There certainly is nothing new about the proper tuning procedure for a tetrode.
The following was taken from the 1963 National NCL-2000 manual who also had
plenty
of prior experience building military tetrode amps from 1-10KW. National
certainly was
not alone with tetrode amps as the 572B and 3-400Z had just been introduced in
1963
as the new wave of GG amps, and other choices such as the 811A , 813, 4-400A,
PL-172,
4-1000A, etc, had their own problems in AB1/AB2 and GG......depending on tube
type.
" Note carefully that the NCL-2000 screen current will be your most valuable
tuning and
loading indication during the following steps. The object of the tune up
procedure, as you
will see, will be to strike the perfect balance between plate current, drive,
and screen current
for maximum output. Excessive screen current (above 25 ma) indicates
insufficient loading,
and in addition, the screen current will act as an extremely accurate
indication of plate circuit
tuning, by reading maximum at resonance. The NCL-2000 is so designed that
maximum
efficient output may be accurately determined by proper adjustment of plate
and screen current.
In the following CW tune up procedure (note: for the then FCC legal 1KW DC
Input in the 1800V switch position), the
objective is to achieve a plate current at resonance of 550 to 650 ma with
10-25 ma of screen current.
Remember screen current will increase with light loading and will decrease as
the NCL-2000 is
loaded more heavily. Screen current will be maximum at resonance"
Even with such a concise procedure customers couldn't stop blowing 8122's since
they didn't read
and absorb what they were reading or maybe had too many adult beverages. Some
things never change.
As Service Tech and later Service Manager I had to type up a shorter procedure
using BLOCK letters
at times plus explain what light and heavy loading in relation to the Load cap
actually meant.
Light Loading means MORE Load capacity and Heavy Loading means LESS Load
capacity. The fact
that the 0-10 markings on the front panel or knob skirt are often the reverse
of the Tune cap position
further confuses those who don't bother to check before even plugging the amp
into the AC outlet.
Carl
KM1H
--------------------------------------------------
From: "John Lyles" <jtml@losalamos.com>
Sent: Tuesday, February 03, 2015 1:06 AM
To: <amps@contesting.com>
Subject: [Amps] {Collins} Understanding Screen Current in a Tetrode
> Last week I read with interest the discussion here. There were numerous
> replies, each with a bit of good information. Gary, K4FMX, nailed it
> spot on, with this explanation reproduced here. This agrees with my
> experience with tetrodes, where the plate dip is difficult to measure.
> Screen current peak always seems to stand out.
>
> 73
> John
> K5PRO
>
> --------------------------------------
> A little clarification is in order here:
> You tune the PLATE tune control for a peak in screen current the same
> way as you would dip the plate current. The screen current is much more
> sensitive than the plate current.
>
> You adjust the loading control (less capacitance) to load the amplifier
> heavier. The screen current will decrease as you load the amplifier
> heavier. Again adjust the plate tune control for a peak in screen current.
>
> Keep increasing loading (decreasing load capacitance) for maximum power
> out. Keep doing this until desired power output is reached and maximum
> screen current is not exceeded. Keep increasing grid drive while tuning
> with out exceeding maximum grid current or maximum screen current.
>
> Too much drive when the amp is not loaded heavily enough will cause the
> screen current to go very high.
>
> With a tube like a 4CX1000A it is rated at zero grid current but slight
> grid current of around 1 Ma is allowed while tuning.
>
> When you are done tuning the screen current may be negative with tubes
> like the 4CX1000A. you always peak the screen current in the positive
> direction with the plate tune capacitor even if the screen current is
> negative.
> 73
> Gary K4FMX
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