[AMPS] SB 220 vice SB221

[Steve Harrison]

At 08:10 AM 11/23/97 +0000, W8JI wrote:
>Hi Bob and all,
>
>You wrote:
>\
>> Late versions of the SB-220 and, I suppose all of the SB-221 runs, had a
>> different capacitof for C1 which had just a little bit wider plate
>> spacing in the design. This was an attempt to stop arcing and spitting
>> when the parasitics might occur.
>That's utter nonsense.
>
>The spitting and sputtering is not due to parasitics at all.
>
>The plate variable/ tank arcing problem most often is incorrect 
>tuning, and secondarily incorrect relay sequencing in the relay 
>contacts.

That's strange; every time I've had an amp do the "arcing" and "spitting"
thing, it was when either/or both the plate and loading caps were at or near
minimum; and almost always when the bandswitch was on 15 or 10m. Sometimes,
I've had a TV running in the shack (an outstanding indicator of...shall we
call them, "arcing" and "spitting"... incidences); and on occasion, have
even had my GDO sitting on the bench in "diode" mode if I was working on the
amp. When the "arcing" and "spitting" occurred, the TV would go nuts (didn't
matter what channel, anything from channel 2 through 6 would do just fine)
and sometimes the GDO would violently deflect. Once in a while, the
parasitic suppressors would even smoke after the "arcing" and "spitting"
incident. Usually, though, the incident was over so quickly that things
exploded rather than smoked.

Now, years later, I sure wish I'd known that mistuning the amp(s) was doing
nothing more than mistuning the amp at the fundamental frequency. I always
figgered that since the incidents occurred while "mis"tuning the amp very
near minimum capacitances of the output tank caps, I was causing a resonance
in the plate circuit at or near a VHF frequency where the input circuit
happened to have a resonance, too. So the amp was taking off, oscillating
like mad for a brief instance while things arced and other things burned up
and still other things popped since there was no load to dampen the VHF
oscillation.

And sometimes, yes, the same would happen during a T/R relay switching
cycle. I always figgered that while momentarily unloaded (the relay contacts
were closing or opening), the arc across the relay contacts was changing
resistance (and capacitance), causing the apparent load on the amp to
change; so when the arc's resistance (or capacitance) reached a particular
point where the amp was *especially* unloaded at VHF, the plate tank,
momentarily resonated at the VHF parasitic frequency, would do that nasty
unpleasantness and BANG!!

Ha ha...that's funny, isn't it? Weird how people can come up with such wild
misconceptions, isn't it?

Seriously, however, it seems to me that what happens when there *is* a load
on the amp (i.e.; when the relay contacts are solidly closed, as they
normally are while tuning-up), and what happens when the relay contacts are
opening or closing, result from two widely different, yet related,
circumstances. In the former, there *is* a load on the amp's output; but in
the latter, there *isn't* a load.

So if my amp "arcs" and "spits" while tuning-up with a solid load (and I
happen to be on 15 or 10m where the plate and loading caps are normally near
minimum), is it not possible that a poorly-built tank circuit *could* have
VHF resonances? Assuming I'm tuning the amp into an antenna (at QRP power
and on an unused frequency, of course, so I don't bother anybody else!),
isn't it also true that *most* 15/10m antennas probably don't present a
low-Z load to the amp other than within the 15/10m bands; so that at VHF,
the actual load impedance *could* effectively be extremely high? In that
case, it really doesn't make much difference where the output tuning is as
far as resonating at 15 or 10m is concerned, does it? As soon as the caps
cause a resonance near some VHF frequency where the input tank resonates and
there is nothing in either the plate or grid tanks to "dampen" that
resonance, BANG!!; right?

This, then, would be the same thing that happens when the amp is operating
while the T/R relay contacts are not completely closed, no? So at some time
while the contacts are still moving either way, there is no load on the amp,
particularly at VHF? And if it happens that when perfectly tuned for 15 or
10m, that my plate tank *also* is resonant at or near the same VHF frequency
at which my input tank is resonant, things go BANG!! just as they do when
the T/R relay is solidly closed.

If my amp isn't taking off at a VHF parasitic at those unopportune
"mistuning" times (evident by the TV going nuts and the FM BC radio nearly
blowing the speaker out), then kindly, dear Sir, explain what generated all
that energy in the VHF bands? Harmonics, due to mistuning of the tank??
But...but...EVERY low-band TV channel (and sometimes some high ones,
too)....the WHOLE FM BC band...gets clobbered! Surely, harmonics would be
rather narrowband, and evident only at multiples of the 15 or 10m
fundamental frequency??

[snip]

>The same is true if your exciter has a power transient (common with 
>many rigs), because overdrive is  like underloading so far as the 
>anode system is concerned. 

Interesting: how could this happen, particularly since most ham exciters
used to drive low-mu triode amplifiers usually will saturate within less
than 150% of the output power required to drive the amp to nominal output
power??

Lessee: Assume that we want an input drive power sufficiently high to cause
the output voltage across the amp's loading cap to reach three times normal.
That's a voltage gain of...hmmm...ahha, three! And when the voltage is
increased by three times, then the power is then...ah...yes, 9.5 dB higher!

WOW...Tom me boy, are you trying to say that ordinary, run-of-the-mill
ham-type radios are capable of emitting an output power spike of almost TEN
times what they can do when SATURATED???

Sorry, I don't buy that explanation as a reason why the output loading cap
in the SB220/SB221 could arc over, Tom.

73, Steve Ko0U/1


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