At 07:12 PM 4/27/97 +0000, you wrote:
>Hi Phil,
>
>I appreciate your technical contributions, but I respectfully
>disagree with some of your comments or observations.
No need for the above, Tom. This is why this reflector is so valuable.
None of us ever reach a point where we "know it all." We come from two
different schools, and sometimes there is more of a communications
problem than a technical disagreement.
>Two other common causes of switch failures are relay transfer not
>completing on the antenna contacts BEFORE the input contacts
>close, and exciter transients.
No disagreement so far! However, my SB-1000 never had a transfer relay.
I used the factory wired Heath QSK board with pin diode switching. The
output of the amp was "hard-wired" to the antenna jack.
>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.
This can be detrimental to the health of the 3-500Z if the exciter is
capable of 150-200 watts, and no cathode padding or no external ALC
cut-back is used.
Again we agree. I have experienced the transients. That is why I believe
the cathode padding resistors in Rich Measures kit are a valuable addition
to this, and any other amp that can be over driven by modern day transceivers.
It certainly does not hurt anything, and almost assures a clean signal in the
"heat of battle."
>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.
That is correct. The failures to my switch were on the 15 meter and
40 meter tap positions, and on the 10 meter position. The 10m and 40m
contacts were nearest the ceramic support insulators, and it melted
the ceramic! The only failures I have had on 160 meters were the door
knob padding capacitors. That was before I obtained a book from HEC
and found out how low the current limits really were on such devices.
ETO, in their infinite wisdom, failed to supply higher current padding
caps with their 77DX-to-SX conversion kit. The drift in the original
cap due to heating was horrific! No wonder..its current rating is only
1.6 amps @ 1 mhz!
>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.
This simulation may be of value during initial design, but when you
put the power to the circuit, strange and different things begin to
happen. In a Pi-L circuit the weakest and most vulnerable point is
the band switch, In a Pi network, it is the tune cap. (the SB-220
immediately comes to mind) What I am saying is that the failure of
band switches started the day amplifiers hit the market with 10-160
meter Pi-L networks. It takes no test equipment or rocket science to
deduce that this combination, in addition to smaller cabinets, cheaper
smaller components, etc., are a major cause of failure. If not, why
did ETO, Ameritron, et al spend extra bucks on special-order band
switches with multiple rotors?
>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.
We still agree here..when that happens on a Pi-L, you loose a band
switch if you allow the arc to continue. On a SB-220, you see flames
on the tune cap.
>> 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 L section is NOT the problem, I agree. It is a player in the result!
No strait Pi network design I have ever seen bothered to short any turns;
commercial or otherwise. It made no difference back then because only
5 bands (80-10m) were involved. (B&W 850 series coils, et al)
>The voltage headaches are all in the pi section of the network.
Certainly no disagreement here!
>In the pi-section, the voltages from inductor end to end are actually
>higher than the voltages from any point to ground.
In the pi-section of a Pi-L network, the voltages on the Pi coil from
end-to-end are actually higher than in an equivalent strait Pi network.
(((73)))
Phil, K5PC
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