[Amps] Anyone build a GU5B?

Roman Hanovych romanh at kb.lv.ukrtel.net
Fri Oct 5 13:40:00 EDT 2007

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
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
 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 SSB
 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 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
 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 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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