[R-390] Re: Miltronix EAC

[Drew Papanek]

Barry (Hauser),

The single "tube" you refer to on the modified power supply unit is
probably a thermal time delay relay.  This would allow sufficient time for 
tube warm-up then would apply B+.  An example of an R-390A power supply 
modified this way can be found at: members.aol.com/ac5zt/r390a.html

One opinion holds that application of B+ before cathode warm-up causes 
deterioration ("stripping") of the emissive coating.

Another says that stripping is not an issue with indirectly-heated small 
receiving-type tubes but does need to be considered for large 
directly-heated filament power tubes.

However, it is noteworthy that tube-type Tektronix oscilloscopes (which use 
indirectly heated tubes) have a delayed B+ feature.

Drew




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