[Heathkit] SB-200 Question

[Carl]

Don, I wrote a 1966 National Service Bulletin about different suppressors 
when using the NCL-2000 on 10M. I discovered the problem when using my 
engineering prototype (still have it) in a CW DX contest sort of pushing it 
a bit over 1KW DC input (-;

It was 3 120 Ohm 2W carbons in parallel with a big 1T loop of wire. No more 
problems even as an AM linear. The amp would have been completely stable if 
marketing hadnt forced us to take out the screen ring bypass capacitor and 
had us just bypass the socket pins. It saved maybe $20.

I know what you mean about the steel absorbing lead atoms. Took a customers 
amp out of the box a few weeks ago and groaned but Im still good for trips 
down and up the basement stairs!

Carl
KM1H
National Radio 1963-69
Member of NCL-2000 and NCX-1000 Design Teams
Service Tech and Service Manager


----- Original Message ----- 
From: "Donald Spoon" <drspoon at sbcglobal.net>
To: "Heathkit" <heathkit at mailman.qth.net>
Sent: Sunday, February 28, 2010 7:06 PM
Subject: Re: [Heathkit] SB-200 Question


> Thanks Carl!
>
> I shall transcribe your reply onto gold leaf and place it in a
> hermetically sealed container for preservation of the "wisdom of the
> Elders"!  ;-)
>
> I managed to  fry a couple of those resistors on my NCL-2000 back in
> 1976 while operating on 10 Meters.  I was experimenting a lot trying to
> learn how to tune and load the thing  into a home-made ground plane, and
> probably abused the amp horribly.  Fortunately, I didn't burn all the
> paint off of them and was able to determine their values.  A trip to the
> local part store and a careful replacement solved the problem!  I was a
> tad more careful after that!  I need to re-furbish that amp, but that
> thing as sure gained a LOT of weight over the years!
>
> Cheers,
> -Don Spoon-
> K0APK
>
> Carl wrote:
>> The coil can vary depending on the self parasitic frequency of the
>> tube. This can vary from around 70-150 mHz for the usual glass tubes
>> and higher for the ceramic-metal versions.
>>
>> The coil is there to be resonant at the parasitic and the resistor is
>> there to prevent it by "suppressing" it from starting. The resistor
>> does not absorb parasitic power but can get hot on 10M if the coil is
>> resonant too low. The tube is the determining factor but sloppy
>> construction can aggravate the severity.
>>
>> If the tube for the HB amp has a recommended coil in a construction
>> article then use it. If not 3-4T of #14 roughly 3/8" ID and spread to
>> resonate is generally used up into the 3CX800 power region.
>>
>> With the suggested 5W resistor the caps work with any coil; Ive not
>> tried different resistors but have used the method for 572B, 4-400A,
>> 3-500, 813, 3CX800 and even 250TH's. Trying to find NOS 33-51 Ohm 2W
>> carbon comps that havent radically changed value is becoming a chore
>> unless you have a stash of Allen Bradley.
>>
>> Carl
>> KM1H
>>
>>
>> ----- Original Message ----- From: "Donald Spoon" <drspoon at sbcglobal.net>
>> To: "Heathkit" <heathkit at mailman.qth.net>
>> Sent: Sunday, February 28, 2010 5:57 PM
>> Subject: Re: [Heathkit] SB-200 Question
>>
>>
>>> I wasn't watching the EMail  addy and this didn't go to the right place,
>>> so here goes again.
>>>
>>> This raises a more "generic" question that I have been meaning to ask
>>> the Elmers/Engineers/Wizards that hang out here.  Is there a rule of
>>> thumb in making  these parasitic chokes, or are they specific for each
>>> tube and/or circuit?  Can I use Carl's circuit with 3 turns of wire in a
>>> home-brew linear and be assured that I am reasonably close?
>>>
>>> -Don Spoon-
>>> K0APK
>>>
>>
>>
>
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