Test / GU74B tubes
¿ªÔÆÌåÓýAgree about the sad loss of Rich there Carl. ? 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. ? 73 ? Leigh ? From:
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? ? YAWN ? At least Rich and I were friends since about 1996 and I miss him ? I cant say the same about you. Go back under your GU-74B rock and behave ? Carl ? ?
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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 withdiametrically opposite opinion.? This little tetrode is the best ham radio amplifiertubes 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 receivedfamily of 1 and two tube amplifiers, as well as others, and hordes of amplifierhomebrewers. 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