Hi,
In June there was a post to the group from someone looking for information on
the conversion of a Motorola Quarter Kilowatt high band VHF PA which uses two
small conduction cooled tubes that are replaced with two 8877/3CX1500A7 triodes.
You can find pictures of the first PA converted at this URL:
http://webhome.idirect.com/~ve3kh/amplifiers.html - I have since built a second
unit which is much nicer, but has not been tested yet. After testing is
completed I intend on drawing it up on autocad and providing pictures of the
new version early next year. The differences fromt he first to the second PA
are as follows:
Two fil transformers, one for each tube instead of a single shared xfmr.
Two separate B- bias circuits for each tube instead of one.
Filament feed throughs moved from rear of unit to left and right side of
chassis, re-using the four original ceramic chassis feeds throughs (moving two
of them to the right side).
Two large holes cut into the top of the PA (with chrome fan guards to keep it
safe) to let air out instead of a large small diameter hole pattern drilled
into the top of the unit.
A single large dual "squirrel cage" fan attached directly to the underside of
the PA instead of using a hose coupling from an external blower.
Metering jacks moved to left and right side of PA for left and right tubes
instead of one grouping of connectors on one side.
Two large handles attached to the front of the PA to make it easier to carry
and mount the PA to a 19 inch rack.
Input tuning control knob moved to the front of the PA instead of being located
on the rear side of chassis.
At the time I built this I had intended on using G3SEK's triode board for this
PA, but at the time he didn't have one to run two 8877's off of one board so I
just duplicated the W6PO bias and control design twice.
I hope this post doesn't show up twice because I tried sending it late last
night right after joining, but believe it was probably rejected because I
either did something wrong, or sent it too soon after submitting my request to
join.
I have a few pictures of the 2nd PA if anyone wants to see them email me and I
will email the pictures, but they are high resolution and will require several
meg of download.
73 Chris KL7FB Palmer, Alaska.
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