[Amps] Parasitic Suppressor 4-1000A

[R at contesting.com]

On Mar 10, 2006, at 2:12 AM, Steve Thompson wrote:

>
>
> Saandy wrote:
>> ....yeah! Now, you have arching, like you wanted (HI).
>>
>> It probably needs more inductance in the suppressor. According to 
>> W6SAI
>> (blessed his memory), the 4-1000 needs  4 turns, 16 gage, ½ inch 
>> diameter
>> suppressor coil with 2 47 ohms 2Watt resistors in parallel.
> I'm intruiged by this, and I'm wondering if I'm misunderstanding 
> something.
>
> As I understand it, the tube doesn't oscillate in isolation - it's a
> combination of several factors, including the impedance presented to 
> the
> plate. At some frequency there's an impedance in the wiring from the
> plate to the tune-C and then to ground that allows oscillation. Knowing
> the frequency and impedance allows you to design a suppressor to 
> prevent
> oscillation, but the frequency and impedance can vary a lot with wire
> lengths, component choices and layout - in which case, how can there be
> a specific design of suppressor for a particular tube?

The optimal suppressor design is that which almost melts down the 
suppressor resistor at 29MHz,  Adding L increases dissipation at 29MHz, 
decreases the parallel-equivalent resistance seen by the anode at its 
VHF self resonance, and that decreases VHF gain at that frequency -- 
hopefully making the amplifier less squirrelly.

> Out of interest, I'd love to know the frequency of oscillation and the
> impedance at the plate at that frequency - anyone out there got an
> oscillating amp and the equipment to measure impedance?

A dipmeter will indicate the anode's self resonance, but measuring the 
parallel-equivalent R presented to the anode by the VHF parasitic 
suppressor takes a Z-analyzer.
>
> Steve
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Rich Measures, 805.386.3734, AG6K, www.somis.org