[Boatanchors] Tube filaments

[ronw8]

"Seem to happen" is an objective statement.

I ran all sorts of life tests.
It happens to all filaments .... even lamp filaments with no external 
current.
"Early" is a relative term.  AC run filaments just have a longer life 
due to the migration physics.
---
Ron



Carl wrote:
> That doesnt seem to happen with indirectly heated filaments that have 
> a long and reliable history in mobile enviroments.
>
> In fact a tube such as the 6V6 which is known to be prone to heater 
> cathode leakage resulting in hum in AC sets merrily chugs away on DC.
>
> Carl
> KM1H
>
>
>
>
> ----- Original Message ----- From: "ronw8" <w8ron at sbcglobal.net>
> To: "Singley, Rodger" <rbsingl at ilstu.edu>
> Cc: <boatanchors at mailman.qth.net>; "Ed Berbari" <eberbari at indy.rr.com>
> Sent: Saturday, December 05, 2009 7:31 AM
> Subject: Re: [Boatanchors] Tube filaments
>
>
>> 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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>>
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>