[AMPS] parasitics
km1h @ juno.com
km1h@juno.com
Mon, 20 Apr 1998 12:29:12 EDT
On Mon, 20 Apr 1998 12:06:40 +0100 Peter Chadwick
<Peter_Chadwick@mitel.com> writes:
>The worst case of parasitics I ever encountered was in a 300 MHz
>tunnel
>diode oscillator; we got non - harmonically related outputs on 5
>frequencies
>from 10Hz to over 3 GHz, and all the intermod products.
>
>Refering to Terman, he says:-
>
>'In fact, parasitic oscillations can be expected as a matter of course
>in
>any new design of amplifier, either radio- or audio-frequency, or of
>power
>oscillator, involving large tubes'.
>
>I've had VHF parasitics in an 80m VFO using a 5763.
>
> Back to Terman on the subject of parasitic suppressors:' At the same
>time,
>they are not in the tank circuits for normal operation and so have
>little
>effect on the desired oscillations'.
>
>A further quote from Terman:' They are to be avoided because they
>absorb
>power that would otherwise go to generating useful output and because
>they
>also often produce excessive voltage stresses in portions of the
>circuit'.
>
>That last quote should bring a smile to Rich - it seems to be what
>he's
>saying!!
I have no idea what Rich is saying most of the time but lets take this
back to the old days basic tests for parasitics. The way I was taught is
that you key the amp with no drive and vary the caps all around and look
for a kick in the plate or grid current. Do this on all bands and into a
dummy load. If all is quiet then hook up the antennas and do it all over
again.
If you are really asking for it then try it with no load.....this is the
way I final test any amp.
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.
Arcs during tuning or operation are purely a result of high voltages in
the tank circuit, poor tank circuit design/components or another
oscillation ( NOT a parasitic in the tube) caused by stray LC in the
tank ONLY. These arcs cause switch and capacitor damage but rarely, if
ever will cause tube or grid choke damage.
Unfortunately there are a few out on the reflector that tend to confuse
the two modes and each other.
Tom Rauch also was 100% correct when he stressed that any amp should be
checked for neutralization. There are amps on the market that will not
even pass that test on 15M much less 10M.
m to remember that the 30Kw tx done 30 years ago at Marconi's
>didn't
>need parasitic suppressors, but it used vacuum variables, which tend
>to be
>physically smaller and have less inherent inductance.
>
>And if the amp needs opening for servicing, it's worth going round
>with the
>vacuum cleaner and a dust brush, (as carl and Dick suggest) and
>checking all
>the resistors while you're at it. I know 'if it ain't bust....', but
>sometimes preventative maintenance saves later problems.
I remember one original L-4 with the 3-400Z's that belonged to a 3 pack a
day smoker. The inside of the chimneys and the tube glass were so coated
with nicotine that the tubes failed. I dont even smoke but I vacuum out
all of my external anode tube amps every 3 months and also clean fan
blades and Windex glass tubes/chimneys every 6 months. I have not had a
tube failure or a parasitic ( in an operational amp) in the 16 years I
have been back active.
If you've got
>it
>open, it takes less time to measure the resistors now than to have to
>open
>the box to do it later. But I'm an idle so-and-so, so anything that
>saves
>work.........
Since the earliest days of this reflector I have stressed checking and/or
replacing suppressor R so I'm at a loss as to which one amongst the
un-informed credited me with the opposite statement. Really doesnt
matter but just wanted to make that clear.
73 Carl KM1H
>
>
>73
>
>Peter G3RZP
>
>
>73
>
>Peter G3RZP
>
>--
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>
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