>Hi all,
>
>Recently I blew up my L-7 by tuning up to max in the SSB position.
*** 3-500Zs can be driven into the distortion region (above 1.3A-peak
anode current) - but overdrive does not make things blow up - nor does it
harm the cathode like it does in heater-type tubes.
>I'm having difficulty finding two components which need replacing.
>
** Knee-jerk/automaton replacement of kaput amplifier parts without
doing some thinking may someday result in a replay. The typical
0.82-ohm, 2w MF-R can handle 7w intermittently. A pair of 3-500Zs do not
have enough emission to damage such a resistor, My guess is that there
was an anomaly which caused a HV arc to chassis-gnd which blew the
0.82-ohm resistor.
>The first is the green Drake-made capacitor C41 47 pF between the top and
>the middle of the HV plate choke.
** I can think of no reason for adding a capacitor to a HV-RFC. I
wonder what Drake's amplifier experts were thinking? -- or were they? If
they were trying to move a resonance out of a Ham-band, removing a few
turns works well.
>Mine arced through and doesn't seem
>repairable. Since this is a "home-brew" component made by Drake (to save
>$$ ?) I cannot rush out and buy one. I'd like to stick to original
>components if at all possible.
** Non incinerating HV-RFC's are not difficult to make if you have a
local electric motor repair shop, a plastics supplier, know which
materials tolerate RF, and have access to a dip-meter. Bring your own
spool and winder to the motor shop, unless you need 36-lbs of wire.
Otherwise, they sell it by the pound on your spool. Note -- it helps if
you explain what you are repairing and tell them that the high-temp
silicone-varnish Cu wire used in electric motors is highly resistant to
RF.
>
>The second component is the R12 0.82 ohm 2 watt power supply fuse
>resistor.
** This component is not much of a fuse since the arc that takes place
as the resistor explodes does almost nothing to limit peak fault current.
Asking a 2w resistor to absorb a 120 or so Joule glitch is clearly
looney-tunes. A sturdier resistor is needed. I would go with at least
one 10w glass-coated 10 - 15 ohm resistor in series with the HV+. [note
-- molded ceramic restors should be avoided because of the potential
schrapnel problem]
>Harbaugh Electronics is the usual reliable source for these.
>However, he just moved and won't be open for business again until
>mid-December.
>
>Anyone who can help me get my hands on these components? It would be
>immensely appreciated!
>
*** If you like surprises, use the same components.
-- There are no perfect amplifiers and there are no amplifier experts
who know everything.
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