[Amps] Filament Voltage Question

[peter chadwick]

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 at miamioh.edu>
 To: "'Joe Subich, W4TV'" <lists at subich.com>, amps at 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 at contesting.com] On Behalf Of Joe Subich,
 W4TV
 > Sent: Tuesday, April 22, 2014 9:34 AM
 > To: amps at 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 at 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 at 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
 > >
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