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

[Carl]

Thats a perfectly normal result as the 4-400's have less gain in GG.
Im assuming the grids are also now directly grounded.

Reduce the bias a bit and shoot for about 200ma idle for decent IMD. Replace 
the zener with reverse connected 1N5408's, about 4 should do it and anything 
between 180-220ma idle is fine.

Nobody will hear the 200W less and the result is a cooler running amp.

Carl
KM1H



Carl
KM1H
----- Original Message ----- 
From: "John King" <k5pgw at yahoo.com>
To: <amps at contesting.com>
Sent: Sunday, December 04, 2011 4:20 PM
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
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