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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