[ARC5] (Fwd) Re: R-10 in the receivers.

[AKLDGUY .]

> The voltage at the high side of the pot is about 30 volts, which
> is about right. The bias is applied to the cathodes so it must be
> a positive voltage. Probably the voltage divider from the B+ is
> used to insure that enough voltage is available to cut off the tubes
> even if the total cathode current is too low.

Yes. By making the cathodes more positive with respect to ground,
you effectively make the grids more negative with respect to them
and reduce the gain.

Perhaps the addition of R-10 can be explained this way:

Disconnect R-10 completely from the circuit. Set the gain control
pot to a non-zero resistance. Note the receiver's gain. Now connect
R-10 with clips.

I guarantee the gain will drop, since R-10 is now feeding more +ve to
the cathode circuit and raising it further above ground. The connection
of R-10 cannot suck +ve voltage from the cathode circuit (which would
increase the gain), since current cannot flow from the cathode circuit
(low potential) to the B+ line.

So the addition of R-10 magnifies the rate at which the pot reduces
the gain, and the resistance values chosen ensure the pot does cut
off the tubes somewhere near max resistance.

I must say I'm astonished at the suggestion that convicts were given
the task of writing technical manuals. Where did that come from?
Oh wait. Was it meant that Australians wrote them on behalf of the
Americans?

73 de Neil ZL1ANM


On Thu, Sep 25, 2014 at 7:38 AM, Richard Knoppow <1oldlens1 at ix.netcom.com>
wrote:

>
> ----- Original Message ----- From: "Kenneth G. Gordon" <
> kgordon2006 at frontier.com>
> To: <ARC5 at mailman.qth.net>
> Sent: Wednesday, September 24, 2014 9:08 AM
> Subject: [ARC5] (Fwd) Re: R-10 in the receivers.
>
>
> Gents:
>
> I BCCed the reply from Neil which included his thoughts on the purpose of
> R-10 to my friend, Roger Kuchera K1TG, who is the person I mentioned who
> is attempting to restore to working condition his first "ARC-5" receiver, a
> badly mauled BC-453.
>
> He did some digging in one of the SCR-274N manuals I sent him and came
> up with this below....which I should have read before I asked the question:
>     Probably answered in other posts. I got curious and
> looked at the schematic, the gain control is actually pretty
> straight forward but I found another puzzler.
>     The set has no AVC, gain control is completely by means
> of bias to the RF and first IF stages.  These are 12SK7
> tubes which require about -30 volts on the grid to cut them
> off.  The gain pot is part of a voltage divider from the B+
> line to ground. It consists of R-10, a 390K fixed resistor
> and R-25, a 50K variable resistor which goes to ground. The
> voltage at the high side of the pot is about 30 volts, which
> is about right. The bias is applied to the cathodes so it
> must be a positive voltage. Probably the voltage divider
> from the B+ is used to insure that enough voltage is
> available to cut off the tubes even if the total cathode
> current is too low.
>     Now, in looking at the circuit I find that the RF and
> both IF stage grids return to ground through a common 100K
> resistor, R-11.  There is a note in the introduction
> indicating that while there is no AVC some means is provided
> to prevent overloading.  I think this may be it since any
> grid current developed from an external signal or a gassy
> tube will tend to cut the tubes off. Very curious circuit
> but I suspect it may be in the Radiotron Handbook. I think
> the idea may have been to keep the receivers operational
> even if tube conditions, etc, were not optimum.
>
>
> --
> Richard Knoppow
> Los Angeles
> WB6KBL
> dickburk at ix.netcom.com
>
>
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