[R-390] 6080 in place of 6802 - RMS ???

[Bob Camp]

Hi,

High power tubes do sometimes use direct heated filaments with DC on them.
It's not to common but it is done. In that case standard operating practice
is/was to reverse the filament leads once a month to even out the "wear" on
the filament. One interesting place it's documented is in a series of ads
that RCA ran in QST at the opening of WWII.

    Enjoy!

        Bob Camp
        KB8TQ


----- Original Message -----
From: "mikea" <mikea@mikea.ath.cx>
To: "James Miller" <JamesMiller20@worldnet.att.net>
Cc: <r-390@mailman.qth.net>
Sent: Wednesday, January 09, 2002 10:56 PM
Subject: Re: [R-390] 6080 in place of 6802 - RMS ???


> On Wed, Jan 09, 2002 at 08:52:04PM -0500, James Miller wrote:
> > I did not see the strobe effect between the filament glow and the TV
> > scan rate.  Frankly, I think you're pulling our legs about that factor,
> > since the glow is a heating effect and would probably not dissipate that
> > rapidly (60 Hz). You are pulling our legs, aren't you?   But the slowly
> > walking "standing wave" seen in the vibrating filament illuminated by
> > the TV flicker was awsome to watch.  So that begs the question.  It
> > seems to me that using DC rather than AC would create less mechanical
> > stress on the filaments, I wonder why they didn't do that.  By the way,
> > where in the 390a is this 6080 tube used?
>
> No, I definitely was not pulling your leg. I was demonstrating by
>  experiment, in the manner of all good empiricists, that the tube
> cathode and filament could not possibly cool down enough that the
> resistance would change materially.
>
> Probably the mechanical stress on the filaments isn't very much:
> they're insulated, and pushed as a bundle into the cathode, or so
> I remember from my dissections of dead 6SN7s and 12A(U,V,X,Y)7s.
> Really low-noise gear tends to use DC on the filaments to reduce
> the possibility of AC hum leaking thorugh from the filaments and
> the filament power supply.
>
> IIRC, there is a preference for AC over DC (or vice-versa) in
> tubes in which the filament is also the cathode. IIRC, it's
> DC, but I could be wrong. Certainly the most negative part of
> the filament would "wear" (lose emission) faster if it were
> DC-heated, while an AC-heated filament would be more likely to
> lose emission more evenly.
>
> Barry Ornitz may have a better idea of what happens here, and
> I'd be happy to hear what he has to say on the subject.
>
> Ah! A short Google search on `"power tube" filament supply yields
> lots of relevant hits. One of the more interesting ones is about
> the Burle S94000E Power Tube, Its filament supply must provide
> 3.5 VDC at 4200 Amperes. Yes, that's Four Thousand Two Hundred
> Amperes to heat the filament. This is a _big_ tube: it weighs
> 325 pounds out of its crate, and 870 pounds crated. The tube's
> data sheet says:
>
> A DC filament supply is required. Filament excitation with an AC
> supply may generate mechanical resonances in the filamen
> structure. A three-phase, full-wave rectifier supply is
> recommended.
>
> The operating region on the voltage-current chart tops out at
> about 90 A with a drop across the tube of 20 KV. At 30 KV, the
> current is reduced to "only" 65 A or so.
>
> This is typical of large power tubes, judging by what else I
> found on Google. Still, I hope Barry O. will give us the benefit
> of his education and experience.
>
> --
> Mike Andrews
> mikea@mikea.ath.cx
> Tired old sysadmin since 1964
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