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

[Kees - pa7two]

And next some HAMs disable the EBS in their Acom 2000a because they think they hear 'clicks' in the RF they XMIT... Oww dear!

I am so glad to read this forum! This is the reason, well explained btw, why the the variable EBS is (and should be) in the Acom 2000A amps. Conclusion confirmed.

Thank you all who contributed to this topic!


Vy 73, de Kees - PA7TWO - M5TWO
DX IS and CW RULES!

Twitter:  @PA7TWO



> From: invertech at frontierisp.net.au
> To: 4cx250b at muohio.edu; amps at contesting.com
> Date: Fri, 26 Oct 2012 21:36:29 +1030
> Subject: Re: [Amps] Additional comments, re GU-74B/4CX800A
> 
> 
> Hi Jim,
> 
> Instead of a 3-holer GU74B amp why not use a single large GU84B tube with
> its 2500 Watt rated plate dissipation?
> 
> I reckon if you need to use more than one tube to meet a given Po target in
> a QRO amp, then you've arguably selected the wrong tube :-)  An exception
> might be if one has a glut of smaller tubes available in the junk box.
> 
> Yes indeed the use of degenerative RF negative feedback via an un-bypassed
> cathode resistor will reduce the need for such a high value of ZSAC and yet
> still achieve good IMD performance.  The use of a properly designed and
> implemented tri-state EBS will significantly reduce plate dissipation with
> SSB speech and the bias transition artefacts will be virtually undetectable.
> 
> 
> One will learn a lot by carefully studying the ACOM-2000 amp schematics;
> their designers have skilfully balanced all the parameters to optimise
> performance very nicely around the GU74B tubes. 
> 
> Cheers,
> 
> Leigh
> VK5KLT
> 
> 
> -----Original Message-----
> From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of Jim Garland
> Sent: Friday, 26 October 2012 5:25 AM
> To: amps at contesting.com
> 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. 
> 
> 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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