There was an interesting article on cqham.ru website by EX8A describing
Output - 3000W++ on HF with 24/7 duty cycle...
Key points are: 2 GU-81 in parallel, 2 pi-networks help to neutralize the
tubes, high grid voltage provides 1500W per tube instead of 700W in classic.
The article is in russian, but worth looking -
2007/10/5, Roman Hanovych <email@example.com>:
> I think it is too big for 1500W PA,
> Heating and cooling alone will take not far from 400W, and
> then the 200W driver TRX eating much above 500W from mains
> during transmit at full output.
> 10-11 db is obtainable gain with best units of GU-5B even at 4 kV,
> but yes its not enough for 100 W radio. I am just guessing and want to be
> mistaking here,
> honestly never tested real number because then I had a 1 kW driver loaded
> to roughly 50-60%, no RF PWR meter in between, and never care to check
> tech specs.
> 1990's were still years of a very low culture in QRO PA building in
> EU in 90%
> of cases.
> My GU-5b was biased at 100-200 mA CW/SSB, getting 1.1-1,3A Ia in CW,
> current not exceeding 300-350 mA. That amp is far away now and I can't
> recheck anything.
> Have no data on its IMD, but I would go rather to 20 then to 30 db,
> heard a nice clean signals out of those medium-monster amps, but it still
> be a matter of a low culture in design, building and operating.
> Many GU-5b's here are used in soviet and bulgarian RF heating generators,
> but some are
> still present in broadcast and communications equipment, including 5 kW
> Viaz-M2 built in 70-80's.
> As of cheap triodes GS-35B can start transmitting after abt 30 seconds in
> cases if you really need it
> few times a year, thats better then spec's required 120 s for normal
> regular service :).
> But if you ask me in the middle of the night what to use for instant-ready
> 1.5-2.5 kW amp, I would suggest a pair of GU-81 or GU-46. They are
> but very easy
> to use and very reliable, esp. GU-81. These run thousands of hours in
> A-like audio
> amps or poorly built old military HF transmitters with little or no any
> Only few years ago it was possible to buy them in hundreds for only 2-3$
> today it is a bit more, but still they cost very little for what they can
> As for me I often prefer to build grid and screen supplies with basic
> protection for GU-81
> than to play with 9 input pi-networks for GU-5B, and I like to keep my
> at 25W output.
> It is only my opinion, not trying to start any discussion triodes versus
> pentodes here :).
> 5 years ago local guys would start throwing stones in me if I start
> about this
> "old junk" like GU-81, but since the trendy GU-74-78-84 now almost totally
> washed away
> from our fleamarkets and cost sometimes a monthly salary of an everage UT5
> amateur :(,
> many change their mind about forgotten for some time post WW II era
> I am too long, and sorry didn't found all the answers...
> 73, Roman US5WDX
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