[Amps] SB-220 filament pin cooling was: Re: Replacing 3-500Z tubes with 4-400A tubes

[Pete Smith]

I have had recurrent trouble with filament pins coming unsoldered in my 
SB-220 during heavy contest operation, even though I have the 
replacement Harbach fan that is supposed to move something like 30% more 
air.  I recall, years ago, reading that a lot of the heating might be 
due to relatively high resistance between the pins and their socket 
clips.  The same author, if I remember right, also opined that the 
heating would, over time, cause the clips to lose their temper, 
aggravating the problem.

I went in and swapped two of the grid clips for the filament clips on 
each socket, polished the pins and the inside of the clips, and put a 
small fan on top of the tube compartment to help move air through.  
Haven't melted a filament pin since.  Fingers crossed.

73, Pete N4ZR

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On 12/6/2011 4:32 PM, Carl wrote:
> The SB-220 cools the filament pins.
>
> Carl
> KM1H
>
>
> ----- Original Message -----
> From: "Bill Fuqua"<wlfuqu00 at uky.edu>
> To: "John King"<k5pgw at yahoo.com>;<amps at contesting.com>
> Sent: Tuesday, December 06, 2011 9:15 AM
> Subject: Re: [Amps] Replacing 3-500Z tubes with 4-400A tubes
>
>
>>   One possible issue with the 4-400A is that it has a metal shield around
>> the glass seals on the base.
>> The cross flow cooling used in some amplifiers does not work well and an
>> additional fan
>> may be need to cool the seals.
>>    Maybe someone can comment on this that have had personal experience
>> making the conversion.
>> 73
>> Bill wa4lav
>>
>>
>>
>> At 04:20 PM 12/4/2011 -0500, John King wrote:
>>> I have several very good 4-400 A tubes and have replaced a pair of bad
>>> 3-500Z tubes with a pair of 4-400A tubes in a Heath SB 220 amp.
>>>
>>> I have noticed much lower resting current being drawn when keyed up. I
>>> have noticed about 200 watts less power output with 100 watts drive. The
>>> 4-400A tubes are as new.
>>>
>>> I made no changes to bias or other parameter adjustment. I would
>>> appreciate the benefit of your knowledge and experience regarding the tube
>>> substitution. Thanks and 73, John, K5PGW
>>> _______________________________________________
>>> Amps mailing list
>>> Amps at contesting.com
>>> http://lists.contesting.com/mailman/listinfo/amps
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