[ARC5] Fw: Receiver current drain (AF output)

[Leslie Smith]

Hello group,
Followed this thread with interest, and every possible angle has been covered, or so it seems.  With this in mind I add MY bit.

Was very interested, Dennis, to read that you got additional gain by running your set at 90V, as I run my sets at 60V DC.  

As I understand the original design, early sets were designed to drive headphones with 2k ohms impedance.  Later this became 600 ohm headphones.  Since the output impedance of early transistor radios was about 500 ohms I ran a raiding party on my local radio club store, and got a handful of output transformers "rescued" from broken transistor radios.  I drive this directly from the output transformer of the "command" set.  Works well with a good 8 ohms speaker.

I measured the output at 40 to 60mW.  This is a suitable for a quiet room.  Alternatively on "old" amplified PC speaker set will work well - and give more volume.

The efficiency of various speakers vary from 88 to 90 dB (at the low end) to 98 to (perhaps) 102 dB (at the efficient end).  Therefore the choice of speaker is as important as the choice of output transformer.

As for current consumption of the "command" sets, I collected notes and comments on this some time ago.  


Denny, AE6C, wrote:
"With about 60V B+, the speaker can be driven adequately without the aid of an external amp.  If your B+ current, while running 60V, is more than about 10 to 15mA then you probably have leaky caps.  I suggest you slowly apply B+ while monitoring the supply current when powering it up for the first time.  You may get lucky and be able to ramp up to a sufficient voltage without having to replace original caps."

The various sets I checked drew 20 to 25mA, (measured) including current drawn by the screen divider circuit.  Since the screen divider circuit draws 8 mA (60volts/7k) my figure matches the values above.  I short the top 7k resistor in the screen divider chain, putting a full 60 volts on the screen grid.  

Best wishes for 2011 es 73s


Les
formerly vk2bcu




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