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

[Richard Solomon]

When I ran a pair of 4CX250B's on Two Meters, I tuned the Amp
for identical Screen Current, within Tube Limits.

73, Dick, W1KSZ

On Tue, Jan 27, 2015 at 4:51 PM, Mike Waters <mikewate at gmail.com> wrote:

> 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 at 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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