>> 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 could have a relay out of sync, an
>exciter transient, an antenna fault, or even incorrect loading.
>
He stated that the loading was correct, Mr. Rauch. There was apparently
no antenna fault.
...
>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.
>
Have we heard about amplifiers that had parasitic problems at the instant
they switched from receive to transmit?
>You are blaming something else on an "oscillation", just because you
>have no idea what the real problem is.
>
Based on the evidence at hand, this seems unlikely.
...
>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.
>
Rauchian gas: First you have it, then you don't. Pull that gassy tube
out of the amplifier, and before the tube can be connected to a high-pot
tester, that elusive gas mysteriously vanishes.
... ...
>Resonances do not mean "oscillations". ... ... ...
>
. OTOH, oscillations mean 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!),
>
>You are tuning the amp WRONG. NEVER tune an amp at low power.
agreed
>If you
>tune it at low power, you are asking for arcing problems.
Apparently not in my SB-220.
>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!
Good point, well put.
>
>> 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)
>
The 110MHz energy from the parasite can not pass through the low-pass HF
tank.
>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.
>
This would be 'absolutely correct' IF tune capacitors had no VHF
resonances. Mr. Rauch conveniently - but probably not mysteriously -
ignores such resonances here even though he previously claimed to have
measured the VHF resonances in a number of tune capacitors. .
... ...
>So after going past 35 ohms to ground at the anode, ... ...
It is not going to be 35 ohms in the vicinity of a tune-C resonance.
>
>>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 big bang appears to come from an arc in the anode circuit.
>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.
Semi-thanks on the semi-compliment, Mr. Rauch.
- I proved that you were absolutely right when you said one could design
a parasitic-suppressor with copper or silver conductor material that
would have an even lower vhf-Q and lower vhf-Rp than a resistance-wire
suppressor. The 200nH, Ls, 200 ohm Rs clearly beat the pants off of the
resistance-wire suppressor in Wes' tests. . . . All one needs to make
the winning suppressor is a 200 ohm, under11nH resistor that can
dissipate 43 w at 28MHz.
>
>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.
I measured under 100MHz. Has anyone else made this measurement?.
... ...
>If the anode Q is high, the impedance is high, and the gain is
>high the stage might oscillate.
>
True enough. However, there is more going on here than meets the eye.
During the grate VHF parasitics debate I became aware of several
considerations that I was unaware of when I wrote the article on
parasites in the September and October 1990 issues of *QST*.
> ...snip...
R. L. Measures, 805-386-3734, AG6K
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