>
>
>-----Original Message-----
>From: Jon Ogden [SMTP:jono@webspun.com]
>Sent: Monday, April 20, 1998 12:16 PM
>To: km1h @ juno.com; amps@contesting.com
>Subject: Re: [AMPS] parasitics
>
>>If it passes those tests AND as long as the suppressors are OK, I can in
>>NO WAY accept that a parasitic will occur during use. Arcs on standby are
>>gas or barnacles in the tubes....note that those arcs rarely if ever are
>>associated with capacitor or switch failure but can blow grid chokes and
>>tubes.
>
(Richard W. Ehrhorn)
>I've read Rich's information on his webpage and I think I can repeat his
>theory:
>
>He says that whenever you have a change in anode current in a tube that a
>small, damped VHF signal will result. He says you can observe this with
>an O-scope or a spectrum analyzer. If there is enough Q in the VHF tank
>circuit in the PA, then an osicallatory condition could occur.
>
agreed
>While I agree that the whole tank circuit in a tube IS VHF resonant,
I do not. The HF tank circuit is a lowpass filter. The VHF resonance
at hand involves the anode-grid C, the total inductance of the conductors
between the anode connection and the Tune C, and the capacitance of Tune
C. Typical resonance frequencies are from roughly 65MHz for largish
tubes like the 8171, to roughly 170MHz for small ceramic tubes like the
3cx800A7. . The VHF resonance picture is further complicated by VHF
self-resonances within the Tune C. . . Murphy was seemingly right.
Things are usually more complicated than they look.
> I am
>not sure of how a transient current condition in the tube can produce a
>VHF ringing.
The same way that striking a bell with a hammer produces a damped wave
oscillation. This is the principal behind a spark transmitter.
cheers
Rich...
R. L. Measures, 805-386-3734, AG6K
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