I removed the LH channel earth return where it connected to the Tail resistor, then cross coupled it to the corresponding tag on the RH channel. Thus only having one common return from phase splitter and preamp section.
Result!
And a lesson learned. Star earthing isn't always as simple as it sounds. Good bit of practical eddication.
There's a very slight residual hum on the LH but only audible with ear pressed to loudspeaker. I ain't worrying about that.
When I get back in later tonight i will tidy the earthing up properly and take both returns to a common point on one of the PSU tags and thence to the star. It'll look neater and should share the current more evenly.
I think there's mileage in doing the same treatment to the output valve section ie the returns from the cathode resistors and grid leaks. if nothing else it will look a lot neater with one less wire run to the star earth point.
Thanks to everyone for their input.
The amp will now go on the scope again and i can check the open loop gain and work out a sensible feedback ratio.
I have a few tricks up my sleeve still to try as I hope to get the amp down to around a volt sensitivity or up to 1.4V which will be right for my preamps and high output sources. This should if my electronic theory is right reduce unnecessary background noise to a minimum.
Andy (happy Bunny)
r