Hi Phil,
I appreciate your technical contributions, but I respectfully
disagree with some of your comments or observations.
> At 11:14 AM
4/27/97 -0400, you wrote:
> >The SB-1000 band switch, when properly installed, will handle normal
> >operating voltages. Unfortunately several things went wrong in the
> >SB1000. Heath decided to NOT install an anti-corona washer. That reduced
> >the voltage breakdown.
>
> I built one of the last SB-1000 kits. It DID specify the installation
> of the washer on the switch contact. It sticks in my mind so well because
> I had never seen this done before, and Heath did not explain what function
> it had.
The washer reduces the voltage gradient around the terminal for the
160 padding capacitor. That terminal has the most voltage, since it
connects directly to the anode.
While this greatly reduces the tendency to arc around the most
troublesome terminal, it is not a cure-all for improper loading or
for excessive quiescent voltage caused by Heath's
transformer.
Two other common causes of switch failures are relay transfer not
completing on the antenna contacts BEFORE the input contacts
close, and exciter transients.
Exciter transients can be a major source of arcing. That's why
Carl's suggestion of loading the PA real heavy at high drive is a
good idea. That's also why the Heath manual tells you to load the PA
at full exciter power and to reduce the exciter power until plate
current is 400 mA CW.
> >None of this stuff is magic or
rocket science. It's all common sense and
> >normal engineering, and can be verified by traditional math. I've
> >measured PA's producing over ten times the normal operating voltage in
> >the tank during load faults.
> >
> I have burned up several band switches on a 10-160m Pi-L by not having
> multi-fingered rotors on the band switch to short out coil turns up line
> from the band in use, as the Ameritron AL-1500 and Alpha 77D/SX et al do.
> The arc has nothing to do with load faults, it is strictly due to the
> "apparent "Tesla effect" in the tank. Removing the 160m coil and the
> L coil, and reverting back to a Pi network cures the problem, and the
> same exact band switch can be used with no glitches, even with severe
> miss-tuning
On 160 meters the voltage steadily decreases from tap to
tap as you move along the switch wafer. There is no "tesla effect" on
160, because the inductor does not have self resonant sections at the
operating frequency.
You can measure this by driving the tank with RF from a generator,
and probing it with a hi-Z RF voltmeter. You can either drive the
output port with a 50 ohm generator, terminating the anode in the
operating R, or you can drive the anode through a series R equal to
the anode source impedance from a low impedance source and
measure voltage with the tank terminated voltage.
I'm sure the tanks had something else going on. The pi-L does
increase voltage across the loading capacitors greatly, and that may
have been the initial arc source. Once the loading cap
arcs, the output of the tank is effectively grounded. The anode
voltage of the tube(and hence across the tank components) is
guaranteed to soar uncontrollably when the load is removed.
> As I stated in an earlier thread, in a Pi network it makes
> little difference whether you short unused turns or not, but it is
> mandatory in a Pi-L, especially with a 160m coil hanging out there!
I disagree with that totally Phil. It makes a great deal of
difference on the higher bands if tank inductances are left
hanging open, no matter what type of network it is. A pi-L is a pi
network, driving an L network. The L network section is NOT the
problem, since it operates with very low Q.
The voltage headaches are all in the pi section of the network.
In the pi-section, the voltages from inductor end to end are actually
higher than the voltages from any point to ground.
73 Tom
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