If? you use? GM3SEK's? ?pi? spreadsheet,? ?you will have some wiggle room.? The trick is to insert a tiny bit
of uh... between plate block cap and C1 main tune cap.? ?The combined tube C? from both tubes? ( anode? to chassis).....? +? that extra? tiny? uh? between? plate block caps..and? C1? tune cap..... will then form a step down? L network.? ? IE:? it will step down the plate load Z..way down.... to a lower value.? ?Then? the PI net will? easily match the lower value.... down to? 50 ohms.? ?Then you won't end up with a sky high loaded? Q.?
To pull this off,? you need to measure the pf? ?between either anode..and chassis..... with both tubes plugged into sockets..... and also with all 6 x grid pins? bonded to chassis.? ?Knowing that info..it's then? plugged into the spreadsheet? ?as? ? "stray pf? between anode and chassis".? ?
Enter calculated plate load Z... and let it spit out results.? ?You may? well have to increase the loaded Q? on the spreadsheet? to get a C1? value? that your tune cap can handle.? ? ?Then start inserting a tiny bit of? uh,? like? .1 uh....? in the stray uh column for the spreadsheet.? ? It will then spit out the lowered plate load Z the PI net is now trying to match.? ?At that point,? you should also be able to also lower the Q? in the spreadsheet.?
Then? try a tiny bit more? uh before the C1 cap.... like say .15 uh..or? even? .17 uh.? ? You want to insert just enough? uh? ?to do the job, and no more.? ?That extra uh? should be made of? strap material...and ditto with the main? 6m? coil? ( between? tune and load cap).??
Same trick is used on the YC-156? to get it to work on 6M....? dues to the c 50 pf? ?between? anode and chassis.? ?YC-156? is? 35 pf as is.... on a wooden bench..... but rises to 50 pf...when bolted to chassis.? The extra 15 pf comes from the proximity of the lower anode fins to the chassis below the fins.?
later.... Jim? ?VE7RF?
