?.i?ll try to explain.
First of all, it?s perfectly true that each and every amplifier has its own
peculiarities and its own particular solution.
What I tried to stress is the fact that this tube needs those values as a
starting point. At least I can vouch that those values ate found in mine and
working well. The one problem with the grounded grid is that there is no
phase reversal in the stage. That means that any small unwanted coupling can
wreak havoc.
In your case I tend to estimate that the plate has a parasitic resonance
around 21MHz and that?s the problem.
BTW, the suppressors are usually quite wideband devices, so that except for
the fact that more inductance is probably needed, the damping produced by
the 23.5 ohms resistors should be enough. Actually the inductance value of
the suppressor dictates the coupling of the resistors to the plate circuit,
thus the loading on the plate line. in any case , this is just a starting
point. It is simply one corroborated by theory and actual fact.
Saandy 4Z5KS
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of R@contesting.com;Measures
Sent: 2006. március 10. 13:31
To: Steve Thompson
Cc: amps@contesting.com
Subject: Re: [Amps] Parasitic Suppressor 4-1000A
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
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