On Tue, 27 Jun 2006, wrote:

First, my thank you's are in order to all who have answered my posts
concerning this amplifier...I have not yet fired it up, but I'm getting
there! (Taking it easy and following the good advice I've received).
Anyway, I've browsed the Heathkit manual and their instructions for
tuning the amp seem rather complicated....they tend to focus on tuning
for output and very specific plate and grid currents. Would it be safe
for me to instead use the following procedure (which is likely to get
me to the same point, but is easier to remember....it's the technique
we used t for the old club amplifier):

1) Turn on amplifier (of course) and allow it to warm up.
2) Set exciter for 80 watts output on cw (amp is supposed to take 100
watts, but I want to be on the safe side).
3) Set pulsed tuner aid so that I am running about a 25% duty cycle
(average power into amp will be 80 watts peak, but only 20 watts
average).

I might even forget the pulser, and just use pure CW and start at 20 watts
of pure CW until you get the feel of "amp on" vs. "amp off" and the
effect of tuning.

4) Set amp tune and plate controls to suggested starting points.
5) Switch amp out of bypass.
6) Tune amp "Load" for max power out, adjust "Tune" to dip plate
current, check grid and plate currents.

I would recommend setting the loading control far CCW to start (i.e. low),
and tune the plate for maximum RF output. You may or may not see much of
a "dip" in plate current as you go through resonance (depending on what
"class" the amp is really running in), but you should see RF output go
from zero to something high.

7) Increase pulsed exciter power gradually while repeating step 6.
7) Repeat step 7 until desired power out is achieved (keeping in mind
that I need to stay well within allowed grid and plate currents).

Another issue you left out but you need to watch is what the SWR is
between the amplifier and its load. Quite a few of us (including me) use
an antenna tuner between our rigs and the transmission line. You almost
don't care what the SWR is between the tuner and the antenna, but on the
coax between the rig and the antenna tuner, you better be low (i.e. SWR
2:1 or lower, depending on the rig) instead of high (i.e. 3:1 or more)
because that reflected power will end up being dissipated in the
tube/heatsink. And, another thing: once I was a little off resonance on my
SB-230 and after transmitting for 1-2 minutes, the overtemp relay kicked
in and put the amp into bypass until the heatsink cooled off (this will
take a few more minutes, maybe less if you put a small fan on it) at which
time the bypass kicked off and I had power again (leave all the switches
on). So, you have to watch your plate resonance. If you raise loading,
always recheck plate resonance.


In a nutshell, that's how I remember tuning our club amp (although it
was near 20 years ago when I last tuned one.) Does this sound about
right? It seems much faster than the steps outlined by Heathkit and
safer too...also, would there be any problems with my signal if I keep
the exciter power fairly low...say at around 50 to 80 watts rather than
the 100 watts specified?

Not a bad idea until you get a feel for how the amplifier behaves in
tuning, how your lights dim on voice peaks, whether your circuit
breakers/fuses blow when you talk up the amp (have a flashlight handy at
night?), etc.

(Granted, my output power will be lower, but
that is fine...especially while I'm still just testing the amp and if
any RFI will be generated by the amp.)
Thanks a bunch!
Craig KB8FGC

Straydog wrote:



because that reflected power will end up being dissipated in the
tube/heatsink.

Another "Old Wives Tale"! Tubes are only "matched" to the point where
the desired output is obtained. Reflected power will see the amp as a
mismatch and will again be reflected back towards the antenna.

73, Roger

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On Wed, 28 Jun 2006, Roger D Johnson wrote:

Straydog wrote:



because that reflected power will end up being dissipated in the
tube/heatsink.

Another "Old Wives Tale"! Tubes are only "matched" to the point where
the desired output is obtained.

All depends on your definition of matched.

Reflected power will see the amp as a
mismatch and will again be reflected back towards the antenna.

