[Amps] Tuned Input

Bill L. Fuqua wlfuqu00 at uky.edu
Sun Jan 22 16:08:33 EST 2006


Don't forge coax is has both inductance and capacitance. In fact the characteristic impedance is the squareroot of 
L/C.  L and C are inductance per a unit of length. 

73
Bill wa4lav


-----Original Message-----
From: "R.Measures" <r at somis.org>
To: Karl-Arne Markström <sm0aom at telia.com>
Date: Sun, 22 Jan 2006 10:26:59 -0800
Subject: Re: [Amps] Tuned Input


On Jan 22, 2006, at 3:19 AM, Karl-Arne Markström wrote:

> The reason why the type of input tuned circuit in a GG amplifier
> becomes critical is that the input and output circuits are effectively
> connected in series.
>
> This means that the plate current pulses also flow through the cathode 
> circuit,
> and if it presents an appreciable reactance for the harmonic content 
> of these pulses,
> the tube will not have the proper RF operating point.
>
> A shunt C at the cathode end of the input network is the easiest way 
> to ensure
> the proper path for the harmonic currents around the network.

True, K. A.,. however, the needed amount of shunt C for 10m is not even 
close to the amount needed for 80m.  Thus, most of the shunt C needs to 
come from C2 in the tuned inputs for the lower bands.
>
> The use of a long coaxial line between the driver and the untuned 
> input of a
> GG amplifier has other drawbacks.
>
> For the fundamental frequency component, the line may appear as a 
> shunt capacitance,
> but with increasing harmonic numbers the effects
> of the coaxial line electrical length and its termination
> may result in high equivalent series impedances in series with the RF 
> plate current flow.
>
> 73/
>
> Karl-Arne
> SM0AOM
>
>
> ----- Original Message -----
> From: "Gary Schafer" <garyschafer at comcast.net>
> To: <gbadger at sbcglobal.net>; "'Tony King - W4ZT'" 
> <amps080605 at w4zt.com>; "'AMPS'" <amps at contesting.com>
> Cc: <skipp025 at yahoo.com>
> Sent: Sunday, January 22, 2006 4:59 AM
> Subject: Re: [Amps] Tuned Input
>
>
>> Could you elaborate a little more on this?
>>
>> Why is the efficiency less when a coil input is at the cathode?
>>
>> Also wouldn't "too much coax" provide extra capacitance to ground?
>>
>> I assume that the harmonic that you're talking about is second 
>> harmonic
>> energy? Why do we care if it is attenuated or not other than IM 
>> performance?
>>
>> Thanks
>> Gary  K4FMX
>>
>>
>>> -----Original Message-----
>>> From: amps-bounces at contesting.com 
>>> [mailto:amps-bounces at contesting.com] On
>>> Behalf Of George badger
>>> Sent: Saturday, January 21, 2006 12:08 AM
>>> To: Tony King - W4ZT; AMPS
>>> Cc: skipp025 at yahoo.com
>>> Subject: Re: [Amps] Tuned Input
>>>
>>> I have been using an antenna tuner as an input circuit
>>> for my HB 8877 lineart for years. When I was at EIMAC
>>> I learned that when linear amplifier OEMs complained
>>> of low efficiency it was often caused by too much
>>> inductance in the input cathode drive cicuit. For
>>> example, a T input circuit (LCL} was a disaster on 40
>>> and 80. Similarly, if the lead from the input pi
>>> network  to the cathode is too long,  a problem is
>>> created on 10M. The reason is that the cathode current
>>> is not sinusoidal so harmonic current must have an
>>> easy path to ground. Unless there is sufficint
>>> capacitance to ground, efficiency suffers.
>>>         To avoid this on my linear,I experimented with
>>> a variable capacitor to determine the largest fixed
>>> capacitor I could get away with and still be within
>>> the range of my drake tuner on all bands The
>>> capacitance turned out to be 50 pfd. It is connected
>>> directly from cathode to ground with short leads.
>>> W6TC
>>> --- Tony King - W4ZT <amps080605 at w4zt.com> wrote:
>>>
>>>> Ian White, GM3SEK wrote:
>>>>> Roger D Johnson wrote:
>>>>>> Tony King - W4ZT wrote:
>>>>>>> I don't know if any of you have looked into this
>>>> particular product but
>>>>>>> at first glance it appears that this is one
>>>> solution for the tuned input
>>>>>>> problem many of us face and at the right price
>>>> too:
>>>>>>>
>>>>>>>
>>>>
>>> <http://www.ldgelectronics.com/manuals/AT-100AMP%20Manual.pdf>
>>>>>>>
>>>>>>> 73, Tony W4ZT
>>>>>> The purpose of the tuned input is to provide a
>>>> "flywheel" effect to
>>>>>> smooth out the variations of amplifier input
>>>> impedance over the
>>>>>> operating cycle. The Q of the circuit determines
>>>> how much "flywheel"
>>>>>> effect we have. The lowest recommended value I've
>>>> seen is 2. Solid
>>>>>> state transceivers want even more...perhaps 3 to
>>>> 5.
>>>>>>
>>>>>> The LDG tuner is an L network in which the Q is
>>>> dependent on the ratio
>>>>>> of impedances to be matched. For normal input
>>>> impedances, the resulting
>>>>>> Q will be very low. It's hard to write the
>>>> formula in text format but
>>>>>> in words it's as follows: divide the higher
>>>> impedance by the lower,
>>>>>> subtract 1 and then take the square root. For
>>>> example to match an
>>>>>> impedance of 100 ohms to 50 ohms, the resulting Q
>>>> is only 1!
>>>>>>
>>>>>> With a 3 terminal network you can choose the Q
>>>> independently of the
>>>>>> matching and would seem to be the way to go.
>>>>>>
>>>>>
>>>>> The recommendation for an input tuned circuit Q of
>>>> 2-4 came from a 1961
>>>>> article by Eimac authors in QST. That
>>>> recommendation was specifically
>>>>> based on a pi tank, and it was also pointed out
>>>> that the output
>>>>> capacitor provides a direct shunt path from
>>>> cathode to ground for
>>>>> circulating harmonic currents. But when an
>>>> L-network is being used in
>>>>> impedance step-down mode, it doesn't even have an
>>>> output C.
>>>>>
>>>>> Now it may be that the designers know all about
>>>> these things, have
>>>>> checked their effects on amplifier IMD
>>>> performance, and have found some
>>>>> reason to ignore the Eimac recommendations. If
>>>> that is the case, it
>>>>> would be good to hear why.
>>>>>
>>>>>
>>>>>
>>>>> 73 from
>>>>> Ian GM3SEK
>>>>
>>>> These are very good points that raise questions that
>>>> must be answered
>>>> before I would consider using such a tuner as the
>>>> input circuit. I'm
>>>> with you Ian, I'd like to see the answers to the
>>>> questions.
>>>>
>>>> It's been said many times that the best input
>>>> circuit is a pi-network
>>>> which will provide the fly-wheel effect to help
>>>> smooth the dynamic
>>>> changes in input impedance. As Skipp025 said, on
>>>> another list, "The
>>>> automated antenna tuners might tend to hunt around
>>>> looking for the best
>>>> match."  If that happens, we've just moved the
>>>> problem from the
>>>> transceiver to the amp.
>>>>
>>>> Bill, WA4LAV, has an interesting suggestion about
>>>> shunting the cathode
>>>> with a capacitor and I wonder what everyone thinks
>>>> of that?
>>>>
>>>> 73, Tony W4ZT
>>>>
>>>>
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Richard L. Measures, AG6K, 805.386.3734.  www.somis.org

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