[ARC5] Audio stage impedances.

[Neil]

> From the tube data, the output RESISTANCE of the 12A6 audio stage at the
> tube's plate is around 7.5K.
>
> However from the turns ratio vs output impedance of the transformers listed
> in the service manuals, it appears that the 12A6's plate output-IMPEDANCE
> is around 20-24 Kohms.
>
> I took the turns ratios of the transformer data given, squared that value, then
> used the resulting value to calculate the plate impedance from the output
> impedance. The result in all cases was 20-24 Kohms.


Your method is correct. The discrepancy may be due to the fact that
the 12A6 is not being operated in maximum power delivery conditions
(it doesn't need to deliver much power, only a few tens of milliwatts
to headphones).
The tube data says that when biased as follows, the 12A6 delivers 3.4
watts output.

Bias 12.5V
Screen current 3.5/5.5 mA
Plate current 30/32 mA

For a total cathode current in the vicinity of 35 mA, the biasing resistor
is therefore 12.5 / 0.035 which is 357 ohms. However, the resistor
actually fitted is 1600 ohms. I think this implies that the operating point
of the tube has been shifted towards voltage amplifier mode with a
consequent raising of the plate resistance.

Maybe the receiver's designers chose to use off the shelf transformer
bobbins and their windings and these transformer ratios dictated a high
plate impedance and thus a raising of the cathode resistor to shift the
tube operating conditions.

Oftentimes the component fitted is not what the designer would have
originally chosen, but is what a supplier can supply at the right price.
For example, if the transformers were originally selected because it was
the intention to fit a low power voltage amplifier tube, but late in the day
a supplier offered to produce umpty million 12A6's for 10 cents each for
5 years...you can guess the rest.

73 de Neil ZL1ANM