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.
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.
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.
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.
73,
Jim W8ZR
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