[Amps] {Collins} Understanding Screen Current in a Tetrode

[Manfred Mornhinweg]

Under the assumption that the guys who designed and built amplifiers for sale 
knew pretty well what they were doing, it might be a good idea to simply look 
into the user manuals of some tetrode amplifiers.

The manual for the NCL-2000 states that:

- 40mA is the absolute limit for screen current;
- while tuning in CW, one should keep screen current between 10-25mA;
- screen current is always peaked with the tune control, then controlled to the 
desired level by the load control;
- Both screen and plate current are of course affected by drive level;
- while operating CW, it's best to adjust screen current to 10mA and plate 
current to the correct level to get 1kW input (that was the limit back then), 
and this should be done by adjusting drive level and load control as needed, 
while keeping the tune control at the screen current peak;
- During SSB operation, optimal peak screen current should be 25-35mA, at 2kW input.

In my practical use of the NCL-2000, I have to deviate somewhat from these 
figures. This is because the designers also ask to set the idling current to 
225-250mA, which is about 600W idling plate dissipation! The blower was 
dimensioned for 60Hz operation, but I have 50Hz, making the blower a lot less 
effective. At 600W I would soon melt down the tubes! So I set the idling current 
much lower, like 60mA. This should result in higher IMD, but I have asked many 
hams to check my splatter, and they all report they can't hear any, so it should 
be fine.
Anyway, at this lower idling current, full power SSB operation at what seems to 
me to be the best settings happens with the screen current meter only 
ocassionally deflecting from zero, never more than 5mA or so. Probably this is a 
peak screen current of about 15 to 20mA. When I tune up (briefly) with a carrier 
with the amp set to SSB mode (full voltages), I set it for 15mA screen current.

The power output and efficiency are both good, IMD seems to be good enough to 
keep the other hams happy, and I have been using the tubes for 15 years now and 
they seem as good as new, and they weren't new when I got the amp. So my guess 
is that my tuning practice is good enough.

I would imagine that the actual optimal screen current for other amps depends to 
a good degree on the ratio between screen and plate voltage, and of course on 
the specific tube.

The old handbooks, and the tube data sheets, also list recommended values for 
all voltages and currents, under several different operating conditions.

Manfred

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