[Amps] Anyone build a GU5B?

Peter Voelpel df3kv at t-online.de
Fri Oct 5 14:43:36 EDT 2007

They do quite well, a few are running also in Germany,

Example loaded up:  http://www.pbase.com/df3kv/gu81_at_friend


-----Original Message-----
From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
Behalf Of Alex Malyava
Sent: Freitag, 5. Oktober 2007 20:25
To: Roman Hanovych
Cc: Amps Amps
Subject: Re: [Amps] Anyone build a GU5B?

There was an interesting article on cqham.ru website by EX8A describing
contesting amplifier.
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 <romanh at kb.lv.ukrtel.net>:
> 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 GU-5B tech specs.
> 1990's were still years of a very low culture in QRO PA building in 
> eastern EU in 90% of cases.
> My GU-5b was biased at 100-200 mA CW/SSB, getting 1.1-1,3A Ia in CW, 
> keeping grid 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, 
> because never heard a nice clean signals out of those medium-monster 
> amps, but it still could 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 SSB transmitter
> 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 inexpensive
> 1.5-2.5 kW amp, I would suggest a pair of GU-81 or GU-46. They are 
> pentodes, but very easy to use and very reliable, esp. GU-81. These 
> run thousands of hours in class A-like audio amps or poorly built old 
> military HF transmitters with little or no any cooling.
> Only few years ago it was possible to buy them in hundreds for only 
> 2-3$ each; today it is a bit more, but still they cost very little for 
> what they can do.
> 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 TRX's 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 
> talking 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 
> tubes...
> I am too long, and sorry didn't found all the answers...
> 73, Roman US5WDX
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