Why AC on heaters

From trsimo–(at)–nterlog.com Sun Aug 27 18:33:38 CDT 1995
From: trsimo–(at)–nterlog.com (TR Simon)
Newsgroups: rec.audio.tubes
Subject: Re: Heater Voltage AC or DC??
Date: Mon, 28 Aug 1995 01:11:31 GMT

snarfsnou–(at)–ol.com (SnarfSnout) wrote:

>My question: is there a difference/advantage to using AC or DC for for
>the heaters. (my shot in the dark… DC is better but in the interst of
>simplicity and cost the St 70’s use AC????)

>Thanks
>Dave
My understanding in this matter is the main reason that DC (for heater
supply) should be avoided, is because of an action similar to
electro-plating which causes uneven migration of metal due to the
voltage potential across the heater. This in effect causes one end of
the heater grow thinner while the other end grows larger.

nb. The cathode provides an intgrating function due to its thermal
mass.
TRS.