RadioBanter - View Single Post - SB-230 tuning

June 27th 06, 09:36 PM posted to rec.radio.amateur.boatanchors

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