[R-390] B+ current

[David Wise]

Long-term Standby is also rough on the cathodes.

The Supertex LND150 is a small, 500V depletion-mode mosfet that's
made for current limiting.  It's in stock at Mouser for 69 cents.
Connect it in series with C553, with drain facing V601 and
source+gate facing the filters.  Normally it looks like a 1K
resistor (negligible compared to V601's plate resistance), but
if the cap shorts, it will limit the current to a couple mA.

I used these to protect the photo-fet chopper I retrofitted
into my HP 740B Voltage Standard/Differential Voltmeter.

Dave Wise

>-----Original Message-----
>From: r-390-bounces at mailman.qth.net 
>[mailto:r-390-bounces at mailman.qth.net] On Behalf Of Tisha Hayes
>Sent: Friday, March 16, 2012 7:56 AM
>To: r-390 at mailman.qth.net
>Subject: Re: [R-390] B+ current
>
>Don,
>That was a great technical write-up of choke input filters. You should
>write that section up for a university book as many EE's have a
>problem understanding the concepts you detailed out. It also speaks to
>one of the reasons on why not to use the standby mode on the radio as
>the B+ voltage rises into risky areas. Radios that still have the
>"killer cap" on the IF filters should be particularly aware of that
>risk as well.
>
>Tubes are forgiving of minor excursions but the idea that the input
>windings of an IF filter could handle the several hundred mA of B+
>current from a shorted coupling capacitor can cause you to lose sleep.
>As the radio sits in standby for hour or days all of those B+ caps
>pick up a nice charge that is probably well in excess of the 250 mA or
>so normally supplied by B+. Even if you have the B+ 250 mA fuse
>retrofit I bet that the IF filter wire will burn out before the fuse
>wire even gets slightly soft.