Mag loop aerial for the Den

[Murphyv310]

Presumably this is also why a ferrite rod works so well as it also receives the magnetic component of the wave.

Tranistors can give a lot more gain than valves since, as Gary implies, gm (mutual conductance) is proprtional to emitter current. Gain  = gm * collector loadresistance. This is just expressing Gary's rule of thumb in a more usual form. It also requires that the fT of the transistors is much greater than the frequencies of interest which will always be true for modern transistors and medium wave. Transistors are also low noise, perhaps not in absolute terms but much lower than the atmospherics etc found at these frequencies. The downside may be if strong signals are present as you can get overload and intermodulation. I understand that this is why RF power devices were often used at the front end of solid state comms recivers.

Am I right in thinking that if you wanted to use this sort of antenna with a valve front end you'd probably need to use a grounded grid stage to get a low input impedance? Or have I got something horribly wrong.

Hi Jeffrey.
Indeed its the magnetic component of the signal we want to use, the tunable loop falls down a little here in comparison to the single turn active loop. I personally wouldn't consider a valve preamp, faffing about with supplies to the loop is an added complication. Don't forget the transistors in the amp are driven fairly hard (they run warm too) to keep their strong signal handling capabilities up, this reduces intermodulation distortion.
Ideally RF medium power transistors should be used and in fact the humble BD131 works very well in the circuit.
I also think some experimentation on the RF output transformer could yield even better matching and thus performance.