[Amps] Additional comments, re GU-74B/4CX800A

Fri Oct 26 10:56:24 EDT 2012

----- Original Message ----- 
From: "Jim Garland" <4cx250b at muohio.edu>
To: <amps at 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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