[AMPS] Shorted Pi-Network Turns

[Tom Rauch]

> The progressive shorting of the coil presents problems for the amplifier.
> In particular, shorted turns will act like a shorted turn of a transformer
> and can lead to large circulating currents.  

Only in cases where mutual coupling between turns is nearly unity, 
very high, and/or there is considerable dissipative losses in the 
system somewhere. 

This is not normally a problem with any air wound inductor, 
because flux leakage is generally pretty high. It is a problem with 
toroids, especially if loaded Q is high. 

> May, 1963 issue of 73 magazine by Bert Green, W2LPC.  Bert wrote the
> article while working for Amperex.  He made distortion measurements of the
> amplifier after construction and stated that "the distortion was decreased
> even further by leaving the unused turns on the plate tank coil unshorted
> when switching from band to band instead of shorting them as was done when
> the amplifier was first constructed".

Losses and circulating currents don't cause non-linearity or 
distortion. More than likely he was seeing normal measurement 
scatter.

How much change did he measure, and what was the test setup?
 
> I installed the replacement wafer with no shorting contacts and had no
> problems.

You are very lucky. The arcing problem occurs because at some 
point in the center of the tank there are back-to-back L networks 
formed by stray capacitance and the series inductance to that 
point. Since C is low and L high, the voltage step up is extreme.

This effect has little to do with mutual coupling in the inductors, is 
is a series L shunt C problem.

If you operate the amplifier on a frequency where the tank system 
is resonant in that mode, it will destroy the switch.

Normally that happens when on 15 or ten meters. The effect is the 
same effect that does not allow you to use split chokes to avoid 
series resonances.

Antenna tuners have this same problem.

You can clearly see the frequencies where this will happen on a 
network analyzer.


73, Tom W8JI
w8ji@contesting.com

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