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Re: [Amps] QRO HF2000 tank coil Q question

To: Jim Thomson <>
Subject: Re: [Amps] QRO HF2000 tank coil Q question
From: MU 4CX250B <>
Date: Tue, 23 Mar 2021 16:45:56 -0400
List-post: <>
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.
Jim w8zr
Sent from my iPhone

> On Mar 23, 2021, at 12:07 PM, Jim Thomson <> 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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