[Amps] 8877 Grid Current

[Byron Tatum]

Hello-   I am commissioning a newly built 144 Mhz 8877 amplifier and am seeking advice on grid current. My tube is a known good, low hours Eimac YU-209. The amplifier is the W6PO design. I rebuilt a Scientific Radio SR-416 NOAA Weather unit RF deck. If you have ever viewed one of these RF decks it is apparent that S/R pretty much copied the 144 Mhz W6PO design from Eimac construction article by Sutherland, excepting the input circuit design.     I tore the RF deck down to bare chassis and rebuilt using the input circuit and bias, control circuits from the K1AGB stripline amplifier article from 1975 QST. Years back I built one of the K1AGB amps and it worked out very well. The input circuit tunes nicely for very low input SWR.    I modified the S/R RF deck to use an Eimac output coupling probe assembly robbed from a 900 Mhz cavity, with a 2" OD round silver plated brass disc. I adjusted the input and output tuned circuits using load resistors and antenna analyzer looking into input and output, both tuned nicely for very low SWR. I was surprised at how well the amplifier came up and the tuning was real close as-is. The HV power supply was built around the Basler transformer supplied in S/R SR-416 unit. I left out the choke that S/R had in negative lead (they targeted 2750 VDC) and am using primary connections to yield lowest output voltage, which is 4200 VDC with 45 uFD GE oil cap and 300K in bleeder resistance. That Basler transformer is a good one, a Hypersil design. Voltage drops from 4200 VDC unloaded to 3800 VDC at 1400 watts output.    To make a long story short during initial tests of amplifier I noticed the grid current was just about non-existant, actually saw it go slightly negative during tuning. The cathode current was not excessive running around 600 mA with 50 watts drive but power output was running about 1000 to 1100 watts. So efficiency was not real good, I started backing the output coupling probe, in very tiny increments, away from plate tank. I noticed that after I retuned the plate tuning for maximum output that the output power increased, cathode current decreased slightly but the grid current moved on up to around 20 mA. I carefully continued backing the output coupling probe away, seeing the same effects, until grid current reached 30 mA. I stopped there, with power output at 1500 watts with around 50 watts drive. I went through this same thing in tuning my K1AGB stripline. My question is this -- what is the best compromise involving grid current? It appears that I must run a certain value of grid current to have decent efficiency but where do you draw the line? Sorry for all the details but thought it might have bearing on any advice given.Thanks,Byron W5FH