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

[Richard Knoppow]

----- 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