[AMPS] Clipperton L amplifier

[Ian White, G3SEK]

Barry Kirkwood wrote:
>
>The last tetrode amp of any significance I built was almost half a century
>ago.  Used an 813 in class C.
>
>The texts at the time made much of the role of the screen in the possible
>production of VHF parasitics. Various fixes based mainly on crude empiricism
>were offered. Amongst those suggested was running the screen feed in
>shielded wire. Presumably one effect of this would be better VHF bypassing,
>as the bypass capacitors used at the time were a bit suspect at VHF. Hence I
>am not surprised to hear that a fix of a presumably VHF parasite was
>achieved as described. (e.g. See pps 22.9-10, Orr, W.I. , Radio Handbook
>19th edn 1975)

There can be a problem of parallel resonance between the screen bypass
capacitor and the self-inductance of a higher-value capacitor somewhere
else in the screen supply circuit.

For example, the self-inductance of typical 0.1uF 1kV tubular plastic
capacitor can resonate with the screen bypass in an Eimac SK-610 socket
at about 15MHz. At that frequency, the screen is effectively un-
bypassed, so HF oscillation is a real possibility - especially in an HF
amp, of course.

The cure is a series "stopper" resistor of about 100 ohms, low
inductance, between the screen bypass and the larger capacitor. This
kills the parallel resonance completely, so the screen bypass works
correctly. To avoid introducing DC resistance (which increases the
source impedance of the screen supply, and may increase IMD) the
resistor can be paralleled by an RF choke, or you can wind about 30-50
turns of wire on the resistor body. 



>----- Original Message -----
>From: Tom Rauch <w8ji@contesting.com>
>To: Peter Sundberg <sm2cew@telia.com>
>Cc: <amps@contesting.com>
>Sent: Saturday, 20 January, 2001 9:23 AM
>Subject: RE: [AMPS] Clipperton L amplifier
>
>
>>
>> > When rebuilding the screen supply for my amp I suddenly developed
>> > arcing BIG time in the tank circuit something that had never happened
>> > before. Screening the rectifiers and bypassing the cable feeding the
>> > screen stopped all arcing. No changes what so ever were done in the
>> > tank circuit to stop arcing.
>> >
>> > There is no doubt in my mind that parasitics caused the tank arcing.
>> >
>> > 73/Peter SM2CEW
>>
>> Hi Peter,
>>
>> I'm missing something.
>>
>> Are you actually telling us you had a parasitic in your screen
>> supply wiring and rectifiers? Please tell us why you think it was a
>> parasitic, and what frequency it was on.
>>
>>
>> 73, Tom W8JI
>> w8ji@contesting.com
>>
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>>
>
>
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>

73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                          'In Practice' columnist for RadCom (RSGB)
                           http://www.ifwtech.com/g3sek

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