That add-on will definitely be a Lamb Noise Silencer. It has the same valve lineup as the setup described in the 1937 article that introduced it.

A very radical and very successful idea that found its way into many high-end HF and VHF receivers. Tait 2 way radios used to have it.

That add-on will definitely be a Lamb Noise Silencer.

Patented we hasten to add by James J. Lamb, not Hannibal Lecter.

Very droll GTC!

"I must go now Clarice, I'm having a friend for dinner".

Hi All, I do a bit on the set from time to time between other things, slowly working my way through the stages and getting into the designers mind.
The most intriguing part of this set is the "S" meter and how it functions. Basically it responds to the IF valve current variations and is an ammeter or more correctly a milli ammeter. It is connected in a weird current dividing circuit that allows you to "null" the meter reading to zero in the standing no signal condition and then respond only to the change in valve current on signal. This then by magic gives a derivative reading in "S" units or dB.
Well that's how it seems to me, anybody ever worked out exactly how it works?
I've seen a couple of references to this meter in the various web sites but no one gives an exact account of how it works, or maybe its so obvious no explanation is needed.

The set is now running but very poorly so I have to go into each stage carefully and gain check etc.

Just a safety thing:, the S meter null pot pokes out the back of the set as a screw driver slotted brass spindle. As always i'm very suspicious of pots connected to the HT and checked this one closely for safety. The pot shaft is LIVE direct connected to the wiper, the pot mounted on a Bakelite washer and that folks is how it was made! Touch that baby with your finger and you are connected to 250 volt HT!
They must have been tough in those days or radio guys just assumed that anything could bite you and it was normal.
Fred.

Hi Fred,

Mmmm .... the circuit is a bit complex, but your understanding seems okay to me. I usually think of it as a voltage difference generated by the amplification of the signal by the IF valves ..... this voltage difference generating the current draw through the Meter.

I have a Hammarlund Comms receiver with the Meter across the two IF cathode circuits based on the same principle (much simpler circuit). I needed to 'match' two IF valves ( ie they develop the same Plate current under same operating conditions) to get to a reasonable 'no signal' zero point.

Another web site with some RME 69 info for you ..... http://www.skywaves.ar88.net/commrx/RME/RME.html

Is that normal for the pot shaft to be connected to the wiper????? Although mine is low voltage, it has red tape on it now to remind me to check these sort of things in future.

Hi Ian, yep I had a good look at it today and when you redraw the basics its a bridge circuit.
The upper arms of the bridge are the 1000 ohm resistor and the null pot, the lower arms are the 100k resisitor to ground and the impedance of the valves drawing current.
I pulled the meter apart and cleaned it and then tested the FSD, with a coil of about 32 ohms it had an FSD about 1.5ma.
I then hooked it up to a test circuit driven at 260 volt and with the original vaues and you can null the meter reading at say 3ma load and then the load going to say 12ma can give you FSD. Of cause the thing is like a seesaw and the meter can read in reverse very easily.
The HT varying in value does not make the meter reading change much and a change of 50 volts from the power supply only shifted the reading one unit.
As the load on the bridge is the AGC controlled valve plates the meter then gives a derivative of the plate current and so carrier level.
I put the meter back in the set and can null it and wind the meter up and down with the manual gain control.
The set still is like a lame duck 2 valve TRF set so I have a long way to go.
Fred.

I cannot add any information as to this radio, BUT... my years in manufacturing in the US exposed me to some technology that may not have been considered.

Emerson Electric used a motor/dynamo combination for quality testing and inspection of their 50Hz powered products for export, delivering a pure 50Hz sine wave power supply from three phase 60Hz. I am sure other companies would use the same or similar system. It looked very much like some large 3 phase DC welders.

Lindsey

Hi Lyndsey, I know exactly what you mean re the motor/dynamo set.
I worked for a firm back in the 1970's that specialised in "MG" sets for running American computer equipment in Australia. Usually 208V 3ph 60Hz and up to 250kVA in size.
The computer stuff was usually Honeywell or IBM. The MG sets were 415V 50Hz motor belt coupled to a alternator with a ratio step up to get the 50/60Hz increase. And yes the belts were very big! Usually a flat multi vee belt about 10 vees or so.
The RME set was my first dabble in a "communications " style set and you have probably read the saga in the "special projects " section.
Cheers, Fred.