WEM Custom 15

[valvekits]

I doubt that the transformer is the issue.
Check the continuity of the ground wire from the 50uF caps.
The heaters centre tap is white according to post #91
It might not be anything to do with the work you have done, for example a pin in one the valve sockets might not be making a good connection.

Eddie

[Bushbaby]

Thanks for your continued help Eddie.
This is so annoying - I just cannot envisage where this is going awry.

Best Regards

[Mike Watterson]

I agree with Eddie. Unless the voltages are madly wrong or transformer getting very hot, it's just a bad connection, or a connection to a less than ideal place.
Try these four:
1) No earthing of heaters.
2) Earth CT only and only at -Ve of main electrolytic.
3) No CT connection, but each of the two heater connections, in turn, only to -Ve of main electrolytic.

The HT - of transformer should connect direct to -Ve of main electrolytic and nowhere else. Then -Ve of main electrolytic goes to 0V/ Earth end of the o/p valve cathode resistors. Then onward to the 33K and cathode resistor of V3.

It's certainly a misplaced earth or bad connection.

[Bushbaby]

I have tried disconnected the CT to the heaters (white cable). Made absolutely no difference with regards to the mains hum.
I reconnected the white cable and disconnected the red/yellow centre tap. I am sure after a few moments I saw a flash in the rectifier tube followed by no hum or any noise whatsoever. Interestingly all of the valves where still lit up but even putting the volume to max there was no hum and no output to the speaker (I tried with a jack plug connected too but no output).
I reconnected the red/yellow CT with the white CT - no output again. I changed the EZ81 and again the mains hum reappeared. Obviously, I've damaged the old EZ81 albeit it was still glowing.
I would try this without the earth connected but I am concerned that I might blow another rectifier valve.
What does this suggest??

[Mike Watterson]

The EZ81 should be on it's own isolated 6.3V that optionally has one side connected to the cathode of it.
The EZ81 cathode is at +HT, so earthing any aspect of the 6.3V heater supply to it may cause heater cathode leakage.
see https://www.radiomuseum.org/tubes/tube_ez81.html

[ppppenguin]

The heater cathode insulation of the EZ81 (and EZ80) is good enough for the heater to be at 0V and share its supply with other valves.
This happens to be a Mazda data sheet but the Mullard one is the same: http://tdsl.duncanamps.com/pdf/ez81.pdf
In the unlikely case of using the EZ81 with more than 500V on the cathode then it would need a separate heater winding. But you'd be overrunning the valve anyway.

[Bushbaby]

Many thanks
What is my next step to try and determine what is causing the short/mains hum?

[valvekits]

Next step is re-read what Mike said in post #103 where he was talking about the heaters and centre tap configurations. No mention of removing the red yellow wire which is for the high voltage.

When that is complete we can discuss the best way to proceed, but it will be a process of elimination.

[Bushbaby]

The only centre taps are the white lead to the 6.3v heaters (that feed the EL84s etc) and the red/yellow CT on the HV side. This is the same as the original transformer. I have already tried this for the white lead.

[Mike Watterson]

The heater cathode insulation of the EZ81 (and EZ80) is good enough for the heater to be at 0V and share its supply with other valves.
This happens to be a Mazda data sheet but the Mullard one is the same: http://tdsl.duncanamps.com/pdf/ez81.pdf
In the unlikely case of using the EZ81 with more than 500V on the cathode then it would need a separate heater winding. But you'd be overrunning the valve anyway.

So something else got connected wrong. I suppose the Philips chassis I had with a separate 6.3V was because they'd always done it that way, or maybe originally used something with a lower or no isolation.