> To: amps@contesting.com
Hi Steve,
> 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.
You have no idea what the exciter power or other transient
conditions were at the time. You could have a relay out of sync, an
exciter transient, an antenna fault, or even incorrect loading.
On ten or 15 meters, just a little too much load C goes a long long
way.
>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.
When an arc occurs, it generates broad band noise. Much broader noise
than a "parasitic". As a matter of fact, if it was a parasitic it
would be a coherent oscillation, not the broad band mess you
describe.
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.
I see. So we have a intermittent parasitic that is long enough in
duration to overcome the thermal lag of a two watt carbon
comp resistor and overheat it to the point of failure..........but it
is also so short you can't measure it or see its effects on meters.
Sounds fishy.
> 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.
Since the amp ALWAYS has maximum gain at minimum power, why can't
you make it oscillate at zero drive? The reason is the LOSS exceeds
the gain.
You are blaming something else on an "oscillation", just because you
have no idea what the real problem is.
> 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!!
Come on now. Cycle that relay all day long without drive and listen
for the arc. If you don't have a tube that has excess outgassing, you
won't EVER get an arc from cycling a relay.
Once loss exceeds, gain a circuit won't EVER oscillate. If loss is
less than gain, it can always be made to oscillate.
> 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?
Resonances do not mean "oscillations". As a matter of fact, they can
mean just the opposite!
> 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!),
You are tuning the amp WRONG. NEVER tune an amp at low power. If you
tune it at low power, you are asking for arcing problems.
The plate impedance of the amp changes greatly as you tune the PA,
and if drive is light when tuning you guarantee the PA will be
undercoupled for higher drive power levels! That's the very thing you
don't want!
> 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?
Reality check here Steve. (It won't work for Rich)
Let's assume the tank C and tube stray C is 25 pF. At 180 MHz that's
35 ohms from the anode to chassis. Now let's add C. Make it 250 pF.
The impedance is 3.5 ohms. Big deal. Both look like near-shorts at
VHF. The resonant frequency moves very little, and very little
voltage can develop across such a short.
Next along the line we hit a tank coil. The series inductance is
probably about 1 uH. That's 1.13 k ohms at 180 MHz.
Next we have a shunt C of perhaps 300 pF in the loading control.
That's about three ohms of shunt X at 180 MHz.
So after going past 35 ohms to ground at the anode, over 100 ohms in
series, and another 3 ohms shunting the output line you are worried
about a little load resistance error or relay capacitance?
I don't think so.
>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?
No. No bang from that. The relay and the load are far beyond
affecting the VHF resonances near the tube.
The problem is you have been reading Pathological Science texts by
one man out west who really is a good writer, but who has no idea how
things really work. You've been sold on perpetual motion by a good
salesman.
The tube has some frequency at which the grid becomes parallel
resonant. In a 3-500Z with short grid leads to the chassis, that's
about 180 MHz or so. At that frequency, it looks like a parallel
tuned grid with high Z. The dominating component is the grid C.
The anode lead and anode stray C can also form a resonant tuned
circuit. The dominating component is the anode C, NOT the tuning C.
If the anode Q is high, the impedance is high, and the gain is
high the stage might oscillate.
The suppression circuit adds series resistance to the anode path.
Ideally it would add a moderate resistance, so the tube
drives a pure resistive load at VHF. The resistance can't be too low
or it looks like it "isn't there".
It's better to err on the high side, since the resistance is in
SERIES with the anode load. That prevent the tube C from connection
to the inductance of the tank leads to the tuning cap, and lowers
gain and Q.
It's just like an audio amp, the lower the anode load resistance the
BETTER the higher frequency response. The higher the anode load
resistance, the worse the high-frequency gain (response).
What you do at the tank coil, the relay, the load, etc does NOTHING
to this system. The system is at the tube, and ONLY between the tune
C and the tube connection.
> Sorry, I don't buy that explanation as a reason why the output loading cap
> in the SB220/SB221 could arc over, Tom.
That's OK. If you don't buy it, then you don't "buy" simple
reactance formulas or how oscillators work.
There is absolutely nothing I can do to prove UFO's don't abduct
people to a true "believer".
73, Tom W8JI
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|