rca: Modification to improve Band B sensitivity
Band B on this set (Medium wave on the Q109X variant) is designed so that the first RF stage is always disabled and therefore this band has very poor sensitivity. It is assumed that this feature was incorporated to avoid overload of the input stages with strong local stations that more typically exist in the USA.
I modified my set so that the first RF stage is active for all bands by isolating the S3 front wafer (tag 5 on the Q109X or tag 4 on the Q109) on the wavechange switch so that the 6SG7 RF valve always has cathode to ground via the 10 ohm resistor R2.
The first RF stage can still be disabled for any band by switching the Local/Phono/Distant switch to local.
This provides full sensivity for all bands whilst retaining to option to disable the first RF stage if required.
It's common on UK sets with an RF preamp for SW to disable it on MW &LW. It's not just about the signal strength. A loop aerial (external or internal, or internal ferrite rod) has a good output on MW & LW. SW usually uses a whip or a wire. This picks up local electrical interference much more than a loop does. Some sets with internal FM dipole even use this to cancel out local electrical interference, E field.
Also atmospheric / cosmic interference rises rapidly with lower frequency, such as MW.
The reverse of your supposition about USA sets is true. There are many with BC (MW) only that have an RF preamp, especially battery tube sets with a shielded loop aerial. All UK sets I know disable the RF preamp on MW & LW. Sets with an external wire pick up too much local interference on MW and if there is a loop aerial or rod aerial, they don't need the preamp on.
UK / Irish model has RF preamp only for SW 39 JH/E
The RCA portable with preamp and only MW. Globe-Trotter 8BX6
In general in the urban and suburbs, adding the RF preamp increases noise as much as signal. In the UK it might help in a rural situation, if tuned, which needs an extra MW coli for the three gang tuner. If it's only a dual gang tuner then the situation is worse and the RF preamp is only good for higher frequency short waves and a suitable wire aerial.
I used to design RF systems and I'd put it back the way it was. Since it had the cost of the RF preamp, it was deliberately not used on MW, which is the most common archtecture on valve/tube sets in USA or Europe with SW and MW.
Thank you for you very comprehensive explanation and background on sources of noise and operation of RF stages. I wasn't previously aware of the benefits of loop aerials for minimising noise interference on MW. I shall try an external loop aerial with a view to re instating the original circuit in this RCA set.