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

Fri Oct 26 09:42:01 EDT 2012

Sell them for a nice profit and start over (-;

Carl
KM1H


----- Original Message ----- 
From: "Jim Garland" <4cx250b at muohio.edu>
To: "'Leigh Turner'" <invertech at frontierisp.net.au>; <amps at contesting.com>
Sent: Friday, October 26, 2012 8:39 AM
Subject: Re: [Amps] Additional comments, re GU-74B/4CX800A


> Thanks for your comments, Leigh. I agree with you about the benefit of
> single tube amps vs multiple tubes. However, I bought a dozen GU-74Bs back
> when they were dirt cheap, along with three sockets and chimneys, so I 
> need
> to do something with them!
> 73,
> Jim W8ZR
>
>> -----Original Message-----
>> From: Leigh Turner [mailto:invertech at frontierisp.net.au]
>> Sent: Friday, October 26, 2012 5:06 AM
>> To: 4CX250B; amps at contesting.com
>> 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
>>
>> _______________________________________________
>> Amps mailing list
>> Amps at contesting.com
>> http://lists.contesting.com/mailman/listinfo/amps
>>
>>
>
>
> _______________________________________________
> Amps mailing list
> Amps at contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
>
>
> -----
> No virus found in this message.
> Checked by AVG - www.avg.com
> Version: 10.0.1427 / Virus Database: 2441/5355 - Release Date: 10/26/12
>