[Amps] Parasitics & Filament Sag

[Colin Lamb]

>The most stable tubes are those with the highest operating 
>frequency limits.
>
>73 Tom 
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Old time home brewers will remember the 6AG7.  I think the max frequency for full ratings is something like 10 MHz (whoops it was Mc).  They are one of the worst offenders I have ever met at parasitic oscillations.  Used in the Central Electronics rigs, they needed resistors across the tank coils.  Even then, they would still take off.

As a kid, and not knowing what I was doing, I tried parasitic suppressors in the cathode, screen grid, control grid and plate - sometimes all at the same time.  By the time I got one stabilized, I was down to 1 watt output.   If you could get one of those tamed, the 811 and 812s were easy.

Colin  K7FM