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Message #73102 of 126777
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Date: 2005-03-06
From: Mike Hawkshaw
Subject: Re: [EE] Wire wrapping insight needed!

From: "Paul James E."
Subject: Re: [EE] Wire wrapping insight needed!

>  Negative on the DC for the filaments.   Modern (60's to now) tubes have
>  heaters that are designed for AC.   If you use DC, there is a chance of
>  premature failure of the heater because of migration.
This is true for directly heated cathodes, but there are some tubes that
have an indirectly heated cathode and it is quite OK to use DC on those. For
example high power klystrons for UHF TV that we have in service use DC
fills. The warm up time is of the order of 5 minutes.

>  The heater is
>  designed to run off AC. The cathode then is the common return point.
>  This way, you don't get the AC hum. The 12AU7 may have a center tapped
>  heater.......
I've never heard of this, but for AC heaters in tubes where hum is a
problem, it is usual to connect the centre of a linear potentiometer to
ground, and then one of the fills connections to each of the ends. The pot
is then adjusted for least hum on the output. This circuit was called a hum
dinger, though the term has fallen into dis (re) use.

Cheers...Mike.


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