Carl, your and Val's comments below have nailed the story of the GU74B
precisely; that is what I'd meant in my earlier comment about good tubes
being in the hands of good amplifier designers who properly understand the
tube they're working with and accordingly go to the trouble to produce a
superb amplifier product around it. ACOM and Emtron understood this and
between them produced thousands of successful amps for a global market based
on this fine tube. The popularity of the ACOM-1000, ACOM-2000 and DX-1 amps
speak for themselves.
You're spot-on when saying that not all amp manufacturers using the
4CX800A/GU74B did this, and consequently many hams used these amps above
their capability and experienced short tube life. Caveat emptor!
You must remember that the GU74B was specifically made for the military, and
is very conservatively spec'd and rated at 600 Watts plate dissipation. Take
one GU74B tube in one hand, and the famous Eimac 3CX800A7 rated at 800 watts
plate dissipation in the other hand, and judge for yourself.
By the way, who won the space race?? The US nearly browned their pants, when
Jurij Gagarin was circulating the earth :-)
Leigh
VK5KLT
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Carl
Sent: Wednesday, 24 October 2012 10:30 AM
To: Val; amps@contesting.com
Subject: Re: [Amps] Question re GU-74B grid current
Val, I suspect the real reason is that the Russian engineers designed the
tube to a specific military specification and it was rated to perform
reliably at that point.
Part of the problem is the anode structure/cooling fins/socket which create
more back pressure than say a 3CX800A7.The 3CX800A7 is actually
conservatively rated compared to the first Eimac spec sheet which got the
FCC upset. OTOH, Ive run 3 of the 3CPX versions at 5KW out on HF and also 6M
conversions that have been shipped to many countries; they have been
reported as reliable. If that tube had a "ham" rating the dissipation would
easily be 1000W for SSB service
None of the memos I have from Russia to George Badger indicate it is
overated, just the opposite and the Russians suggested not pushing it. I
dont blame the Russians for the rather short tube life when run hard. In
fact the reason to increase by 200W was to justify the much higher idle
current. It appears that Acom realized that and used the 3 stage bias to
restore some of the reliability and/or reduce the cooling/air noise
requirement and that is not in most of the other 4CX800 amps on the market.
In reality the 4CX800 was strictly Svetlana USA market driven as they
realized that intelligent hams would see thru the 600W rating as pushing it
too hard.
Carl
KM1H
----- Original Message -----
From: "Val" <val@vip.bg>
To: <amps@contesting.com>
Sent: Tuesday, October 23, 2012 3:14 PM
Subject: Re: [Amps] Question re GU-74B grid current
> Carl,
>
> The 800W labeling of GU74B wasn't fraudent, but a pretty fair. If this
> tube was designed by EIMAC, or other western company it would have been
> undoubtedly labeled 4CX800. Compare its size, grid and screen to 4CX600
> and 4CX1000.
> Underrating of GU74B proceeded from the political and economic system in
> Russia. It wasn't market driven, there was no competition, the price of
> the tube and the price per watt weren't of big importance. However it was
> really dangerous for the designers if the tube failed to meet its
> specifications. That's why they rather insured their lives by rating the
> 800 watts tube at 600 watts. This was corrected only 25 years later.
>
>
> 73, Val LZ1VB
>
>> Yeah sure. A 600W rated tube fraudently labled 800W to grab US dollars.
>
> _______________________________________________
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> Amps@contesting.com
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>
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