On 1/27/2015 9:26 PM, Carl wrote:
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
Agreed. Most of the Tetrodes I've seen used at ham power levels and a
bit beyond like the 4CX100, 4CX1500B, FU728F and others exhibit negative
screen current with low drive, but around a KW out, (more or less) they
go to positive screen current. At that point you tune and retune for
maximum screen current while staying within the manufacturers ratings.
At-any-rate, "in general", you tune for max output as you increase drive
until positive screen is observed, You then tune for max screen current
with plate tuning and loading (while staying within the rating of the
tube) as you increase drive. None of the ones I use goes beyond AB1 so
no grid current is present. A tiny amount of grid current shuts them down,.
Few really worry about efficiency any more as the power limit is based
on output regardless of mode. OTOH when running Class AB1 rather than
AB2 the amount of heat dissipated can be quite high. In class A it's
enough to heat a good size shack in mid winter. IE, at 1500 out @25%
(class A) efficiency it takes around 6,000 watts input -1500, or 4500
watts of heat out. That's a lot of heat and requires a lot of air at a
pretty good pressure, which can make for a lot of noise. Even at 60%
that is still a KW of heat out for 1500 watts out. Class AB1 for a
couple hours is the limit in winter. Class A, summer or winter is out of
the question.
A 4CX3,000 is really working and 1500 out Cass A is a lot more than the
dissipation than the tube is rated for.and that is really noisy!
With an Emtron DX-2sp running the legal limit with my voice
characteristics and a fair amount of processing, the blowers will kick
into high and the outlet air is quite toasty. (and noisy in this small
den. It's too many BTUs for this small den and it gets wayyy too
warm/hot in here in mid winter. Hence contest operation is strictly
from the shop. A 28 X 40' well insulated and open room with additional
heat in the winter and air conditioning in the summer.
As a comparison, I used to heat a 13 X 21' room and a 13 X 30' room in
the basement with a 4500 watt heater mounted in the wall. For class A
I'd really want a tube larger than the 4CX3000 and now we are getting
into tubes that take 500 to 750 watts just for the filament! <:-))
IIRC the 4CX3000 and larger do not show negative screen current. which I
understand (have read) is eliminated by the precision alignment, or
placement of the grid and screen wires.
I have to ask the more knowledgeable, if the negative screen current is
from secondary emission?
73
Roger (K8RI)
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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