[Boatanchors] Tube filaments

[ronw8]

Running DC on filaments would cause the filament material to migrate to 
one side with early failure.  No issue with AC.
---
Ron


Singley, Rodger wrote:
> 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
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