OK thanks Andy I do have the circuit diagrams but I understood that the clamp only operated if the drive was lost. The description was not that clear to me but a probe with the scope may clear that up. I have added resistors across the fet gates  so that if the transformer fails the fet should turn off. I think I was lucky when the transformer failed and the fet was not  killed! The insulation in the transformer is only what is on the litz wire and it has sleeving out of the coil. It failed as the ferrite is earthed and it had degraded the insulation over time. It was a pig to wind but was worth it, no spares are probably available unless from a scrap unit.
Dave

-----Original Message-----
From: Andy Talbot <[email protected]>
To: rsgb_lf_group <[email protected]>
Sent: Wed, 13 Jan 2016 10:45
Subject: Re: LF: Decca 5501

I've just looked again, and there's more to to the drive circuitry than that.  It is single ended drive for current into the inductor, as I stated, but there is another IRF520 used as an active clamp (the circuit diagram was drawn a bit ambiguously, so didn't spot the arrangement straightaway).  So when these two FETs are driven alternately there will be a proper equal amplitude symmetrical drive waveform generated.

It is definitely used as an energy storage inductor though, hence a gapped core is essential. 

'jnt