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

To: <amps@contesting.com>
Subject: [Amps] Replacing 3-500Z tubes with 4-400A tubes
From: "Jim Thomson" <jim.thom@telus.net>
Date: Wed, 7 Dec 2011 21:18:54 -0800
List-post: <amps@contesting.com">mailto:amps@contesting.com>
Date: Sun, 4 Dec 2011 13:20:38 -0800 (PST)
From: John King <k5pgw@yahoo.com>
Subject: [Amps] Replacing 3-500Z tubes with 4-400A tubes

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

## Stuffing 4-400A/B/C's  into SB-220's etc, is nothing new.   That's been 
going on since the 70's.  I assume you directly bonded the 6 x grid pins to the 
chassis. Wire a 22-24 ga jumper across
the stock 5 v zener..and try it with no zero bias. 

##  What was the idle current, when you had 3-500Z's ?  The Eimac 3-500's had a 
low mu = 130.  The amperex/chinese 3-500 are higher, mu=200.  The higher the 
mu, the lower the idle current.
What idle current do you have with 4-400's ?   At most, you might need 1-2  x 
1N5408's /6A10's. 

Jim  VE7RF



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