>From measurements performed here whilst building "The Ultimate
Amplifier" a tank inductor with a Q of less than 400 doesn't warrant
using!!!! Measured using a Marconi Q meter I found that a Q of 600 and
above is practical with normal aspect coils. The position of the coil
relative to metalwork has a large effect on the final Q value, the
values quoted are for no degradation due to poor mounting.
John ZS5JF
----------
> From: Peter Chadwick <Peter_Chadwick@mitel.com>
> To: 'amps' <amps@contesting.com>
> Subject: RE: [AMPS] Plate load charts
To: <amps@contesting.com>
> Date: 05 January 1999 03:06
>
>
> As Rich says
>
> ? In any event, reducing RF resistance in L1 improves tank
efficiency.
>
> Get this in perspective. If you have a 4CX1000 with 2000 volts plate
swing
> (Peak, not peak to peak) and a 2000ohm plate load with a Q of 10, and
Ip
> peak of 1 amp, the circulating current in the tank is 10 amps. With
a coil
> Q of 100, on 20m the reactance is about 175 ohms and the series
resistance
> 1.75 ohms. This gives 175 watts loss in the tank coil. No wonder they
get
> hot!
>
> If coil Q is 200, the series resistance is .875, and the loss 87.5
watts.
>
> Now take a tube with 4kV plate swing, and Ip peak 0.5 amp. The
circulating
> current for the same Q is 5 amps. The inductance is about 350 ohms of
> reactance, so for Q = 100, R series is 3.5 ohms, and the losses are
> 87.5watts.
>
> Now look at the losses in the padding capacitor on 160. With a Q of
500 and
> a 100ohms of capacitor, R = 0.2 ohms and in the first case, losses
are 20
> watts.
>
> In the second case (the 4000 ohm load) we may not need the capacitor.
>
> So doubling the Q of the tank coil (NOT the working Q of the tank
circuit)
> halves the losses, and this is more important proportionally with low
plate
> load impedances.
>
> All of which says in a lot of words (but hopefully amplifies with
> explanation) what Rich said in a few.
>
> And also explains why good amps have big plate coils with high Q.
>
> 73
>
> Peter G3RZP
>
>
>
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