TopBand: Re: [AMPS] TL922 and burned grid choke

[km1h @ juno.com]

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



 
>Regards, 
>Martin ZL1ANJ
>
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