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

[Jim Thomson]

Date: Sun, 4 Dec 2011 13:20:38 -0800 (PST)
From: John King <k5pgw at 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