[Amps] Replacing 3-500Z tubes with 4-400A tubes

[John King <k5pgw@yahoo.com>]

I have several very good 4-400 A tubes and have replaced a pair of bad 3-500Z 
tubes with a pair of 4-400A tubes in a Heath SB 220 amp.

I have noticed much lower resting current being drawn when keyed up. I have 
noticed about 200 watts less power output with 100 watts drive. The 4-400A 
tubes are as new.

I made no changes to bias or other parameter adjustment. I would appreciate the 
benefit of your knowledge and experience regarding the tube substitution. 
Thanks and 73, John, K5PGW
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