[AMPS] Transformers

[Peter Chadwick]

Carl says:

	>Peter, do you mean completely potting in tar as was done ages ago?
	>I have not had or noticed problems with open frame xfmrs that are
vacuum
	>sealed/impregnated.

	My failure was a transformer that had no apparent impregnation:
possibly a thin layer of varnish. 
	'Proper' transformers with vacuum varnish impregnation are another
matter, but some of the cheaper 
	xfmrs don't bother with that - they just have a lick of varnish. The
old 'tar' potting really needs to be a
	vacuum job for best results. Some UK WW2 xfmrs were tar potted in
vacuo: the ones for the Admiralty 
	seemed to always be generously rated, with current densities in many
cases of 750amps/square inch.

	Potted xfmrs do run hotter, but if they are designed for CCS,
contesting shouldn't bother them. One of 
	the UK built amplifiers that ran a pair of 572B's has a history of
the xfmrs failing. The major reason for 
	this was that the original design specified 120 deg C insulation:
this was expensive, and buying 
	decided on something cheaper under pressure from the boss.

	Shades of Carl's stories of National!!

	If you can get them, mil spec xfmrs in sealed cans full of oil are
the thing......

	73

	Peter G3RZP




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