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

[mikea]

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