Jim,
In a directly heated cathode, i.e. a filament, the higher the filament voltage,
the greater the induced grid swing caused by the filament voltage. If you had a
100 volt heater and the grid return to the centre tap, the grid-filament
potential at one end of the filament would swing 70 volts in one direction and
70 volts in the other direction to part of the filament, the actual voltage
depending on the voltage distribution along the filament.
Where you have an oxide coated cathode and a heater, the cathode is a
uni-potential cathode, i.e. it has the same potential (pretty well) from all
parts of it to the grid, which is how things like 117L7 get away with it. That
also made tubes like the 28 volt heater 4CX250 and the UK 5B258 (a loktal
version of the 807) variant possible - both meant for aircraft use. But
filament type tubes need not to have too big a swing on the filament voltage.
The directly heated 6146 (the Mullard QZ06-20) had a filament voltage of 1.6
volts at 3.2 Amps, partly because the bias voltage would be around 45 volts, so
the filament volts were small in relation to the bias. The battery operated
pre-WW2 QPP (Quiescent Push-Pull) AF double triodes and pentodes run in Class B
(to save B battery drain) had in some cases, a recommended filament + and -
pins because of minimising the effects of filament voltage on bias.
Oxide coated cathodes run at around red heat, quite a lot cooler than thoriated
tungsten, for the same emission in amps/sq. cm. On the other hand, excess
current can literally strip the oxide coating off the cathode, which is why
they need a longer warm up period. Some 'dull emitter' (as they were called in
the 1920s) filament tubes have an oxide coated filament e.g. 1T5, 1R5 etc and
run at just perceptible red in a dark room.
Are you going to be at Dayton this year? I'll be on the IARU booth for much of
the time - there's very little I want other than some good quality phono plugs.
vy 73
Peter G3RZP
========================================
Message Received: Apr 22 2014, 05:09 PM
From: "Jim Garland" <4cx250b@miamioh.edu>
To: "'Joe Subich, W4TV'" <lists@subich.com>, amps@contesting.com
Cc:
Subject: Re: [Amps] Filament Voltage Question
A note from WC6W pointed out that the particular tube in question, a
4CX3500A, actually uses a directly heated cathode, although I'm not sure
that makes a lot of difference (except that the filament presumably runs
hotter than it would otherwise). And the ruggedness issue could certainly be
a consideration. Still, 90 Amp filaments strike me as excessive unless
there's some other really compelling reason. Hum rejection (also pointed out
by a commenter) would seem to me to be worse in low V/high I filaments,
since the induced hum field would be greater. In fact, with 90A of current,
I'd think there could be some significant distortion of the electron beam
caused by the filament's magnetic field. I wonder if heat conduction from
the filament to the socket is a consideration? It might be hard to get the
heat out if running a filament at high V/low I.
73,
Jim W8ZR
> -----Original Message-----
> From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Joe Subich,
W4TV
> Sent: Tuesday, April 22, 2014 9:34 AM
> To: amps@contesting.com
> Subject: Re: [Amps] Filament Voltage Question
>
>
> Perhaps because the low voltage, high current filament is more
> physically rugged?
>
> 73,
>
> ... Joe, W4TV
>
>
> On 4/22/2014 11:19 AM, Jim Garland wrote:
> > I was reading the data sheet this morning on the 4CX3500A and noticed
the
> > filament requirements are 5V@90Amps. It occurred to me that I've never
> > understood why so many tubes with indirectly heated cathodes have such
> > low-voltage - high current filaments. Since the only thing the filament
is
> > used for is to heat the cathode, then why not design it to run at, e.g.,
> > 115V@4A? That sure would be a lot easier to implement. I'm sure there's
a
> > reason, howevrr, and would appreciate somebody informing of it!
> >
> > 73,
> >
> > Jim W8ZR
> >
> > _______________________________________________
> > Amps mailing list
> > Amps@contesting.com
> > http://lists.contesting.com/mailman/listinfo/amps
> >
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