[Boatanchors] Tube filaments

[Singley, Rodger]

Tektronix used DC for the filaments in their vertical plug-ins on the 500 series "lab" scopes.  In this case the filaments are in series and few from one of the regulated B+ supplies.  

Given the desired performance of these scopes and the selling price (many of the plug-ins sold for as much as a typical ham receiver/transmitter setup) it was possible to justify the cost of this circuitry.  But that wouldn't be the case for typical consumer gear.

I seem to recall reading somewhere that filaments (tubes or light bulbs) are more prone to developing localized "hot spots" on DC as compared to AC.  Anyone else recall this potential problem?

Rodger WQ9E


-----Original Message-----
From: boatanchors-bounces at mailman.qth.net on behalf of Ed Berbari
Sent: Fri 12/4/2009 7:46 PM
Cc: boatanchors at mailman.qth.net
Subject: Re: [Boatanchors] Tube filaments
 
I would like slightly change the direction of this discussion on tube 
filaments.  My background is in medical electronics and when I first got 
started in this field tube amplifiers for biophyscial measurements (ECG, 
EEG, etc) were still common.  A leading company was Electronics for Medicine 
(E for M, now defunct) and they used DC voltages for filament supplies.  It 
was their way to limit 60 Hz hum from interfering with these low level 
signals.  I always wondered why this practice was not used with receivers 
and other low level signal circuits.  Instead we are burdened with numerous 
bypass capacitors and other means to limit 60 Hz interference.  Any wisdom 
or thoughts out there on this approach?

Ed, W9EJB