: Re: [AMPS] TL922 and burned grid choke

[Rich Measures]

>
>
>At 10:54 12/3/97 -0500, you wrote:
>>
>>On Wed, 03 Dec 1997 12:44:29 +1300 Martin Ellis <jmellis@ihug.co.nz>
>>writes:
>>>Good morning, 
>>>Here are details of repairs I found necessary when one 3-500Z
>>>had a grid-filament short in my TL922:
>>>
>>>Destroyed components:
>>>L7	470uH grid RF choke
>>>R22	4.7K grid resistor
>>>D1	diode 1N4007 type,  110v relay/bias power supply 
>>>C3	33uH 160v electrolytic (replaced, but may have been OK)
>>>The good grid RF choke measured 3.0 ohms (with 4.7k across it).
>>>The 7.5v 10w zener D2 tested OK.
>>>With assistance from Charley W1TE I am planning to install
>>>new RF chokes with higher current capacity.
>>
>>That would be a mistake Martin. In that amp the chokes make poor current
>>limiters. In fact I would suggest even lower current ratings such as the
>>160ma 1MH that Heath used in the SB-220. Those have an R of about 23
>Ohms
>>and I would rather lose an occasional choke as compared to a tube.
>>
>>>
>>>I have one question:
>>>Will modern type metal film 4.7K 1/2 watt resistors be suitable
>>>for the grid resistors R22 and R23?  Or should we still try
>>>to use the old style composition resistors?
>>>What is the best component to protect the filament/bias transformer
>>>is the event of an unexpected grid/filament short in the TL922?
>>
>>Since many seconds of a short will not fry the xfmr a simple fuse may
>>suffice. You would need to choose a Slow Blow type that could handle the
>>initial inrush current but still be able to blow on overload....not  a
>>particularly easy task. If it was my amp I would also add a fuse in the
>>bias line and drill 2 new holes on the rear apron for the fuse holders. 
>>
>>GL   Carl   KM1H
>>
Drill holes, add 2 fuse holders?  A large solution to a small problem.   
 To convert a TL-922 or an SB-220 from V-cutoff bias to R-cutoff bias  
takes about one minute of work with a soldering iron.  No parts are 
needed.  
Rich...


R. L. Measures, 805-386-3734, AG6K   


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