[Amps] 4CX800 2 Meter Linear

[Glenn McNeil <vk4tzl@bigpond.net.au>]

Hi Gang..

I've seen a lot of good advice metered out here, and quite a few problems 
solved....so here goes with my present headache.

I've just finished a 4CX800 2 Meter linear...it's a near copy of the 
JH2CLV device, but with a "standard" slab line anode with flapper tuning. 
The plate line is U Shaped with a sliding assembly with fingerstock at the 
earthy end of the line to couple the RF out to the world.The line bends 
around from the DC Blocking Caps to adjacent the Socket where it is 
earthed..makes for a short RF Box. The Tube end has two 330pf Doorknobs 
attaching it to the Tube.

DC conditions are 1850 on the Anode ( when tuning with constant carrier), 
350v on the Screen and the grid bias set to give 150ma idling current. All 
the above is controlled/supplied by G3SEK Boards.

The thing fires up no problem, but going to Tx and injecting some rf drive 
yields little in the way of output. After much swearing and testing which 
involved putting a 2k resistor across the Anode/Screen and looking into 
the Output connector and checking for resonance on 144.200 and tuning 
up/down to make sure I had the line the right length and a low SWR, I can 
only get about 120w out with 6w of drive.The thing "peaks" the rf when the 
anode flapper is tweaked, and the flapper sits at about 5mm off the plate 
line when it is tuned.  Sliding the output coupling up and down its 
travel, from furthest away from earth towards earth, yields max output as 
I approach the earthy end of the line, but the screen current does zilch.

When I have max output, I'm looking at about 680ma Ip, very little in the 
way of screen current ( the needle hardly moves) and a big kick 
"backwards" on the Grid Meter...Yep, it attempts to go negative, and the 
more drive, the more energetic it becomes.Incresing drive see's more rf, 
but lots more Ip.

A fellow Ham has one the same...and his works fine. I copied his 
construction details accurately ( I hope ) and the above is the result. My 
Amplifier learning curve is steep, and going more vertical. This thing is 
on wheels and I live on a hill...any ideas before I give it a push ??

73

Glenn
VK4TZL
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