[Johnson] Invader 2000 Idle Current

[David C. Hallam]

I am have ask this question before.  In the linear mode, what should be 
the finals idle plate current?  My plate current meter registers about 
200 mA with a pair of 4-400A's.  Looking at the characteristics curves 
for the tube, that is just about just about right for a pair in AB2 with 
2500V plate and 500V screen.  I looked at AB2 because the 2000 operates 
into AB2 when fully driven.   10 mA is specified as max grid current.  
This is part of the adjustment. I can't find any place in the manual 
where the idle grid bias or idle plate current is specified.  Max key 
down DC plate current should be 480/490 mA.  That's assuming you stay 
within the legal limit for its day and I don't think the power supply 
would provide more anyway.  The plate voltage drops to about 2100V under 
full load.

The only reference to static plate current I can find in the manual is 
the ambiguous reference in the adjustment and operation section E part 2 
linear operation loading current where you measure the static plate 
current and then refer to a graph to find the what they cal Ip loading 
current.  They give an example using 280 mA.  I have no idea why they 
picked that number if it didn't bear some relation to reality.

Johnson used both the 4-400A and Penta PL-175A in the 2000.  There were 
no circuit changes for either tube, that I am aware of.  Maybe someone 
can correct me on that.

I am fairly certain that my 4-400A's are weak and need to be changed as 
the output as measured by my Collins 312B4 watt meter is .  I have a 
pair of PL175A's with an unknown number of hours on them as well as 
several new and used 4-400A's.  Before I start changing tubes, I would 
like to verify that my grid bias is correct. According to Eimac it 
should be ~-75V.  I will also check that the screen voltage is in 
500/525V range Johnson specifies.

-- 
"I predict future happiness for Americans if they can prevent the government from wasting the labors of the people under the pretense of taking care of them."
Thomas Jefferson


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