I have a few questions about an Ameritron AL-80B amplifier that I converted for
50 MHz mono-band service.
The first question goes to tuning the amplifier. I am using a Vacuum Variable
cap, 3 ? 30pF on the plate and the original air variable plate cap has been
moved to the load position. With 60 watts of drive, I achieve full amplifier
output of approximately 900 watts with a grid current of 65ma and a plate
current of 575ma. This happens with the plate cap at minimum and the load cap
at ½ its range or set in the ?middle?. I can achieve the same output power
results by ?screwing in? the plate cap and turning the load cap counter
clockwise. This setting yields a grid current of 100ma and a plate current of
450ma. The latter produces currents more typical of how it operated as an HF
I do not have an O scope to check linearity. That is on the list of items to
purchase. The first tuning method produces a more ?linear? gain reading up to
800 watts output. In other words, if I graph the input wattage against the
output wattage, the resulting gain stays very close up to the 800 watt mark.
The second tuning setting results in this gain measurement fluctuating up and
down and drops of more at the 800 watt mark. Any comments as to how this
amplifier should be tuned?
The second question is that the internal watt meters show about a 100 watts
more output (forward and reflected power) as compared to an external Bird
wattmeter. Can the internal watt meters be compensated for 50MHz to bring them
closer to a true value?
The third question, when using the ALC, the power cuts back as expected in CW.
However, when transmitting SSB, the ALC set point has to be set close to Zero
to have an effect on the exciter?s drive power. While transmitting on SSB, the
grid current will only deflect about 20 or 30ma so the ALC never sees the 100ma
that I had it set to cut back at. Is this a typical issue on SSB?
Lastly, while transmitting in SSB, the 3-500Z does not show any color. I can
transmit a long burst, 30 or 40 seconds, and never see a color change. If I
key the amp in CW, I can see light orange well before this time period. Is it
ok to operate in this manner and periodically operate CW to produce enough heat
to ?condition? the tube?
Thanks for your input ? Doug N9XTF
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