[Amps] QRO HF2000 tank coil Q question
4cx250b at miamioh.edu
Tue Mar 23 16:45:56 EDT 2021
Jim raises an issue which has troubled me lately, and that is whether
the added expense and headaches required to build a bandswitched pi-L
tank circuit is justified by the modest gain in harmonic attenuation
(over a simple, well-designed pi-network). I realize the pi- L is
universal in contemporary commercial amplifiers, but there is a price
to be paid for the added complexity and reduced reliability. For
example, the Alpha 9500 uses a complicated, custom, four-deck
bandswitch to do the job (replacement wafers are $200 each) and it
requires a couple of hours of study just to figure out how it works.
Ditto for, e.g., the old Drake L-75. Such contortions become
increasingly difficult when using heavy Radio Switch Model 86 or Model
88 bandswitches, as might be appropriate for 8877-level amplifiers. I
vaguely recall having Tom, W8JI weigh in on the debate, but don’t
remember the details. I think he doubted whether the benefit of a pi-L
was really worth the effort.
Sent from my iPhone
> On Mar 23, 2021, at 12:07 PM, Jim Thomson <jim.thom at telus.net> wrote:
> ## per Owen Duffy, in VK land, any tube operating in class A, AB, or B already has 7.5 db of 2nd harmonic
> attenuation..... before you add the simple pI net. He claims with the pi net..and a C1 Q of just 8, the 2nd harmonic
> attenuation is 40 db..... which IMO, is more than ample for my needs, None of my ants are resonant on their 2nd harmonic anyway.
> My F12 40m yagi isnt even resonant on its 3rd harmonic either. The 40m yagi does resonate on 24 mhz.
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