[Moon] W6PO amp input mismatch issue resolved !

[Hugh Duff]

A couple of months ago I threw out request for help to get my homebrew 
W6PO 8877 2m amplifier working. I said that I would post the resolution 
to the list for the benefit of others if I got it working.

To recap:
After exploring some other possibilities like re-strapping the 
connections on the tube socket for lower inductance and reinforcing the 
ground I soon realized I was having a mismatch issue with the input 
circuit. Even when built to spec I could not tune it for good power 
transfer from the exciter. My FT-847 would fold back and not provide 
it's full 50w output to drive the amplifier to a reasonable output level.
I tried many variations of the T network input circuit... plus or minus 
turns and I.D. on each coil in many different combinations, different 
shunt tuning caps, different mounting schemes for the cap, relocating 
the complete T network from the front of the input compartment to the 
back, changing all hardware to non-ferrous... I assumed it worked per 
the design for everyone else that ever built one over the years. I tried 
everything and soon became frustrated.

The ultimate solution:
Lionel VE3BQN gets the golden egg !  He told me to scrap the T network 
and try a more complex PI network. He also provided a link to an online 
circuit calculator. Punching in the required input & output impedances 
plus the desired Q  produced the values needed for the PI circuit. While 
searching for a couple of 150pF tuning caps I happened across an MFJ-921 
2m/220 antenna tuner. With the tuner between the FT-847 and the 
amplifier I am now able to obtain 1:1 input SWR.
I'm happy to report that the amp now puts out 1100w with 50w drive & 
2900v on the plate and I've seen as much as 1700w out with a little more 
drive power (small brick amp between  tuner and 8877 amp).

So it seems the original T network per the W6PO amp design does not 
appear to be sufficient to maintain satisfactory impedance for the 
interaction between the ultra sensitive FT-847 transmitter output and 
the tube cathode. The slightly more complex PI network handles the job 
just fine.

For those interested, pix of my amp here:  http://www.va3to.com

73 de Hugh VA3TO