> >Maybe Bill could explain to us how there is going to be more voltage
> >appearing across the plate choke than there is across the tank circuit
> which
> >is in parallel with the choke. :>)
>
> REPLY:
>
> There isn't more. Where did I say there was? They are the same, but
> the tank components are far more likely to withstand the excess than
> the choke is.
>
> 73, Bill W6WRT
My point is that you are worried about parallel resonance in the plate choke
and damage to it because of that. I am pointing out that the voltage across
the choke will be held in check by the tank circuit no matter if the choke
is parallel resonant or not.
Your example of disconnecting the antenna and driving the amp full power is
an extreme one. Many things would probably arc in that instance including
the plate tune capacitor and probably the bandswitch. The arcing of the
plate tune capacitor in some amps protects the bandswitch from arcing in
those situations along with other components. The plate choke being parallel
resonant or not will usually not be a deciding factor.
73
Gary K4FMX
## well, I have yet to see a plate choke explode, with drive applied, and
NO ant connected. I can't fathom why a plate choke, that's parallel resonant
on a ham band is going to be a problem [ vs series resonant] . It seems to
me,
that if the plate choke is parallel resonant on a ham band, that the plate choke
would present a HIGH Z . IMO, a high Z is a desirable characteristic of any
plate choke. Even Orr sez words to that effect.
## Unless the value of the choke is huge, it will parallel resonate with the
tune cap anyway. A 200 uh plate choke will... 'eat' 40 pf out of any tune
cap
and a 100 uh plate choke will eat a lot more. [ 160m] When we tried the
2x piece plate choke set up [ 135 uh for the big choke... and 45 uh for the
small
plate choke, mounted at right angles] both chokes used on 160m only. Small
choke used on 80-10m. At one point, the 135 uh choke was out of the circuit
[ on bench].. leaving just the 45 uh choke. The amp was fired up on 160m..
with JUST the 45 uh choke, and it ran all night long, just fine. The bypass
caps
for the small choke [ 3 x 500 pf doorknobs] didn't blink, or heat up, or
anything else.
The vac tune cap used had plenty of extra C, as a lot more tune C required,
when
only 45 uh is used on 160m.
## I contend in this case the 45 uh plate choke and the tune cap will parallel
resonate anyway, leaving the top of the plate choke at a high Z point. We
couldn't
cook the magnet wire on the plate choke, nor cook the bypass caps.
## Don't have big enough bifilar for 160 conversion project? No big deal.
Just use
more C2 cap on the tuned input circuit [cap closest to the cathode]. You will
have to use
more C2.. to obtain a flat input swr. You have just parallel resonated the
bifilar on 160m.
## Tube amps have no problems with a dead short on their output [infinite
swr].
Tank voltages will all drop to zero... no big deal. BTW, if the C1 tune
cap is way
off resonance, tank voltages will also drop way down.
Tube amps will develop sky high tank Voltages with a wide open on their output,
[also infinite swr] . The simple fix? Install an adjustable spark gap,
directly in
parallel with the C2 load cap. Adjust the gap, so that the firing point is
the equivalent
of a say .. 4:1 swr. The idea here is to adjust the spark gap, in such a
fashion,
so that it fires well above any external swr protection you might have.[ say
2.5:1].. BUT
BELOW the max V rating of the C2 load cap.
## Then in normal operation, no problems. Now if swr is too high, [ > 2.5:1],
external
high swr protection kicks amp off line, and also apply -10 vdc to xcvr ALC
buss, shutting
the xcvr down. [ then latch the entire mess]. If a wide open occurs [
infinite swr]
spark gap fires off, tank voltages drop to zero, everything protected.
later....... Jim VE7RF
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