The tuning procedure for a 100MHZ 35KW FM Transmitter with a 4CX20,000
tetrode is to tune the loading and tuning for a screen current peak.
The tuning is so broad that you can't see the plate dip but you can
see the screen peak. The screen current protection circuit is 1/2 the
allowable screen current per Eimac. Typically around 150ma.... set by
the screen variac.
Don W4DNR
Quoting Peter Voelpel <dj7ww@t-online.de>:
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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