I think that is only partly true. Yes, there will be reflectd power going
back out to the antenna, but there was an article in QST back a number of
years ago. Yes, the mismatch also results in higher plate dissipation,
too. I will correct myself about reflected power being disipated in tube,
but the mismatch will increase the plate dissipation. The other issue is
being off plate resonance. I had my SB-230 trip the overtemp relay when I
moved frequency and did not retune the final.

You also need to allow for loss in the transmission line. Not all of what
gets reflected at the mismatches ends up at the other end of the
transmission line. SWR (i.e. reflected power generated at the
antenna/feedline) would not be a problem if the transmission line
were long and lossy.

73, Roger

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Reflected Power usually gets dissipated in the coax, but may result in
unusual currents and voltages in the Plate output circuit and may not allow
a proper load.

Whatever your Amp or proceedure, Pi-network outputs require maximum
loading. It results in maximum output = maximum efficiency = minimum
harmonics and distortion. Even a minor loading misajustment results in a
big rise in harmonics and distortion. Your Club tuneup routine is an
excellent proceedure since you would be providing sufficient drive at peaks
and If a peak reading meter were handy, very meaningful. CW at the 80-100
watts would also be useful but there is little need to lean on it for more
than a couple of seconds while looking at meters and tweeking a knob.
Always keep an eye on grid currents as they are the most fragile they will
be metered if it matters in your Amp (or protected somehow). Once tuned to
maximum output in this fashion, you can reduce drive, confident that the
final stage is fully loaded. Do this with a 50 ohm Dummy load so that you
will know what this is supposed to look like as it is easier to do a little
tweeking on the air versus tune-up from scratch into an impossible load.


"Straydog" wrote in message
x.com...


On Wed, 28 Jun 2006, Roger D Johnson wrote:

Straydog wrote:



because that reflected power will end up being dissipated in the
tube/heatsink.

Another "Old Wives Tale"! Tubes are only "matched" to the point where
the desired output is obtained.

All depends on your definition of matched.

Reflected power will see the amp as a
mismatch and will again be reflected back towards the antenna.

I think that is only partly true. Yes, there will be reflectd power going
back out to the antenna, but there was an article in QST back a number of
years ago. Yes, the mismatch also results in higher plate dissipation,
too. I will correct myself about reflected power being disipated in tube,
but the mismatch will increase the plate dissipation. The other issue is
being off plate resonance. I had my SB-230 trip the overtemp relay when I
moved frequency and did not retune the final.

You also need to allow for loss in the transmission line. Not all of what
gets reflected at the mismatches ends up at the other end of the
transmission line. SWR (i.e. reflected power generated at the
antenna/feedline) would not be a problem if the transmission line
were long and lossy.

73, Roger

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JB wrote:
Reflected Power usually gets dissipated in the coax, but may result in
unusual currents and voltages in the Plate output circuit and may not allow
a proper load.

Why would reflected power be any more dissipated than forward power?

73, Roger

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Roger D Johnson wrote:
JB wrote:
Reflected Power usually gets dissipated in the coax, but may result in
unusual currents and voltages in the Plate output circuit and may not
allow
a proper load.

Why would reflected power be any more dissipated than forward power?

Only because it takes 3 trips through the coax before it reaches the
antenna. Forward power only takes one.

-Chuck

On Fri, 30 Jun 2006, Roger D Johnson wrote:

JB wrote:
Reflected Power usually gets dissipated in the coax, but may result in
unusual currents and voltages in the Plate output circuit and may not allow
a proper load.

Why would reflected power be any more dissipated than forward power?

I would ask the same question. Or, if you had 100 watts going forward from
the transmiter end of the coax on a 100 feet of 3 db loss/100 foot coax, then there is only 50 watts at the
end of the coax. If half of that gets reflected (i.e. 25 watts coming
back) from ant-coax mismatch, then it suffers another 3 db loss by the
time it gets back to the entry point on the coax (or, 12.5 watts +/-). At
the transmitter end, Roger and I are having substantial discussions by
private email.

73, Roger

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