[Amps] HIGH INPUT SWR PROBLEM

[Bill McHugh]

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