[Amps] 4CX1500B help needed...high screen current in HF-8020 PA

TexasRF at aol.com TexasRF at aol.com
Mon Jan 14 13:33:53 EST 2013

Hi Paul, I have no direct experience with your amplifier model but do have  
a couple of general comments about the tetrode tube in use.:
!> 4CX1500B is rated for 12w screen dissipation, Ay 225v, that is 53 mA  so 
your 40mA reading should pose no problem, especially in a cw or ssb  mode.
2> elevated screen current comes with high drive power and/or less plate  
loading. If the auto tune circuit is working then you might try reducing 
drive  power slightly. Sometimes a small change in drive power results in a 
surprising  change in screen current.
Gerald K5GW
In a message dated 1/14/2013 11:48:53 A.M. Central Standard Time,  
radioboy at telus.net writes:

Hi  Guys,

I have been struggling trying to figure out why I have  high screen current
in my Collins HF-8020 1KW PA. It uses the 4cx1500B and  is an autotune PA
built like a tank. This one was built in 1990 and I only  had it working 
about 7 years ago, but can't remember if I had any  issues then or if this 
new.  I just reconnected the whole system  and it works fine, although one
day I went to tune it up and I got a fault  light for high screen current. I
went thru a rough alignment of standing  idle current, set to 300mA, and
checked all voltages and this is what I  have

Plate 3000V

Screen 225V

Bias -35 keyed/-75  unkeyed

Standing idle 300mA

Keyed full poop .75A

Screen  40mA  and this is what concerns me. The fault light is off now but  
is still way high, I think. Should it be about  15mA?

Anyway, any help would be appreciated. The tube is NOS  1983 with very few
hours on it. All relays, contacts, switches and  everything I can see
mechanically is spotless inside.  I do have full  manuals for this and have
read thru them but holy cow, they read like a  science lesson. I'm sure I
could eventually figure it out after weeks of  reading and studying...It 
put out full power no problem, over  1KW.

Any help would be greatly  appreciated.




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