[Diabolical Artificer]

I've cracked most of the amplifier but still have a hum issue. At present I'm using a bog standard common cathode triode gain stage as the IP stage to bump the IP voltage up a bit to "drive" a LTP, with a sensitivity of 1.5v RMS in, 3v RMS out, open loop gain. Am using a very small Ra - 6k8 on an ECC82, biased at 5v, HT 250v, so not a high gain stage. Htr's are DC powered.

IP wiring is as shown, attached. Basically a short length (6") of good quality shielded cable grounded at one end only, into a coupling cap etc. I've tried routeing it in various ways, well away from htr wiring etc, flat to the chassis and only crossing DC HT cables at 90 deg. The amplifier is separated from the PSU by 2mm thick ali plate.

With nothing attached to g1 there is only 20mv P-P of 50hz AC waveform on the OP, however as soon as I connect the inner core of the shielded cable to the coupling cap, OP AC goes up to several volts P-P.

Have tried various values of resistor on the LHS of the coupling cap to ground, grounding the LHS inner, like a pi filter, as well as numerous other fixes, I still have unacceptable 50hz AC hum though. The only thing that does reduce hum is to lift or elevate the shield ground with a 22r or 100r resistor, a 1k increases hum.

Am really puzzled how and why spurious 50hz AC is not shunted to ground. I'm hoping someone will point out a glaring error, but at the mo I'm stumped.

Andy.