>
>> With some socket, also the actual method to ground grid is prone to
>> have inductance, and inductive reactance yes increases with frequency.
>> With proper bypassing capacitors and short leads, like with mica
>> multistrate caps, one can aspect that loss (if any) and not reactance
>> increase with frequency.
>
>It is a popular myth that adding a series C will decrease reactance
>in HF amplifiers as frequency is increased and improve stability.
>
>The normal capacitance in the tube parallel resonating with the grid
>inductance is a few pF. Adding a few hundred pF of capacitance
>outside the tube barely changes the VHF anti-resonance, the
>capacitance would have to be only a few pF to move things at VHF
>significantly.
>
True. There is not much improvement in the grid resonance freq. with
200pF caps.
>What the capacitance like use in a SB-220 actually does is add
>series inductance at VHF up near 150 MHz and higher (where the
>tube tends to oscillate) because of the long thin leads and
>excessive value of C. This brings the tendency for oscillation down
>in frequency, where it is harder to suppress.
>
>If the capacitors were low inductance chips mounted directly
>between the socket and chassis, it would be another story. But
>adding any long thin leads to the grid is generally a bad idea,
>whether in series with a capacitor or not.
>
>The capacitors are just a poor idea that made it into many
>amplifiers because no one checked the effects thoroughly.
>73, Tom W8JI
The AL-82 has Rauchian directly grounded grids. Is it stable? See
Figure 17 on my Web site.
end
- Rich..., 805.386.3734, www.vcnet.com/measures.
end
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