Help before I lose my sanity

[RhythMick]

I'm still intrigued by the original fault. The missing earth connection can't have caused that can it?

[RhythMick]

In this page...http://www.valvewizard.co.uk/heater.html

Merlin Blencowe states...

Rule number one is that the heater supply must have a DC connection to audio ground. This may be a direct connection or an elevating circuit (see shortly). This is equally true for AC or DC supplies. Leaving the heater supply floating will result in almighty hum due to primary-to-secondary transformer leakage current, and is a common beginner’s error.

How does this transformer leakage current work and how does a ground reference stop it? Could this be the cause of my fuse blowing when I had missed the ground connection?

[Nick]

How does this transformer leakage current work

Remember there are hidden capacitors between the primary and secondary, and if the secondary is isolated then there is noting stopping the DC level of the secondary floating anywhere between + and - 320v, the range of the 240v sine wave on the primary by the capacitors charging up with static voltage.

and how does a ground reference stop it

Being grounded means that the other end of those capacitors are connected to ground, so any leakage current through them cant charge up with static voltage.

If the heater supply is floating it can assume any voltage WRT signal ground that it wants. That voltage will be expressed across the insulation between the heater and the cathode, coupling to the cathode via the heater/cathode capacitance. So in effect you have a capacitor coupling the incoming AC mains to your cathode. Hence hum.

Could this be the cause of my fuse blowing when I had missed the ground connection

No.