Carl,
Plate Idle Current in SSB position is ~ 120 to 125 mA. I have to check the
grid chokes as suggested by you, how abt grounding the grids directly
without chokes?
Checked the tubes... there is no short.
The coaxial cables looks ok, as the VSWR is far better on other bands like
40 and 80 input can be tuned on 40 and 80.
Will run the amp with individual tubes as suggested by dan.
I am not sure if it is a European build, however it is wired for 230 V no
way to find out if it has 50Hz transformer.
Thanks for all the help.
Regards /// Rahul
On Tue, Sep 8, 2009 at 10:50 PM, Carl <km1h@jeremy.mv.com> wrote:
> A couple of things to check Rahul:
>
> 1. Plate idle current should be 120-150 ma in the SSB position.
>
> 2. One of the grid chokes is open from a tube arc or short.
>
> 3. If it has always been this way since built the coax cables going to the
> input bandswitch may be reversed.
>
> Input VSWR is a function of plate current & voltage so with one defective
> tube it will be quite high and out of the networks tuning range.
>
> Dont go ripping things out, that 100W should be more than enough at 1000W
> out.
>
> Also, unless it is a European built/kit amp from Heath UK, or supplied with
> a 50 Hz transformer it will have to be run at reduced power to keep the
> transformer from burning up.
>
> Carl
> KM1H
>
>
> ----- Original Message ----- From: "Rahul Sathe" <vu3raz@gmail.com>
> To: <amps@contesting.com>
> Sent: Tuesday, September 08, 2009 11:32 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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>
>
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