Bob wrote:
> Hello, I have a question that may be a little OT. I am concerned about
> my SSB audio both before and after my SB-220.
Hardly off topic. Not enough users of amps worry about the quality of
their audio.
> I have a scope but I am
> not sure about buying/building a RF probe.
>
One of the best ways of checking the effect of your amplifier tuning on
your signal quality is to use a "monitor" scope in the so-called
"trapezoidal" pattern mode. The Heathkit SB-610, in matching cabinet to
your SB-220, is ideal for this if you can find one in the used
marketplace. I have exactly this combination of amplifier and scope here.
In this scope, a sample of the RF output from the amplifier is fed
directly to the vertical deflection plates of the cathode ray tube. At
the same time, the RF output from the exciter (in other words, the RF
input to the amplifier) is fed to the horizontal deflection plates via
an amplifier so there's some ability to adjust how much of the
horizontal axis the trace will occupy. The SB-610 provides very coarse
adjustment of the height of the display by a switched capacitive voltage
divider on the rear apron of the chassis. I think there are three,
maybe four, "gain" settings.
If your amplifier is adjusted properly, when you speak you should see a
sideways triangle that grows with the amplitude of your voice. At the
peaks, if you are hitting the amplifier too hard or you don't have the
amplifier tuning and loading controls set properly, the amplifier will
"flat top" and you will see very graphically what this means as the
trace on your monitor scope won't go as high as it goes wide. On my
SB-610, if I "flat-top" with excessive drive from my transceiver, the
two sides of lazy triangle will curve toward horizontal on the left-hand
side of the screen. The amplifier output has quit increasing in direct
proportion to the input (hence the term "flat-topping"), and distortion
(splatter, etc.) is the result.
Most regular oscilloscopes allow you get directly at the deflection
plates, so the vertical part is pretty easy. I don't have any
generalizations about the horizontal input, since that would depend on
the scope and its frequency response. If you had a good 100 MHz scope
with matching probes, you could go in that way both horizontally and
vertically, I suppose. Perhaps others on this reflector can comment on
clever techniques for this with an ordinary scope.
The other half of your concern should be whether you're creating too
much distortion within the transceiver itself. The easiest way to
monitor this situation is to use the "Compression" and "ALC" metering
that may be available in your transceiver. Typically you should be
making those two voltages peak at half or less of their total range.
On my Kenwood transceivers these have always tended to be somewhat
"touchy" adjustments, but they're important and when I'm speaking I
watch the meter readings like a hawk. That way you will avoid having
the sound of overly compressed audio and you will also avoid having the
sound of everything in your shack, including amplifier fan noise, kids
screaming in the background, etc., from coming out your amplifier at
full power between spoken syllables.
> For purposes of setting the mic gain and compression, is a rf probe
> connected before or after the amp a good way to measure this?
You're best off by following the recommendations of your transceiver's
manual.
> If so, is
> before or after the amp a better bet?
As explained above, you need to worry about and check BOTH. You can
create "garbage" within your transceiver and/or within your amplifier.
Bud, W2RU
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