[AMPS] Where is my plate current?

[Steve Bookout]

Hello all,

This is just not my year for working with amps!

Finally got time to work on my 10 meter 2x4-400 GG amp again.  Situation is
I can't seem to get the plate current I should, but it appears I have
plenty of drive.

Can get 300 mils grid current which is full scale on the meter, but only
about 200 mils of plate current. I am running 4-4.5KV (between no load and
700 mils on another amp) with this supply.  The plates don't seem to have a
hint of color (maybe just a wee bit), so I think the indicated 650-700
watts out is OK.  In the past, right or wrong, I've always had plenty of
plate current, even if the tank was all hosed up.  
I used a 3.6K to 4 K ohm resistor with a little bit of capacitance thrown
in in place of the tubes and dipped it all fine.  I made several coils and
they all acted as they should.  The coils with more/less turns resonated
lower/higher.
Using a 100 pf vacuum cap on the C1 and air variable on output.  Added 40
pf series doorknob to the C1 to make the tuning a bit more in the desired
range.
Nothing seems to be getting hot except the suppressors off the plates of
the tube, but this is not excessive.
The tank coil is #2 solid copper wire 2 inches OD; 6.5 turns; 2 1/4 inches
long.  I calculated this to be about 1.5 micro henries and I was shooting
for 1.7 or 1.8.
The input SWR is <1.3 with the matching network right at the blocking cap
feeding the tubes.  I am currently using the black cores (-10 material)
with 6 turns and variable caps.
Using a 8.2 volt 50 watt zener for bias and the ZSAC looks low at about 50
mils.
Amp is stable.  We used it for CQWW on 10 at 600 watts out for hours.
(Kinda hoping the offending item would fail.)
Using separate metering for grid and plate.  If metering was just plain
off, the tubes would be beyond cherry red only putting out 600 watts.  So,
I feel the metering is saying what really is there.
I can take these two tubes and put them in another home brew amp on 10 and
get 1200 watts out at about 650-700 mils at 200 on the grid.  (These are
old pulls that were 'old' when I paid $10 for them in 1981.)

Normally, when I have a problem it's not due to lack of plate current.  

Would it be that because I'm just a bit off with my tank coil it just
wouldn't drive?  I'm about a 1/2 turn less than I need (as calculated.)

This actually *looks* like too much inductor for 10, but if I use any less,
say 2 turns, I can not get it to resonate on ten no matter how I tweak the
tuning caps.  I winds up liking '30 something' megs with less coil.

All leads are short or non-existent.  Maybe I can get a picture, FWIW.

'Frustrated in Virginia'

73 de Steve, NR4M
---------------------------------------------
Steve Bookout, NR4M (ex NJ4F)
Rapidan Data Systems

'DX4WIN...The way logging programs should be!'
Free demo at http://www.erols.com/pvander

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