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From your description, it sounds like you are experiencing the (predictable) effects of tying all the filaments (and cathodes!) together.  Utah Bob already gave some good, proven suggestions for preventions/remedies, a couple of posts up the thread.

Something else I wonder about is whether you have a way to entirely disconnect whichever DHT that is not being used (including eliminating standing current in the "unused" filament/cathode lines, of course).   Otherwise, it is just another nest of EMF/EMI portals, especially with the shared filament/cathode supplies.  Let the DIY/electrical experts weigh in, but I think the sonic pay-off would be well worth the hassle.

You have seen/heard by now what I have long bitched about, that the DHT is very picky about isolation.  However, from your early reports, I'll bet you will be pleased with results when you finally get the usual problems under control.

Best reghards,
Paul S

Rerurn to Romy the Cat's Site