I don't care for that JI answer since grid driven tetrodes are almost always
run in AB1 where there is no grid current and in AB2 there is only a flicker
of current on peaks but a lot more IMD. Tuning is via the screen and power
meters and let the mike gain control take care of the grid. Just remember
that AB1 is less efficient than AB2 or B...or C for that matter
Some tubes exhibit negative screen current, the 4CX1000A is known for that
and it is normal under some conditions and right in the spec sheet.
Many of the small 4X150 class exhibit negative screen current on VHF and if
output doesn't increase when going positive it is OK.
I get a small amount of negative with a 6M NCL-2000 while tuning but output
always peaks in the positive 10-15ma region for 1200W. There is no negative
with the HF NCL's unless it is a sudden downward drift signaling an
impending BANG; this is a fairly common fault with 8122's that have been
beat hard and misused. They often work OK in the CW position at the lower
plate and screen voltages and are still linear.
The AM-6155 I use on 432 and several of the AM-6154 and 6155's Ive converted
or repaired for 144, 222, and 432 all show positive screens when loaded
properly. The 8930 that is used is a 4CX250B with a bigger anode cooler and
the Russian GS-36B/4CX400A is a direct replacement except for sometimes
having to bend the plate cavity finger stock a hair; it performs the same.
Carl
KM1H
--------------------------------------------------
From: "Mike Waters" <mikewate@gmail.com>
Sent: Tuesday, January 27, 2015 6:53 PM
To: <amps@contesting.com>
Subject: Re: [Amps] {Collins} Understanding Screen Current in a Tetrode
FYI, here was my post to the Collins reflector that prompted that reply.
What do you think?
"There is a lot of misinformation out there about screen current. But it's
the most important indication of proper loading!
"When I used to DX on the low end of 144 MHz with my homebrew 4X150A
amplifier (30 years ago), I knew that the amp was tuned just right when
the
screen current was either zero or very slightly negative.
"According to www.w8ji.com/loading_amplifier.htm : 'In grid-driven
tetrodes or pentodes the most important parameter is screen current, with
control grid current a close second, and output power a close third.'
Believe it. :-) "
73, Mike
www.w0btu.com
On Tue, Jan 27, 2015 at 5:51 PM, Mike Waters <mikewate@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@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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