[Amps] Tuned inputs

[Roger]

Dave White wrote:
> I tried one of those Henry input chokes on my 3CX3000 amp and although it held SWR below 2:1 the transceiver's auto ATU was never happy with it and had a nasty habit of whirring away for minutes at a time.  Although I didn't have the kit to prove it I suspect that linearity was impacted too.
>   
I've found that to be the case with almost all the commercial amps I've 
tried even with the input SWR around1.1 to 1.3:1. So, when running 
through the amps I turn off the autotuner.

73

Roger
> The amp is a pi-L circuit with two big rollercoasters to apart from the plate choke series resonance at about 12MHz it should have had pretty much continuous coverage on 1.5 to 30MHz, hence the interest in a broadband match.
>
> I eventually fronted the input with a series of tuned pi input circuits and now things seem to work more as they should.
>
> Dave G0OIL
> -----Original Message-----
> From: "Jim Thomson" <Jim.thom at telus.net>
> To: amps at contesting.com
> Sent: 20/04/2010 02:56
> Subject: [Amps] Tuned inputs
>
> Date: Mon, 19 Apr 2010 12:27:50 -0400
> From: "Carl" <km1h at jeremy.mv.com>
> Subject: Re: [Amps] Tuned inputs
>   
>> Henry uses a broadband input for the 8K that seems to function. It is a
>> stack of 3 ferrite toroids with Hi bands/Low bands switching. It can be
>> purchased as an individual part.
>>
>> As they say, YMMV!
>>
>> Carl
>> KM1H
>>
>> ##  It's a basket case excuse for a 'tuned input'
>> It consist of 2 un-uns nose to tail.  They step the Z Up.. then turn 
>> around,  and step it back down.  Same deal on all bands except 10m.. where
>> they change step down ratio. A relay used to toggle between the
>> 2 x output un-un's.    So 3 x un-un's in total.
>>
>> ## No flywheel action anywhere.  The 8k uses a 3CX-3000A7,.. that has
>> a 47-50 ohm cathode Z.    The 8 k manual states NOT to use the auto tuner
>> in your xcvr.. and to just ..'put up with the high swr'.   Apparently, 
>> things really  go screwy when the auto tuner used.  With a real tuned input,  input swr 
>> is flat,  and way less drive required.
>>
>> later... Jim  VE7RF
>>     
>
> And what is the operating input VSWR?  I haven't heard back about the RF deck 
> I worked on yet. Using the tube as a dummy load and with low power it was 
> well under 2:1 on all bands.
>
> ##  apparently, it will vary per band.. and go as high as 1.8:1   no big deal.
> No flywheel action though, so tank eff suffers a bit.  The point was, it may work
> on a 50 ohm input Z tube... like a 8877 or 3x3..... but It would not be my 1st choice
> on something like a single 4-1000... with a 100 ohm input Z... or any other
> tube that deviated from 50 ohms.   It was only used on the 3k ultra.. and also
> the 8 k ultra. 
>
> Jim  VE7RF
>
>
>
> Carl
> KM1H
>
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