5U4G valve sparking problem

[JulesQ]

Hi,
I have just joined diyAudio on the strength of reading your posting regarding the rectifier valve problem.
I have Quad Forty amps and they have been dogged with problems virtually from new. I won't bore you with the endless list of faults but one I would like some opinion on is this rectifier sparking. After reading what other members have suggested as being possible faults I never considered or thought possible. It’s made very interesting reading.
The rectifier valves in the Quads are 5U4G's and a good while ago I had to replace one due to sparking. This particular fault was due to a fractured connection inside the valve and replacing it put it right. However, there have been two successive cases of sparking but of sort described by one of the guys on the forum. When I first switch the amps on, just as the heater starts to produce enough heat to start the thermionic reaction a shower of miniature sparks (quite nice display) from the top of the metal structure. It doesn’t last more than a few seconds then it's gone and all works absolutely fine.
As I said before, there have been many faults with the amps and I have changed leaky and short capacitors BUT only for ones of exactly the same value, maybe slightly higher voltage but no difference in the capacitance itself.
Phew! Sorry it's been a long-winded post but I am a bit puzzled by this. If anyone can help out or at least point me in the right direction I would be more than grateful.
Many thanks,
Jules.

[eyoung]

Sylvania made some nice 5u4g's without the fire works


Elwood