[Moon-net] W6PO amp help pse !

[Hugh Duff]

Hearing and not being heard (operating "QRP") during the 1st leg of the 
ARRL contest has inspired me to get back trying to iron out the input 
SWR problem on my W6PO 2m amp. I built the amp last year and it has been 
sitting for some time due to my being discouraged after many attempts to 
get it going. I'm hoping to finally solve the problem so that I can have 
it up and running for the 2nd leg of the contest.
Unfortunately I don't have any locals who I can consult so I'm hoping 
some of the experts here on the moon lists can help me sort this out.

The amp is my homebrew version of the famous W6PO design 8877 2m 
amplifier as seen on the W5UN web site.

Problem:
I cannot get the input SWR on the amplifier to be low and stable. As 
such, the FT-847 exciter power folds back so I am not getting full drive 
to the amp. The most power output I've seen is about 450 watts which I 
attribute to insufficient drive power. The maximum drive power from the 
FT-847 is only 50w but I expect that I should see more than 450w out 
even with 50w drive.

I've tried winding different coils of various turns and diameters (+/- 
what is specified), different variable caps, reinforced the grounding, 
moved things around in the I/P circuit...all to no avail.  Sometimes I 
can get the SWR to look decent with low drive power but then it jumps as 
the drive power is increased. First I used an MFJ-259A to check the 
resonance of the I/P cct.  Then I checked it on a network analyzer and I 
was getting the same result. There is something unstable about it. I get 
what looks to be decent resonance, change the tuning cap a little then 
go back to the previous setting and the resonance has changed. I am at a 
loss here.

FWIW, Cathode bias ~12v, supplied by a G3SEK Triode board.  HVPS is 
stiff....it uses pole pig xfrmr good for about 3KV @ >1A.

I'm using a 3CPX1500a7. I have 2 of these tubes and it doesn't matter 
which one is installed, I get the same results. Are there any 'gotchas' 
when using this tube compared to a straight 3CX1500a7 (8877) ?  I 
thought they were interchangeable where there are no physical 
restrictions on the tube height.

I welcome any and all suggestions at this point.

I'd also appreciate hearing from any others who are using a W6PO type 
amp with an FT-847.

In the interest of keeping the moon lists uncluttered, please reply 
direct.  I'll post the resolution with credit where due, for the benefit 
of others should I finally get to the bottom of this.

Tnx es 73 de Hugh VA3TO