[ARC5] Modeling the PA stage

[Dennis Monticelli]

Hi Ian,

The additional bottom turns on the variable transformer should be
included in your analysis.  They are as much a part of the transformer
action as the top 6 turns.

Dennis  AE6C

On 4/14/06, Ian Wilson <ianmwilson at earthlink.net> wrote:
> I spent a few hours on this yesterday. Any numbers in the following are for
> the BC-459 at 7.1MHz.
>
> Tank coil: the business part of this is 6 turns on a 1.75" diameter ceramic
> former, spread over 1". There are also a couple of additional turns
> occupying another 1/2". Estimated inductance: 1.85uH. This would resonate
> with 272pF at 7.1MHz.
>
> Coupling coil: on a 1.25" former. Assuming 1 turn; about 0.075uH.
>
> Roller inductor: 24 turns spanning 3" on a 2" diameter former.
>          25% of coil =  6 turns = 2.2uH
>          50% of coil = 12 turns = 6uH
>         100% of coil = 24 turns = 15uH
>
> Half-roller would resonate with 84pF at 7.1MHz.
>
> Tube source resistance: my pair of 1625's draw about 150mA at 600V.
> Rp (see ARRL Handbooks, etc) = 600 / (2 x 150mA) = 2k ohm.
>
> If you ignore coupling coefficient and just think of the tank + coupling
> coil as a tuned transformer, this provides a voltage ratio of about 6:1 and
> hence an impedance transformation of 36:1. Using the actual inductances
> instead suggests an impedance transformation of 25:1. This would match the
> 2000 ohm source impedance to 80 ohms.
>
> The effect of the less than unity coupling coefficient is to increase the
> transformation ratio. A coupling coefficient of 0.5 reduces the matched
> output impedance to about 20 ohms.
>
> The coupling coil has some inductance, and so does the roller inductance (if
> not on the end stop). These need to see a load that looks capacitive,
> otherwise the inductive reactance reduces the available output power. I did
> some playing around with (simulated) components. A good recipe for 7.1 MHz
> looks like:
>
>   - series capacitance of 50pF (e.g. ARC-5 vacuum capacitor)
>   - 10uH roller inductance setting (somewhat more than half)
>   - 50 ohm load with 560pF parallel capacitance
>
> When I get time (perhaps over the weekend), I'll see how the reality matches
> the simulation.
>
> 73, ian K3IMW
>
>
>
>
>
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