[Amps] Question re GU-74B capability

[Jim Thomson]

Date: Wed, 24 Oct 2012 10:14:20 +1030
From: "Leigh Turner" <invertech at frontierisp.net.au>
To: <amps at contesting.com>
Subject: Re: [Amps] Question re GU-74B capability


Carl, VK land has a 1 kW power limit on the HF bands.

###  My wife’s hair dryer  sucks 1.5 kw CCS. 



However I would not use a single GU74B/4CX800A tube at more than 800W PEP.
Run at that Po level or under, in SSB mode the damn tube lasts forever.

##  when the fil is lit.... the clock starts ticking.  Take any amp, shut off the B+
and just run the fils..and leave em on, and eventually the emission drops off,
regardless of whether drive is applied or not.  Broadcast stations typ get
2-4 years out of a tube b4 it’s rebuilt.   Applying drive does not  wear the tube out,
it’s the emission of the fil/cathode that wears off.  Hence rebuilding the tube...
unless it’s a Russian tube, then you can’t rebuild em anyway, you toss em. 





Which is precisely what one would expect for a conservatively rated Russian
rugged mil-spec tube, i.e. practical experience matches expectations!

Even at today's increasingly higher pricing compared to how cheap they used
to be, the GU74B still represents exceptional value and bang for buck.

##  My problem with that silly GU-74B is that the damn thing IDLES
at a whopping 225ma PER TUBE.   And it has to idle that high, just to get
some decent imd.  The 2 hole QRO amp idles at 450 ma....and the 3 hole version
idles at 675ma.   Check out the original russian imd specs..they suck. 

##  On cw, who cares, just crank up the bias V..and lower the idle current way down.
3 x stage EBS (if you can actually get it to work) will reduce the load on the tube and
HV supply. 

###  My advise is to use a tube designed to do the job. The GU-74B  was never designed
with clean imd in mind.  It’s a surplus russian military tube. 

###  If you want a real tube for the job, use a single 8877.  If you require more power
than the 8877, use a 3CX-3000A7.   In fact, use the 3CX-3000A7 anyway, since you can
drive it to 1.5 kw out..with just 50w.  You can also use the 225 W CCS grid as a dummy load
for your FT-1000D.  You can’t blow it up, and IMD3 is –59db pep. 

## what do you folks do with the 50 cents you saved with your russian tubes... buy a 
cafe latte ? 

##  By the time you spend XXX  on shipping a 1/2 doz GU74B’s to na... then find out 2 are duds,
and one is soft, and then more screwing around shipping  em back, then “new”  ones being shipped back
to you... you save nothing. 

Jim VE7RF  




Leigh
VK5KLT