Hi Jim,
The good thing about the sweet little GU74B tetrode tube is its stable high
gain in grid-driven AB1 configuration with a simple untuned swamped grid
input circuit....no messy band switching required on the input side.
Just a few watts drive from an SDR exciter pushes the HF amplifier's output
well past 400 to 500 Watts at which PEP output the tube will virtually last
forever.
You don't need a "sky high" idle current to get acceptable IMD equal to or
better than most solid state amps; just use some cathode degeneration NFB
and all's well on that score.
If you specifically want a squeaky clean state of the art signal then just
put your SDR exciter in "Pure Signal" mode and a feedback sampling coupler
on the amp's output.....then all IM products are negated for an exemplary
clean signal spectrum.
These military surplus tubes were once very low cost a decade or two back,
but sadly market prices have risen like they have done for all Tx tubes :-(
It's the old argument of selecting different horses for different courses
Jim....each tube whether triode or tetrode and the various circuit
topologies have their merits in particular scenarios and use cases.
The available tube choices are drying up nowadays for tube amplifier
aficionados....everything has either become damn expensive, or is in the
realm of unobtainium..... :-(
73
Leigh
VK5KLT
________________________________________
From: ham_amplifiers@... [mailto:ham_amplifiers@...]
Sent: Sunday, 26 May 2019 1:27 PM
To: ham_amplifiers@...
Subject: RE: [ham_amplifiers] Test / GU74B tubes
Posted by: jim.thom@...
But on the subject of the venerable GU74B tube we contraire with
diametrically
opposite opinion.? This little tetrode is the best ham radio amplifier
tubes around; > on the proviso it¡¯s used properly and within its ratings¡..
as evidenced by amp
manufacturers like Acom in their immensely popular and well received
family of 1
and two tube amplifiers, as well as others, and hordes of amplifier
homebrewers.
Leigh
##? Why even mess with a tetrode ?? ? My problem with the GU74B ......"
4CX-800"? ?is the sky?high?225 ma idle?current requirement PER TUBE !? ? And
that's just to get the IMD?to acceptable levels.? That's?insane.? ?450 ma
for a pair...and 675 ma for a 3 holer.?
##? Don't? mess?with the Acom? SS? amps.....? way too many problems...
and?no?qsk.??
##? Leigh,?with?your 400 W? limit,?anything will last a long??time.? ?With a
400 w? limit, a?SS amp would suffice.?
Jim? ?VE7RF
Leigh Turner