in parallel with the C2 cap of
the tuned input. We also have to factor in the stray C from each fil lug
to chassis, as there is more stray C at the hot end of the fil choke. We
also have to factor in the length of coax from IPA (intermediate power amp)
to input of the tuned input. We also have to factor in the bias used
(fully adjustable), drive level, (also adjustable).. and also B+
used...(can be altered by varying the taps). We also have to factor in
the length of coax from tuned input to the cathode. Ideally, you want the
C2 cap as close to the cathode as possible, esp on upper HF, but that is
not always possible. Typ eff will suffer on 10 + 6m, with 50 ohm coax
lengths > 1' on 10m, and even less on 6M. Typ 'fixes' include using
paralleled 50 ohm coaxes ( which effectively adds C to the cathode), or
adding C to the cathode (cathode to chassis)
In the past, we tested the manually tuned, tuned input by applying a 800
watt cxr for 10 mins on each band. Done with wattmeter's on both sides of
the tuned input....then into a 50 ohm DL. The 3CX-6000A7 (and also the
smaller 3CX-3000A7) have a 50 ohm input Z. With input swr tweaked for 1:1
we get max drive applied to the cathode...and also max grid current.
Trying to design and fabricate 5-9 separate, bandswitched / relay switched
PI tuned inputs....that will each handle 800 watts CCS cxr, is an exercise
in futility. Arco compression trimmers are only good for 200 w pep...and
also have to be padded on 80 +160m. Correct sized toroid's and magnet wire
are a non issue. Been there, done that. If fixed caps are used, they have
to handle the current, and you need a box of em to arrive at the final
tweak...pita. Then either a 2 x wafer bandswitch is used, or a small
single wafer bandswitch is used to activate a mess of relays. You end up
with a convoluted, complicated mess on your hands....and even more
complicated if the tuned input bandswitch is mechanically ganged to the 10
kw output bandswitch.
The dead simple, manually tuned, tuned input is far simpler, and bullet
proof...and fast to design and install. With 800 watts into 50 ohms, it's
only 4 amps into 50 ohms, and only 200 V rms (283 V peak). Those
broadcast style caps typ have .032" spacing, and rated for 1.1 kv. With
older, dirty ones, toss em into the dishwasher....(with plates un-meshed),
and they come out squeaky clean.....and hi-pot test to an easy 1.5 kv.
We intentionally don't mechanically gang the manually tuned, tuned input
bandswitch with the 10 kw output bandswitch. In normal operation, it does
not take long to change bands, with the 2 x bandswitches and 4 x caps,
using cheat sheets for both the input and output tuning. (< 1 min from 15m
down to 160m).
https://www.youtube.com/watch?v=VqmrX1IMFSM
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