On Mon, 20 Apr 1998 16:22:27 -0600 "Richard W. Ehrhorn" <w4eto@rmii.com>
writes:
>
>
>-----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.
>
>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.
>
>While I agree that the whole tank circuit in a tube IS VHF resonant, I
>am
>not sure of how a transient current condition in the tube can produce
>a
>VHF ringing. I would like to know if anyone has seen this on a piece
>of
>test equipment. If true, it does add a lot of credence to Rich's
>theories.
Fast keying the amp control line will accomplish this Dick. Switching to
tetrodes for a moment, this was a way to debug the NCL-2000 in R&D. With
the HF tank in place and a 50 Ohm load we would see arcing at the antenna
relay contacts from the roughly 200MHz energy. Changing the parasitic
chokes did help but only a bit. The final cure was selecting the value
and lead lengths of the screen bypass caps ( 3 screen pins per socket),
adding 2 Ohms in series with the grid and reorienting the plate choke.
Seemingly a lot of work but neccesary when having 20+ dB stage gain and
tubes rated to 500MHz.
73 Carl KM1H
>
>ALL tubes contain parasitics.
>[Dick Ehrhorn:] Jon, I think it'd be much more accurate to say that
>(virtually?) all circuits contain parasitic reactances and therefore
>parasitic resonances, which lead to the potential for parasitic
>oscillations when used with many or most types of tubes.
>
> There is a BIG difference between a
>parasitic and a parasitic oscillation. It's easy to get them
>confused.
>A parasite in a tube is the lead inductance and stray capacitances,
>etc.
>that exist in the real world. Even a leaded capacitor has parasitics
>(lead inductance).
>
>73,
>
>Jon
>KE9NA
>
>
>--------------------------------------------------------------------------
>Jon Ogden
>
>jono@webspun.com
>www.qsl.net/ke9na
>
>"A life lived in fear is a life half lived."
>
>
>
>
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>
>
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