Hi Peter,
> Tom says
>
> >Unless you have a very high Z in the shunt components of the HF
> >tank at VHF, or some pretty poor wiring or connections, it is
> >impossible to have a VHF parasitic arc across a bandswitch.
>
>
> I'll play devil's advocate here.
>
> Assume that the parasitic circuit consists of the tube output capacity in
> series with the inductance of the connection between the plate and the
> tune capacitor (and the inductance of any parasitic suppressor) and the
> tuning capacitor. Bit like we used to 'series tune' things like 6146s and
> 5763s on 2 metres. Now, when we are on 10m especially, and the tune C is
> getting low (in Jon's case, to the same order as the tube output
> capacity), is it not possible that a parasitic under those circumstances
> could have a voltage high enough at the tune Capacitor to exceed the
> bandswitch rating?
If the tune C is equal to the tube output C, then the
> voltages would be about equal, while the 'parasitic' tank would be
> effectively unloaded, and could thus be of high Q.
That's true. In that case the voltage across the tank tuning cap
would be just as as high as obtainable at HF, if the tube operated
just as efficiently as a VHF oscillator as at HF.
A condition like that could be met with a vacuum capacitor and a
low capacitance tube, where the anode to cap path would be low
enough L to resonate at high VHF with the two capacitances.
If the layout is typical, the circuit is a low pass filter operated above
cutoff at VHF. That's what every amp I've seen behaves like,
although there could be exceptions.
For example, I measured one amplifier Rich claims has a "VHF
parasitic" problem, an SB-1000 amplifier. I injected a high
impedance source VHF signal directly at the anode of a dud tube.
Voltage through the tank decreased smoothly with a steep slope
as I moved the probe of a high Z voltmeter along the anode path.
AL-1200's, 1500's, 82's, and SB-220's behave that way. While I
never measured a 922, I'd venture a guess it also would be the
same since the components are the same.
It would be simple to measure a TL-922 and prove or disprove that
bandswitch arcs from parasites are POSSIBLE. Of course, that
still would not prove they occurred.
Most 922 damage is caused by underloading the amp, or driving it
with exciters that have leading edge overshoot. Another common
cause is the antenna relay sequencing, or load faults. Those things
I have seen first hand.
Before I put up a web page, and started claiming certain things
happened, I'd make some supporting measurements that showed
the VHF energy could actually make it to the switch with higher
voltages than the HF energy.
The ratio of operating Q to unloaded Q also becomes an issue in
this.
73, Tom W8JI
w8ji@contesting.com
--
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
|