To see 1.5KW output your anode current must be around 1.2A if you have 2100V
on the plate.
Your load Z will be about 1-1.2kOhm, so on 80m (3.5MHz) your coil should be
about 5.7µH, not 3.7 and never 7.2µH on 3.8MHz.
Tune cap setting will be about 400pf and the load padder should be at least
1200pf.
Your writing is not clear when you talk about grid current, screen current,
tune cap and load cap.
Screen current drop and grid current drop are somehow different as well a
1000pf tune cap or a 1000pf load cap.
Remember, with a tetrode amp you need to tune with your tune cap for maximum
screen current and tune your load cap for specified screen current.
Normally that setting gives the desired output.
73
Peter
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of chuck
Sent: Dienstag, 5. Mai 2009 13:48
To: amps@contesting.com
Subject: [Amps] Tank value problems and grid current
OK, so I had time last night to play more with my tank circuit. I'm having 2
problems I'm wanting to run by you reflector'ees :)
I used several calculators to determine the inductance for the tank on the
amp (gu-84b), running 2100v on the plate when keyed, about 2300 idle. I have
no clue what the current will be at any given drive point.
I tapped the coil around 3.7uh and set the caps before installing it to the
values on the spreadsheet..I forget exactly what they were now but here is
what I have been observing. (and I did a 7.2uh coil for 3.8mhz and observed
the exact thing as below)
While increasing power in 4-8watts in, making slight tune adjustments to see
if power out increases/decreases I notice that the screen grid current
decreases some and then starts to increase, (which is ok) now increasing the
input to 10w I notice about 500w out, hold key down I see the screen current
start to drop BUT the output power seems to stay constant...I'll investigate
that some by monitoring both grid voltages but thought it was weird.
Also now at this point while increasing the input power getting up around
1kw the tank needs MORE tune capacitance and I'm using a 1000pf vacuum cap.
spreadsheets were saying around 600-700pf..I run it to the end and the power
is still increasing.
when I did this on 3.8mhz, I had band switched in a 1000pf doorknob and it
was asking for more capacitance. Is it just values adjusting working its way
to the promised 1.5kw? (values I put in
spreadsheets)
I'm beginning to look at roller inductors and multi wafer band
switches..between little room left and $$ I'm rethinking this whole issue..
if I don't run this full out, the adjustments will always have to be
varied...
All I really would like is 80-15m at this point, adding 160 later if I can
find the components..
IF I stop this now its going to sit till the late fall or winter :)
so! why is my grid current dropping when key down? and why does it ask for
more load capacitance? related? bad vacuum cap?
I measured it but not under power...
de KA1MWP
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