[Amps] SB-220

[R.Measures]

On Nov 3, 2004, at 9:32 AM, kwasny wrote:

>
> ----- Original Message -----
> From: "R.Measures" <r at somis.org>
> To: "kwasny" <kwasny at netzero.net>
> Cc: <amps at contesting.com>
> Sent: Monday, November 01, 2004 8:38 PM
> Subject: Re: [Amps] SB-220
>
> Richard, How do I go about getting these parasitic mods an 
> instructions from
> you?

- - The suppressor retrofit info for 2, 3-500Z amplifiers is on my Web 
site.  The General Instructions are there too, as is an article on the 
SB-220.  If you need a complete retrofit kit and you have a soldering 
iron, my helper is doing orders on Saturday.  E-mail a list of what you 
want, we will ship it, and you can pay from the invoice.

cheerz, Daryl

> I hate to use the smoke test to find where it arced but I will power it
> up without the first and if that goes well will try one tube then 
> both. At
> this time I have a .5mH choke to replace the one that fried and have 
> removed
> that smoked 200pF cap. Should this be OK to try it? I will not drive 
> it at
> this time.
>
>> Problem is that the parasitic may occur next time you try it out.  
>> I'd do
>> the modifications first before any power was applied.  My opinion is 
>> that
>> the Harbach >parasitic mods are kinda expensive; you can get the same 
>> stuff
>> cheaper direct from Rich Measures.  But the Harbach step/start 
>> circuit can't
>> be beat and the >replacement fan from them is tons better than the 
>> original.
>> Bought my 220 used 14 years ago, had spitting and arcing in the tune 
>> cap,
>> did all the mods, it runs like a >champ.  Watch out for that HV!  Gd 
>> luck,
>> Phil  KB9CRY
>>
> Darryl - K5WAS
>
>> On Nov 1, 2004, at 4:50 PM, kwasny wrote:
>>
>>> I smoked one of 1mH chokes on my sb-220.
>>
>> Darryl -- This is caused by a parasitic oscillation ar c. 110MHz.
>> Replacing the choke with resistors will not prevent vhf oscillations.
>> There's an article about parasites in the 220 on my web site that
>> explains this phenomenon and a method of reducing vhf gain in the 220.
>> .
>>
>>> Also saw some arcing around the meter area. I am going to replace the
>>> chokes with resisters. Have to replace one of the 220pF caps that got
>>> smoked up. Does anyone know what may have arced near the meter area?
>>> Fact is, what made the rf choke smoke?
>>
>> The high grid-I during a vhf parasite creates a lateral EMF on the hot
>> filament helix.  This occasionally bends the filament far enough
>> sideways to short against the grid -- which is grounded through the
>> choke.  Since the filament xfmr is connected to the +110vdc power
>> supply on Rx, it shorts the supply to ground, causing a large current
>> flow-- which smokes the choke.
>>
>>> What other damage might I have done. What is a good source for the
>>> 220pF caps and any other parts that I may need?
>>>
>>> Thanks to all, Darryl
>>> _______________________________________________
>>> Amps mailing list
>>> Amps at contesting.com
>>> http://lists.contesting.com/mailman/listinfo/amps
>>>
>>>
>>
>> Richard L. Measures, AG6K, 805.386.3734.  www.somis.org
>>
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>>
>
>
>

Richard L. Measures, AG6K, 805.386.3734.  www.somis.org