[Amps] Question about glowing 3-400z's

[Tom W8JI]

> I'm working for the first time on a pair of 3-400Z's in a 
> Swan Mark I. On both the high and low power position, 
> the amp is making exactly the instruction  manual's 
> specified idle current. Plate voltage is 2900VDC and idle 
> current is 180ma.  With key down and no excitation at this 
> level,  the tubes begin to glow  to a  color that  I 
> consider to be a bright red. The plate current meter shows 
> no rise in current draw during this entire  process.   The 
> amplifier loads perfectly and when driven with 100w , 
> power output is about 1100W. The question is whether this 
> "glowing"  is the normal characteristic of a  pair of 
> Eimac tubes??

Ron,

Some anode color is required to keep the tubes gas-free.

The gettering agent is the gray powdery coating on the 
anodes. If the tubes never showed anode color they would go 
bad.

The color is rated totally to the dissipation in the anodes, 
nothing else.

73 Tom