[AMPS] Band Switch Heath SB-1000

[Phil Clements]

At 10:41 AM 4/28/97 +0000, you wrote:
>
>ALC will not cure the problem, because ALC is traditionally too slow 
>to limit leading edges of waveforms.

Just a little communications problem here...a fixed, adjustable external
ALC supply will cut the exciter back and keep it there.

>
>Be sure you apply that current rating correctly. On lower 
>frequencies current is determined by the I*X voltage drop 
>across the component and the voltage drop across the component. On 
>160, you are well within the rating of the component. The 1.6 amperes 
>does NOT indicate the capacitor is close to HEAT failure from 
>excessive current, al low frequencies the current rating indicates 
>the point of arcing from VOLTAGE failure.

The 1.6 ampre rating @ 1 mhz is the maximum allowable current to maintain
the N750 temperature coefficient rating. Drift goes to pot above this #.
The 1000 pf doorknob in the Alpha 77DX worked fine. In the SX, you need
to replace it with 2-500 pf units to stop the drift problem on 160m.

>Drift problems are a different issue. Drift problems have come and 
>gone with ceramic doorknobs. HEC's capacitors were originally quite 
>stable, while ITT Jennings caps were very poor. When HEC bought out 
>ITT, HEC's stability went to hell.

I hope they get their act together soon, as they are just about the only
game left in town! Prices, by the way, are through the roof!

>> 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

Communications problem again...what I am saying is that the spark on a
Tesla coil is a little more robust @ 1500 watts input than it is @ 1 watt.

>Other than the slightly higher voltage across the loading 
>capacitor, the pi-L is no more likely to produce high voltages 
>anywhere in the tank system than the pi.

Not likely, but possible...been there, done that!

>Virtually all PA's short unused turns. Turns often need to be 
>shorted to prevent unwanted voltages from building up when operating 
>on higher bands. 

I agree totally, but show me ONE article in any Bill Orr Handbook, or
ARRL handbook that specifies a shorting type band switch! I'll save you
the trouble, there are none. This is a costly oversight!

>On a 80 through ten meter tank, the designer might get by without 
>shorting unused turns, but he likely won't get away with it on  a 
>160 through ten PA.

I believe I stated that...I am here to tell everyone you will not get
away with it in most cases.

>> 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.
>
>Not necessarily. It depends on phase shift in the network.  In most 
>design cases,  phase shift is less making end to end voltage less. 
>Traditionally pi-L's operate at slightly less Q in the pi-section 
>that conventional pi's.

A design Q of "X" is a design Q of "X"...in ANY network. The voltages
in a Pi-L might not necessarily be higher, but they can be, at least
in my case.

>In any event, the problem was indicated to be  voltage breakdown to 
>ground..... not end to end across the switch.

That is correct. The rotor remained unharmed in all cases.

>But I can say one thing for sure, the reason was 
>almost certainly not due to the network being a pi-L...unless the 
>pi-L caused a loading capacitor or rotor to ground arc. 

The reason was the Pi-L was hooked to a band switch that did not have
shorting provisions; just like the Handbooks show!

>No matter what the cause of this particular problem, I'd hate to 
>see people dismiss pi-L's as "arc prone" systems.

Pi-L's are very arc prone if you do not short the unused turns. If you
short them, no problem! Build your amp from a schematic from Alpha,
Ameritron, Command, or QSK; not Bill Orr or ARRL, and you will do fine!


(((73)))
Phil, K5PC

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