[Amps] Drake L-7 failure...amp vs L7-PS power supply?

[Michael Tope]

Torrey,

Did you hear a loug band on both attempts, or just on
the first attempt. How about the visible arcing, did you
see it on both attempts?

How about meter readings?  When the close the key line
with no RF drive, are you still getting normal zero signal
anode  current (~140mA). How about grid current?

BTW, you are correct the 0.82 ohm resistor is supposed
to be a high voltage fuse. A number of amplifier gurus
recommend that this be replaced with a high voltage globar
type glitch resistor (10 to 30 ohms). The idea is to limit the
instantaneous anode current to a reasonable value in the
case that a high voltage fault occurs in the RF deck. The
0.82 ohm resistor "fuse" may not blow fast enough or it
may arc across. In either case, this will allow for a very high
instantaneous dump current when the capacitor bank dumps
it charge through the fault which in turn can damage the
tubes.

73 de Mike, W4EF.................................

----- Original Message -----
From: "Torrey L. Mitchell" <torreym at mchsi.com>
To: <amps at contesting.com>
Sent: Thursday, November 13, 2003 7:06 PM
Subject: [Amps] Drake L-7 failure...amp vs L7-PS power supply?


> Good evening all.
>
> As I was tuning up my Drake L-7 amp this evening there was a very loud
bang and a visible flash behind the plate voltage meter on the left.
Foolish or not I reset the power supply circuit breakers and tried again
with low RF drive.  Same story again.  I took the top off the L-7 and could
see no evidence of arcing or burned components.  Final tubes and attached
resistors look normal.  In short, everything looks normal topside.  I have
not removed the bottom panels to look.
>
> The remotely located L7-PS power supply reveals charring and a
burn-through of the R12 resistor (2 watt 0.82 ohm IRC type BWH).  The
charring put a big black mark on the adjacent large tubular capacitor, but
the capacitor does not look damaged and all the large resistors across these
capacitors appear intact.
>
> The R12 resistor appears to function as a protective fuse in the high
voltage output line.   If so, where could such a impressive short circuit
have occurred in the linear amplifier without any evidence of damage
topside?   There is a good chance that I was over driving the L-7 with the
TR-7 which easily puts out 160w instead of the nominal 75w.   If so, what
could have caused the large bang & flash in the linear (the R12 resistor is
located in the remotely located power supply)?   Does it make sense to
replace the R12 resistor, make sure the idling current is nominal, then try
tuning up with 25-30w drive.  How can I determine whether one or both 3-500z
finals are shorted or shot?
>
> I may have to send both units out for repair, but I certainly would like
to avoid it...given all the weight, 3 boxes, etc.
>
> Specific recommendations for L-7 repair?
>
> Any thoughts or help will be most appreciated.
>
> Thanks,
> Torrey N9AUJ
> cell 515-778-2354
>
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