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

[Carl]

The GU-84B is not a 2500W Pd tube, even Svetlana USA realized that when they 
renamed it the 4CX1600B in another attempt to Americanize Russian tubes. Of 
course they had to cheat as usual by raising the maximum Ep from 2200 to 
3300V and reducing the Ip in an attempt to improve efficiency at the sake of 
increasing IMD. In FM BC service that wasnt an issue.

One only has to compare the anode measurements with the Eimac 4CX1500B/1000A 
to realize the foolishness of the 2500W Pd rating.

Carl
KM1H


----- Original Message ----- 
From: "Leigh Turner" <invertech at frontierisp.net.au>
To: "'Jim Garland'" <4cx250b at muohio.edu>; <amps at contesting.com>
Sent: Friday, October 26, 2012 9:51 PM
Subject: Re: [Amps] Additional comments, re GU-74B/4CX800A


>
> Jim, if you operate your GU74B tube at no more than 800W CW or PEP output 
> in
> AB1 SSB service then the tube will have a very long service life. DO NOT
> push it up to 1000 or 1200 Watts Po like some ham-radio clowns do!
>
> For your proposed 3 holer, that equates to quite a healthy 2400 W output, 
> or
> circa 1500 to 1600 Watts for popular two-tube amps like the fine 
> ACOM-2000A.
>
>
> As I remarked in an earlier post, one of my amps with a single GU74B has
> been used as a station workhorse for over 10 years at the 800W and under
> level and it's still the original tube. The amp today happily produces 
> 800W+
> Po and exhibits no sign of power output degradation; so at that rate the
> attainable tube life when used at or slightly above the original
> conservative Russian language data sheet spec appears to be indefinite!
> Precisely as one would expect from a rugged mil-spec purpose designed SSB
> tube!!
>
> There is so much nonsense and misinformation in ham circles about this 
> very
> fine tube.  If you want to know how to optimally deploy these tubes, go 
> look
> at the ACOM-2000A specifications and carefully study the schematics. 
> Salient
> features are cathode NFB, and an efficacious tri-state EBS system to 
> achieve
> a small desktop footprint with adequate blower CFM and quite low fan 
> noise.
>
> When it come to the outstanding performance of the GU74B's bigger brother,
> the 2500W plate dissipation GU84B, then look no further than those 
> exemplary
> amps designed and produced by the Slovakian company OM Power.
>
> Moral of the story is very simple: don't push any tube beyond its sensible
> ratings as per the manufacturer's data sheet. Work within those parameters
> and excellent results will be achieved, especially in respect of the 
> GU74B.
>
> The only folk disappointed by the GU74B are those who flogged them too 
> hard
> and tried to deploy them for purposes never intended. No surprises here.
>
> Yes, best results with EBS stem from using PTT mode. Yes, keep an eye on
> maximum plate/cathode current and always keep it within published ratings,
> and use sufficient CFM cooling.
>
> Leigh
> VK5KLT
>
>
> -----Original Message-----
> From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of Carl
> Sent: Saturday, 27 October 2012 1:26 AM
> To: Jim Garland; amps at contesting.com
> Subject: Re: [Amps] Additional comments, re GU-74B/4CX800A
>
> ++SNIP++
>
>
> ** 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.
>
>
>> 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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