[Heathkit] SB-200 Question

[Donald Spoon]

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
>>
>
>