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

[Mike Everette via ARC5]

One of the Surplus Radio Perversion Manuals, I think Volume 3, has a short article about how to add an AVC circuit to the SCR-274N and ARA receivers.  Simple but effective, and reversible if need be.  It does not require major work.

However, my experience with the SCR-274 and ARA does not lead me to think it's all that necessary.

73

Mike
WA4DLF
--------------------------------------------
On Wed, 9/24/14, Richard Knoppow <1oldlens1 at ix.netcom.com> wrote:

 Subject: Re: [ARC5] (Fwd) Re:  R-10 in the receivers.
 To: ARC5 at mailman.qth.net
 Date: Wednesday, September 24, 2014, 3:38 PM
 
 
 ----- 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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