A couple of thoughts, Drax. First, re the Z-50, as Carl notes, it has a
self-resonant frequency of about 70 MHz, with a note on the Ohmite data
sheet saying that stray capacitance and external circuit components will
likely lower the SRF to about 50MHz. In and of itself, this is not a bad
thing. At the SRF, the choke looks like a parallel LRC circuit (with a one
ohm R), which is desirable in a plate choke because at that frequency it has
a very high impedance. It is a series resonance that would be bad, because
that would effectively provide no "choking" action at all.
Second, if the Z-50 is "exploding," that means that the current through it
is so much in excess of its rated 1A that it is vaporizing the wire and
blasting apart the choke's coating. If it's doing this with only 5W drive,
and IF the 8877 is not experiencing a parasitic oscillation (as indicated by
a high plate current), then it highly unlikely that enough DC or RF current
through the choke would be available to cause this symptom. In fact, if the
SRF is near 50MHx, you should have very little RF current through the choke.
That suggests to me that something is breaking down in the amplifier when
you apply RF drive, most likely an internal arc in the tube. In that case
the Z-50 is acting like a fuse. If you've got another 8877, try substituting
that.
Lastly, the Z-50 is probably underrated for an 8877 amplifier. I'm somewhat
dubious of Ohmite's 1A rating for the choke. Small AWG wire, closely wound
in a 1/4 in diameter, won't be able to dissipate much heat on a sustained
basis. I suggest following the advice of some other commenters and
space-winding a heftier choke, say 10uH, on a 1/2" (or larger) diameter
ceramic or teflon rod.
73,
Jim W8ZR
> -----Original Message-----
> From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Drax Felton
> Sent: Friday, April 12, 2013 6:27 AM
> To: amps@contesting.com
> Subject: [Amps] Plate choke explodes
>
> So I've tried two different plate choke designs on my 6 meter 8877 amp I'm
trying to build.
> Both times the plate chokes have violently exploded as soon as I apply 5
watt or less RF
> input to the amp.
>
> Any ideas what I did wrong?
>
> Sent from my iPhone
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