Tom,
I greatly appreciate you taking the time to answer my post.
Excerpt:
It's odd that you have a second setting. That probably
indicates the tank is going into two modes, or perhaps that
the amp has significant feedback that is changing as you
change tuning settings. You might even have a parasitic
going on. A parasitic is often hidden except for little
quirks like jumping power levels or spurious frequencies
::::> What I have going on here is that I can turn the plate cap clockwise to
raise the grid current and drop the plate current and then adjust the load cap
counter clockwise to peak the output power.
::::> It sounds as if I should go back to the first setting using mininum plate
capacitance.
::::> Any suggestions for recalibration of the wattmeters?
Thanks for your time,
Doug - N9XTF
Quoting Tom W8JI <w8ji@w8ji.com>:
> 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
> amplifier.>>
>
>
> Without putting your amp on a network analyzer or taking
> some time to cut and try, it's hard to tell what is going on
> in the tank.
>
> 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. >>
>
> You measured transfer function. The fact transfer function
> was linear indicates the amp is very linear up to 800 watts
> and 575mA. That's exactly how the 3-500Z's I've tested
> behave. They actually are more linear loaded that way. Of
> course on CW or long carrier you should not run more than
> 400mA or so, you may get into heating problems if you do.
>
> (Thanks by the way for verifying the amp can be linear at
> 575mA! That's an important fact for other people to note.)
>
> 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?>>
>
> It's odd that you have a second setting. That probably
> indicates the tank is going into two modes, or perhaps that
> the amp has significant feedback that is changing as you
> change tuning settings. You might even have a parasitic
> going on. A parasitic is often hidden except for little
> quirks like jumping power levels or spurious frequencies.
>
> 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 directional coupler isn't designed for 50MHz. The
> primary problems are ground loops in the groundplane. It
> just gets by on ten meters, although you could recalibrate
> only for six. That's what I did in my AL800H that I
> converted to six (2kWout on six).
>
> 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? >>
>
> Shouldn't be an issue. The ALC should respond to peak grid
> current, not average, unless something happened from the
> early production stages.
>
> 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?>>
>
> 3-500's are gettered on the anode coating. In order to
> getter the tube, the anode must show some color. If you
> never heat the anode red, the tube will likely fail
> prematurely from gas. This is especially true if the pins
> are contaminated by being in a moist environment and the
> tube sits for extended periods.
>
> Lack of operation that colors the anode for extended periods
> of time will often cause an arcing tube, or a "big bang".
> Sometimes the arc energy will break down the gas and heal
> the tube, but it is better to not let one go that far.
>
> 73 Tom
>
>
>
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