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
> -----Original Message-----
> From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Peter
> Voelpel
> Sent: Tuesday, January 27, 2015 9:54 PM
> To: amps@contesting.com
> Subject: Re: [Amps] {Collins} Understanding Screen Current in a Tetrode
>
> Tuning a tetrode amplifier for screen current maximum is correct.
> Tuning for the maximum screen current allowed by the manufacturer might
> be
> wrong if you like to stay within legal limit and use a large tetrode.
>
> 73
> Peter
>
> -----Original Message-----
> From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Mike Waters
> Sent: Mittwoch, 28. Januar 2015 00:51
> To: collins@listserve.com; amps@contesting.com
> Subject: Re: [Amps] {Collins} Understanding Screen Current in a Tetrode
>
> Are you saying that we should always tune a grid-driven tetrode linear
> amp
> for the maximum screen current allowed by the manufacturer?
>
> Well, that would have been 30 or 40 mA on my homebrew AB1 grid-driven
> 4X150A amp. I think that would have been 30 or 40 mA too much! :-)
>
> It would be interesting to see what the people on the AMPS reflector
> have
> to say about this.
>
> 73, Mike
> www.w0btu.com
>
> On Tue, Jan 27, 2015 at 5:15 PM, David C. Hallam
> <david.hallam@knology.net>
> wrote:
>
> > For a GRID Driven tetrode amplifier, the correct way to tune and load
> is;
> > apply dome drive and dip and load in the normal so that you have some
> > indication of resonance. At that point this is what you do. Keep
> > increasing the drive and adjusting the loading for maximum screen
> current,
> > check the tuning occasionally. You keep doing this until you reach
> one or
> > the other of two conditions. One, You reach the maximum screen
> current
> > allowed by the manufacturers spec or Two, you have applied the maximum
> > drive you have available. Once you reach either of the two
> conditions,
> > reduce the loading just slightly, and you will be properly loaded for
> the
> > best IMD your setup will produce. I've been doing this for years on
> my HB
> > linears with never a complaint about the quality of my signal.
> >
> > This is the point of the QST article.
> >
> > David
> > KW4DH
> >
> >
> > On 1/27/2015 4:45 PM, Tony Sokol wrote:
> >
> >> Hi Kurt,
> >> Good information because it backs up what I have been reading.
> >> EVERYONE tells you that monitoring the screen current is important
> but
> they
> >> really do not say what to look for or what to do about it. From
> reading
> the
> >> spec. sheet on the 4CX-1000A a comment is made that some tubes may
> exhibit
> >> a negative screen current as much as -25Ma. I did read that having
> the
> >> loading capacitor meshed too far will raise the voltages (and heat)
> in
> the
> >> tank circuit. I also have read that when the screen goes positive
> the
> IMD
> >> goes to pot and you start splattering. I guess the way to go is to
> tune
> up
> >> for max output with minimum plate current and then back off the
> loading
> to
> >> keep the meter at or under "0" screen current. As far as the screen
> >> current goes...positive is not good and negative (within reasonable
> limits
> >> ie. >20Ma) is nothing to worry about as long as the screen voltage
> remains
> >> stable. There was one other comment about dipping the screen current
> >> around "0" like you would with the plate meter I assume. I will try
> using
> >> that theory tonight on the net and see what happens. So far it seems
> to
> be
> >> running cooler and I had several unsolicited reports of "great
> sounding
> >> signal". With all of the 30S-1's out there I am surprised that we
> haven't
> >> seen more on proper tuning procedures for it.
> >>
> >> 73
> >> Tony - W9JXN
> >>
> >>
> >> On 1/27/2015 2:51 PM, W6ph--- via Collins wrote:
> >>
> >>> FYI, there is an excellent article about screen current on page 26
> or
> >>> July
> >>> 1961 QST. In the article it says that negative screen current is
> not
> >>> unusual. But it also says that maximizing screen current (within
> its
> >>> limit) is
> >>> the best indication of resonance. It also says that a tetrode
> amplifier
> >>> should never be tuned for maximum output as is common for cathode
> driven
> >>> (grounded control grid) amplifiers.
> >>> Kurt W6PH
> >>>
> >>
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