[ARC5] BC-456 and the microphone circuit.

[Leslie Smith]

Hello James,

You ask if I have a BC-456.  
No, at the moment I don't have a BC-456 modulator.   I may be able to
get one - but that's still future.

Failing that I'll use the modulator you see in the photo.    I think the
power out will be about 12 watts and that should give me about 80%
modulation into a BC-457.    The difference between 20 and 25 watts
doesn't amount to a hill of beans.  The photo has a twin triode  and a
D^D 12J5.  (Oh, delirious joy!  - to have a circuit using a Department
of Defence "tube"!)  

Regarding low modulation in your set - I imagine you have fed a sine
wave into the mic. socket on your set.
If you don't have an audio generator I can send a copy of a "nice"
circuit that will generate a good quality sine tone at about 800Hz.  3
transistors.     Never fails to oscillate, and gives the most beautiful
"sin" signal you'll see your side of the Pacific.

That signal on the mike jack should quickly identify the appropriate
signal level to correctly modulate your set.

I note that the secondary circuit on T51 (drives the grid of the 1625)
has two resistors - R55 and R56.
These form a 2:3 divider, and set the impedance seen by the primary of
the transformer.
With a 10:1 turns ration the "Z" seen by the mic circuit will be 30 ohms
(by calculation this is 3k/100 ohms or 30 ohms.)

Note, however that the transformer has R54 in series with the primary
and this will attenuate a voltage signal significantly.  The signal from
the mike is "current" (or variation in current).  The current isn't
attenuated by R54.  That is quite different from a voltage-level signal.

I think you should see 15 to 50mA in the mic. circuit.

    73 de Les Smith
   vk2bcu at operamail.com
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