Tetrode breakthrough, PO on 10M!!

[Naysayer]

Just a test setup (1 plate xfrmr, limited AC from socket, Load cap disconnected, etc). Two main revisions but somewhere north of 300w from 5w in (cw). Need to button up before going at it again. Previous 10M max was 60-80 so this is a big improvement. I was using a single shared 150R parasitic on the control grids. Changed to separate suppressors for each tube plus an ATU on the input. I suspect the ATU (small MFJ unit) is responsible for most of it. I had no sense of whether the lack of PO above 40 was from the grid or anode side of things. Now, at least I know where the problem lies.

Tempted to get a cheap auto atu for input. While adjusting the MFJ really sharp/sudden changes in PO. Input SWR on radio a big help. I was about to give up. More to come.

 

[Naysayer]

External T-match measurements that permit 10m operation:
Xmit V Cap: 84 pF
Inductor : 2.5uH
Ant V Cap: 70 pF
I have room to add a small T-match w/ these values behind input so-239 but there should be a more efficient way to do it by adapting the Pride input circuit already in place.

 

[Naysayer]

Constructed a small T-match with a small 1 piece but dual 30-130pF air variables and a 6 position rotary switch with a 43-1.40 inductor w/ 6 taps from 2.5 to 56uH. I previously measured 2.5uH as the best inductance value for 10M when the MFJ T-match connected to amps RF input so-239.

10 meter Result: I need a lot more inductance for the home made T-match that connects btwn T-R relays and tube grids. Like 20x as much (>56uH). At least I know where the problem is so it's only a matter of time via trial & error.
Did not expect location to make such a difference. Air variables are fine, tuning mid-range. Tetrode input very picky, finnicky.

EDIT: Silly me. The label I printed for the small T-Match was in reverse order. 2.5uH was actually 57uH etc. Good news I guess. 10M is working (into DL) but input needs a bit more tweaking

 

[nomadradio]

Never tried a T-match with shunt inductor. Always stuck with PI or L-match for cathode drive. The broadband approach with a parallel L-C shunted to the swamping resistor is all I've ever tried for grid drive, a-la the Pride. Tried a neutralized input on a 4CX15000A once. Wanted to keep drive power requirements down. Turns out making it mechanically stable is what makes it electrically stable. That one ended up grounded-grid after finding out how mechanically unstable our input hookup was.

I should post a pic of the John-Boy method. Tends to be unstable, but maximizes drive to the tube.

Damn the red line, full smoke ahead!

Last thought: Want another FT-401? Gotta figure out what to do with one. The space it takes up is urgently needed for more important junk.

73