A while ago, someone was asking if it were possible to replace the old style long wire antenna with an internal antenna, like a ferrite rod loopstick antenna. I just modified a Healing L502E this way, so it now has a loopstick. And the sensitivity is quite good, right now I'm picking up a station 400 km away (it's Saturday night here now in NJ USA).

How to do it: Locate the antenna circuit, the input coil and the section of the tuning cap it goes with, and also the feed to the converter tube, in my case a 6A8. And the AVC line. The ferrite rod should be adjustable in inductance, which is usually done by having the coil slidable on the ferrite rod. My rod came out of a dead solid state AM radio, and had too much inductance to resonate with the Healing tuning cap, but being slidable, I slid the coil about halfway off the end of the rod to get the inducance right.) But I'm getting ahead of things here. Disconnect the old antenna coil from the tuning cap and converter tube. Connect the "hot" end of the ferrite rod coil (the line that went to the tuning cap in the solid state set) to the tube radio's tuning cap and converter tube. The other end of the ferrite rod antenna coil now connects to the tube radio's AVC line, like the old antenna coil connected. In my case, the AVC line was a bit hard to solder to (underneath other parts) so I connected to the old antenna coil's old "hot" end to pick up the AVC voltage, and I bypassed this connection to ground with a 0.01μF cap. Thus using the old antenna coil as an RF filter choke, though this is not electronically necessary. I was not going to remove this coil anyway, so as to allow restoration back later.

Solid state radio loopsticks also have a secondary winding to feed the converter transuistor, but here I could use it as an input winding for an optional long wire antenna. If you want to connect this secondary to the old antenna terminals of the radio, disconnect the wire going to the old antenna coil first.

To adjust the loopstick inductance, tune in a station around 600kHz, and slide the coil on the rod to maximise signal strngth. Then tube a station around 1400kHz and adjust the cap trimmer to maximise signal. You shouldnt need to touch any other adjustments.

I have been considering fitting my STC830 with a loopstck antenna. As you say this is simple in principle but can be difficult in practice due to inaccessibility of connection points. This is so in my case with everything buried between the wavechange switch and the bottom of the aerial coil can, including the attachment nuts for the aerial coil can, so attachment inside the can out as well. I don't want to remove the wavechange switch as this suffers from the same inaccessibility problems, and these switches are fragile - mine happens to be OK so not looking at creating another problem.

So presently am thinking of a separately tuned loopstick attached to the present aerial terminal - not ideal.

You have mentioned that a little lateral thinking may be needed such as your idea of the bypass cap at the hot end of the old coil to allow AGC pick-up. Do you have any other such ideas?