----- Original Message -----
From: "Jim Garland" <4cx250b@muohio.edu>
To: <amps@contesting.com>
Sent: Thursday, October 25, 2012 2:55 PM
Subject: [Amps] Additional comments, re GU-74B/4CX800A
This has been an interesting thread on the GU-74B. Since I kicked off the
discussion with my question about grid current ratings for the tube, I
thought it might be helpful to explain my context.
One problem I've had in designing appropriate amplifier parameters is the
variation in published tube specifications. As noted by others, the
original
GU-74B data sheet gives maximum ratings as follows:
max plate dissipation - 600W
max plate voltage - 2000V
max screen grid voltage - 300V
max plate current -750 mA.
When Svetlana rebranded the tube as a 4CX800A, they upped the published
maximum ratings, presumably because they figured the commercial and ICAS
services were not as stringent as the mil-spec service the tube was
originally designed for:
max plate dissipation - 800W
max plate voltage - 2500V
max screen grid voltage - 350V
max plate current - 800 mA
The ambiguity in tube ratings is also reflected in the way commercial
amplifiers use the tube, with most manufacturers (i.e., Acom, QRO) pushing
the screen voltage to about 350V and the plate voltage to 2500V,
evidentally
with no ill effects on tube performance or tube life. At this high screen
voltage, both the operating bias and resting plate current is quite high.
** How do you justify that tube life statement Jim?
From Alpha on down owners have had reliability problems when pushing those
tubes.
In the real world few hams run them at 1500W a pair and instead drive with
the typical 100W xcvr and tune for full bore in the 2200-2400W range. The
higher power rigs "may" be backed down to 100W but they could also get the
Magnum version. A pair of 3CX800A7's with a real 800W rating can handle that
for years.
For example, QRO specifies about 500W of resting plate dissipation per
tube,
with a claimed operating bias of -70V. High resting dissipation is a
common
problem with many tetrodes. One solution is to bias the tube nearly to
cutoff with no speech, and then lower the bias when RF is detected at the
grid. This is done by Acom and also Alpha (in the 8410), but at some cost
of
circuit complexity. There is also the possibility that bias switching
artifacts might be audable in the transmitted signal.
** It is more than just a possibility, its a fact.
Svetlana recognized this problem and proposed using cathode
degeneration (e.g., inserting a 25 ohm resistor between the cathode and
ground) to reduce the resting dissipation to a reasonable value. This is
the
approach I'm planning to use in my homebrew amp (three GU-74Bs), along
with
reducing the screen voltage to about 250V. Here are some typical operating
parameters predicted from the tube constant current curves, for a plate
voltage of 2500V, screen voltage of 250V, and grid bias of -40V, but no
cathode degeneration. Values are per tube.
Grid current (mA): 32.3
Screen current (mA): 11.7
Plate current(Amps): 0.600
Input power (Watts): 1490
Output power (Watts): 997
Plate Dissipation (Watts): 502
Efficiency: 66.9%
Plate load (ohms): 2160
Grid Swing (Volts): 52.0
Resting Dissipation (Watts): 250
Drive Power (W): 1.68
What's interesting about these results is that the amplifier isn't
operating
in a linear regime, even when each tube is dissipating 250W of resting
power! (If it were, the theoretical efficiency would be closer to 61-62%).
Another interesting result is that appreciable key-down grid and screen
currents are drawn, although both are well within tube limits (2W grid
dissipation and 15W screen dissipation). I don't know how much these
results
would change by adding some cathode resistance, although obviously more
drive voltage would be required. The bottom line, I guess, is that there's
no easy way to rein in the resting dissipation of these tubes while still
preserving linearity. One either puts up with the heat or else devises a
tiered electronic bias circuit, a la Acom's.
** I tried that cathode resistor idea decades ago, strictly to improve IMD,
with the 8122's in the NCL-2000 and lost in the IMD department as you have
already surmised. Eventually Ian, GM3SEK, came up with his tetrode boards
and I was able to improve IMD by ~ 5dB with grounded cathodes.
Incidentally, although some folks have asserted that the GU-74B life
expectancy will be extended if the tube is not pushed to, e.g., 1000W
output, I don't see why that is necessarily true. As the above numbers
indicate, at 1000W output, the tube is only dissipating 500W, and the
other
parameters are well within maximum ratings. Assuming adequate cooling is
supplied and that the filament voltage is maintained, I'd think the
biggest
killer of tube life would be drawing excessive cathode current. At 0.6A
cathode current, the tube is being operated conservatively and still
producing 1000W of RF.
** You are making a lot of assumptions there Jim. One is that the tube is
actually rated for long life at its intended military service. In actuality
tubes are changed when a certain number of hours are reached. The GU-74B
commercial ratings in AB1 MF/HF service is 550W output at 2000V with a 300V
screen.
The all Russian original spec sheet is the same as the later one that is
Russian and English and aimed at capturing some of the commercial market.
Then Svetlana simply made the specs to fit their needs.
The "Limit Operating Values" paragraph is there for a reason as it is in
tubes from all countries. The 600W Pd is an absolute max, not daily
operating.
You have enough tubes, set one up in a test jig as a monoband amp and run
30WPM CW thru it 24/7 and graph the Pout degradation at an initial 1200W
out. Also do IR tests on the anode using the typical ham amp cooling.
When you were involved with the ARD-230 did you and Jerry conform to Eimacs
3CX800A7 specs or push them a bit?
Carl
KM1H
73,
Jim W8ZR
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