[Amps] SB221 Low Power Help

[TexasRF at aol.com]

Rahul, You can't use the internal tuner because the amplifier input  
impedance varies with drive level and the auto tuner tries to follow that. It is  
too slow for that kind of use.
 
The radio senses the higher vswr and reduces the drive power which  
increases the impedance and so on..
 
A manually adjusted tuner will work much better in this application. In  
fact, the amplifier input matching device is supposed to do this very job but  
the Q of the tuned circuits is too low for modern solid state radios as it 
was  designed 40 to 50 years ago when radios were tube type.
 
A small manual tuner will provide the needed Q boost to allow the radio to  
deliver full power to the amplifier.
 
73,
Gerald K5GW
 
 
 
In a message dated 9/8/2009 12:29:36 P.M. Central Daylight Time,  
vu3raz at gmail.com writes:

Yes I  tried that Gerald, the internal tuner eases the SWR and improve 
drive, still  no luck beyond ~ 400 mA with 2 tubes on 20.

Regards /// Rahul
On Tue, Sep 8, 2009 at 10:51 PM, <_TexasRF at aol.com_ 
(mailto:TexasRF at aol.com) > wrote:


If you have a small antenna tuner, it can be used to reduce the vswr  for 
test purposes. 
 
73,
Gerald K5GW
 
 
 
 
 
 
In a message dated 9/8/2009 11:59:00 A.M. Central Daylight Time, 
_vu3raz at gmail.com_ (mailto:vu3raz at gmail.com)  writes:


I will try that tomorrow don. For the input circuit does  nothing on 14 MHz
VSWR is nowhere near 2.

I am thinking of  ripping the input tuned circuit and replacing it with a
simple parallel  resonant ckt as in the eimac design.

Rahul


On Tue, Sep  8, 2009 at 9:59 PM, Paul Playford <_paul at w8aef.com_ 
(mailto:paul at w8aef.com) >  wrote:

> Richard Measures, AG6K, has done a lot of work on  SB-220s and talks about
> the input circuit at:
>
> _http://www.somis.org/SB220ci.html_ (http://www.somis.org/SB220ci.html) 
>
> FWIW,  Richard has his detractors however I have applied his  
modifications
> to several SB-220s and a Henry 2K with great  success.
>
> de Paul, W8AEF
>
> ZF2JI/ZF2TA   FO8DX/FO8PLA  8Q7AA  XZ0A  VU7RG   TX5D
>

> ----- Original Message ----- From: "Rahul Sathe" <_vu3raz at gmail.com_ 
(mailto:vu3raz at gmail.com) >
> To: <_amps at contesting.com_ (mailto:amps at contesting.com) >

> Sent: Tuesday,  September 08, 2009 8:32 AM  

> Subject: [Amps] SB221 Low Power  Help
>
>
>
>  Hello  Experts,
>>
>> My name is Rahul call VU3RAZ. This is my  first post to the list. I am
>> seeking help on fixing my old  SB221.
>>
>> as such the amp works on all bands, no arc  or parastatics, tuning and
>> loading is smooth and easy.. I am  not able to load the amp to full rated
>> plate current even with  a 100 W drive with my 756 Pro3. Input VSWR is
>>  high,
>> I tried tuning the input tuned circuit but no luck on  few bands (20, 
15).
>> With the 756 ATU the things improve but I  can not get plate current 
beyond
>> ~
>> 400 mA. Even  tried disconnecting HT from each tube to see if one if the
>>  3-500 is dead... both work but even with single tube I am not able to  
load
>> the amp fully. Am I looking at dead (weak) tubes ... can  both of them 
show
>> exact same behavior? Or I am missing  something I am not sure if I need 
to
>> re
>> build  the  PSU (replace caps)
>>
>> Please  help...
>>
>> 73
>> DE  VU3RAZ
>>
>>
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