>>>Is it perhaps that the Q of the tank circuit is very large?
>>
>>The Q in the typical ham amplifier is nowhere near very large. 10 to 12
>>is common.
>>
>>>Larger Q will cause the voltages to be higher. And perhaps at the same
time
>>>the
>>>switch that is used is really under-rated?
>>
>>The output bandswitches in the TL-922, AL-80, and SB-220 have a
>>withstanding ability of roughly 6000v.
>
>OK....I think the original question Carl posted though was not about the
>AL-80 but about another Ameritron amp. Is it possible that Ameritron
>uses a different, bandswitch..
>
>So I still have my question on bandswitches that I posted the other
>night, Rich:
>
>If the Pi-network in a PA rejects VHF/UHF signals how does the VHF
>voltage of a parasitic get high enough to destroy a bandswitch?
Due to the parasite freq. being near a series resonance in the Tune C?
The most often destroyed bandswitch contacts are the10m position, which
is closest to the Tune C potential.
>Consider this: The tube has less gain at VHF/UHF, the pi-network is not
matched
>at VHF-UHF. So how does it happen?
Perform the following near-resonance experiment:
Connect 1 ohm of XC in series with1.001 ohm of XL.
Apply a sinewave of one volt rms.
What is the current?
What is the potential that appears across each component?
It looks to me like the current would be 1V/the net X of 0.001 ohm =
1000A-rms,
the potential across L would be 1000A x 1.001 ohms = 1001V-rms,
and the potential across C would be 1 ohm x 1000A = 1000V-rms
>The Pi-network does not "absorb" out of band energy as someone suggested.
> If that's the case RF theory of ideal components would break down.
true
>Unless I am really, really missing something VHF/UHF energy will be
>reflected at the input of the pi-net *back* into the anode circuitry.
>This is how "lossless" networks "attenuate" energy.
>
>If VHF/UHF energy was allowed to freely roam in and out of the pi-net,
>then we wouldn't get harmonic attenuation.
>
agreed
>No one responded to my earlier post on this. Anyhow, after mulling this
>over some more I have another question:
>
>* Just because the VHF/UHF energy is reflected from the pi-net back to
>the anode, does this mean that there is no VHF/UHF voltage across any of
>the components in the pi-network?
My guess is that it bounces back and forth looking for a resistive load.
Metal vapour arcs certainly have resistance.
> I wish I had the capability to do a
>SPICE model
... apparently, so does Prince Harry.
>...on this because if you build up a voltage potential, then
>maybe a parasitic could break down the switch. But I can't prove this
>one way or the other.
>
I doubt that Jon or Rich could prove it to anyone who has committed
themselves to the position that parasites are incapable of doing such
damage. However, for me, finding a damaged vhf suppressor resistor after
a bandswitch arc/meltdown is something that I find to be fairly
convincing.
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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