I have a commercial amp from Europe running a pair of GI7B tubes. The
amp works well and loafs along at 500W output with no problems on any
band, but it has a very high input SWR on all bands. Design of this amp
is such that drive is fed into the filament choke via a BNC connector
and cable from the input matching circuit so it was easy to check the
input pi networks for each band. On all but 15M the input SWR indicated
when tested this way was quite acceptable: 1.3-1.7 as tested with an MFJ
analyzer. Similar results were obtained by attaching the input matching
circuit BNC to a dummy load and feeding a small amount of power into the
front of the input circuit. So, the problem does not appear to be in
the input circuitry.
The first thing I discovered when I opened up the sub-chassis was that
the coupling input capacitor appeared to be too low in value: only 1 nf
compared to many designs for a pair of these tubes which I have seen
using 10 nf. Second, there were no filament by-pass caps installed and
no cap across the filaments. Therefore, as an experiment I installed
by-passing using common values and changed the input cap to 10 nf.
These changes produced no lowering of the input SWR but did increase the
plate current drawn for a particular level of drive, probably due to
much higher drive.
This amp design uses a large fan beneath the tubes which does not leave
very much room for a rod-type filament choke inside the enclosure. The
choke is wound with 19 bifiliar turns of what looks to be #16 wire on a
toroid, 1.75" in diameter by 3/8 inch thick. No information is
available on the core.
My questions is this: if this filament choke is undersized, could this
be causing the high input SWR? If not, does anyone have other ideas
about the problem?
Bill, VE3NH
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