[Heathkit] SB-200 Question

[Carl]

Mark, there is nothing special about 47 Ohms. The idea is to stop the 
parasitic by placing a snubber across the coil. A lower value gives a bit 
more bandwidth to fudge with but increases potential heating, 33 Ohms is the 
practical lower limit. Ive used up to 100 Ohms when the parasitic was known 
exactly and the coil designed smack on it. That doesnt work on multi brand 
572B's nor sweep tubes.

When 2W 47-51 Ohm carbons started getting scarce I bought up packages of 100 
at a time until the prices got ridiculous and then worked out the 
alternative at a job where I lived with a lab full of HP gear.

Carl
KM1H




I'm converting an SB-200 to 6M and did some reading on the construction of 
parasitic suppressors in some of the vintage handbooks;   I could never find 
a close book formula,  just examples and general discussion.   I concluded 
that it was kind of a trial and error process.

For the SB-200 conversion,  I'm using two turns of old radio shack solder 
wick on original out of tolerance 47 ohm carbon comp resistors that measured 
in at the upper 50's.  While working one night,  I busted the lead of one of 
the resistors.   I combined two 1W 100 ohm (out of tolerance) carbon comb 
resistors with the same two turns of old solder wick.  That seems to work, 
but I like Carl's suggestion and may try it.

Indeed there are a few places on the web that sell old "good" 47 ohms 2W 
carbon comp resistors,  but you're looking at $2.50 to $5.00 each plus 
shipping.  The values close to 47 ohms aren't that much.  Based upon my 
testing and Carl's remarks,  I think a lot of people think 47 ohms is a 
"magic" value and thus command a heftier price.   That's my comment only and 
I may be all wet on it,  but that's what I'm guessing.

Mark K3MSB



On Sun, Feb 28, 2010 at 6:25 PM, Carl <km1h at jeremy.mv.com> 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
>
> Ronnie Hull wrote:
>> Gentleman
>> I need to build up two parasitic suppressors for the 572's in the
>> sb200 I'm restoring.
>>
>> heathkit p/n 45-53
>>
>> I know this is a resistor wound with copper wire. WHAT value
>> resistor,  what size wire and how many turns
>>
>> Can anyone help me with this?
>>
>> Thanks in advance
>>
>> Ronnie
>>
>>
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