Hi Mike,
> impedance. The SB-220 suffered no damage, so it is my belief that the
> damage was caused by arcing of the switch contacts that resulted from the
> excessive RF voltage associated with the Hi-Z of the antenna.
If the SB-220 did suffer damage, it would have been from operating
frequency voltages.
If a VHF parasitic would ever NOT be bypassed very well right at
the input of the the tank, the amplifier would be a TVI nightmare. It
would never pass FCC TA testing.
In order to suppress harmonics, the components shunting the RF
output from the tube to chassis must have an extremely low
impedance. The series components must have a high impedance
at all harmonic frequencies.
This whole parasitic thing is nonsense, so long as the parasitic is
upper VHF. Now a low frequency parasitic can fly right through the
tank, but the frequency that will build the highest voltages is
always the operating frequency.
The ONLY exception would be if the tank was so poor in layout or
component design that it actually behaved like a matching and
energy storage system to the tube. We certainly know if that was
the case to any extent at all, the amplifier would also be "harmonic
rich". (no pun intended)
> That is not to say that parasitics aren't in the realm of possibilities.
> Based upon my limited experience, I don't know enough about the impedance
> characteristics of HF tank components in the VHF range to really say with
> any certainty. What is clear to me, is that load mismatches (wrong
> antenna, improper tuning, etc) can generate RF voltages at the operating
> frequency which are well in excess of nominal operating levels. This seems
> to me to be a more likely explanation for switch failure.
It is an absolute cause. All that stored energy that used to be
forwarded to the load has to go somewhere if the load can not
accept the energy.
The weakest link takes the hit.
73, Tom W8JI
w8ji@contesting.com
--
FAQ on WWW: http://www.contesting.com/FAQ/amps
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
|