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

[Dennis Monticelli]

Hi Les,

I agree with everything you wrote.  I believe the reason why the gain on my
sets go up with >60V is due to my not shorting out the top 7K screen divider
resistor as you have done.  The extra screen voltage I get with >60V B+
boosts current draw and gain.  You may not see much change because your
screens are already at a decent level.   I think if one is willing to run
the sets at relatively low B+ voltages at all times, then shorting the top
7K resistor (or perhaps better yet, lifting the bottom leg of the divider if
you can easily reach it) is a good way to go.  Just as long as one doesn't
forget about the mod and put 250V on the set.

Dennis AE6C

On Thu, Jan 6, 2011 at 9:54 PM, Leslie Smith <vk2bcu at operamail.com> wrote:

> 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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