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Yes, it may be a 13 volt filament is easier to run at slightly lower voltage because perhaps the voltage being twice and the current being half of a 6 volt filament would make for a more constant current with line variations. Probably the way tungsten filaments work?

The 8393 was used in early 453's and later 503's, I know. Mebbe other models too.

Reducing heater voltage causes the cathode to produce a weaker electron cloud which reduces grid contact current. A positive voltage on the cathode sets plate current and normally blocks contact current with low DC resistance grid circuits but very high DC resistance grid circuits still suffer from grid current. I know for the 503 input 6DJ8's grid current was one of the selection criteria, too much produces diff amp balance instability.

308's were hot stuff when they came out but a little tricky to employ. Glad better types came out. Once ran into a PhD living in the past that thought a 308 was better than an OP27.

Arden

> Phosphorphile posted
> > Ayup. I always wondered why Tek used the 8393, the 13 > volt filament version of the 7586. Anyone?
>
> I dont know what instrument Tek used the 8393 in, so cant comment on an actual TEK unit, but running a13v volt filament on 12 v (provided the emission is high enough so the characteristics still suit the application ) will have two effects
> 1. All other things considered, it can lengthen the working life.
> 2. It will raise the input impedance much higher than when run at the specified filament watts.
>
> I remember running 6.3 v fil s at 4,0v to get hi Z inputs for electrometer applications in the distant past.
> Cant remember the tube type now, maybe an EF37.
> Underrunning filaments in this way was a common thing to do in high input Z applications, but you had to select the right tube that could handle it, without poisoning etc.
> Special electrometer tubes were made with deliberately underrun filament voltages and some were specially selected types from a production run of normally 6.3 V filament units tyen rebranded to a different type number.
> The anode current was sigificantly reduced when this was done for the same grid base.
>
> A somewhat parallel thing was done with the LM308A IC, which achieved a 40Mohm Dc Zin by running in "current starved " conditions
>
> But maybe TEK had a diferent reason?
> Jojn Byers
>