[Amps] Drake L-7 failure...amp vs L7-PS power supply?
r at somis.org
Fri Nov 14 04:03:22 EST 2003
>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.
** What is the measured resistance of the anode-circuit parasitic
>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).
** In an L-7, R12 is supposed to be the glitch-R. However, at a minimum
it needs to be replaced with a 10 - 15 ohm 10w
glass-coated/vitreous-enamel unit, or some other resistor that can handle
>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.
** Sure, R12 fused the circuit, but the purpose of a glitch R is to
limit peak fault current, and a R can do this better if it remains intact
during the fault.
>If so, where could such a impressive short circuit
>have occurred in the linear amplifier without any evidence of damage
** Since there is c. 110-Joules of stored energy, which is not a lot,
and because aluminum boils off at 2,467°C, the damage is hard to spot
>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.
** At 3kV, a pair of 3-500Zs need 140W of drive. Also,
thoriated-tungsten tubes are naturally emission limited, so they can not
be easily damaged by a bit much drive.
> If so, what
>could have caused the large bang & flash in the linear
** Some kind of anomaly?
>(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.
** When an amplifier is tuned up with reduced drive, the carefully
calculated tank values suddenly become wrong. To reduce stress, tune up
with 50wpm dits or a tuning pulser, but do so with full-throttle PEP
>How can I determine whether one or both
>3-500z finals are shorted or shot?
** with a high-potentialbreakdown tester. 5000v to 8000v breakdown
potential from filament to grid is good news. Grid to anode (gas)
leakage should be under 10uA @ 8kV.
>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?
** Get out your DMM and soldering iron, and put on ur thinkin' cap.
gud luck, Torrey. / If you have a question, please telephone me. (
Finger-talking is like molassas in January.) If I'm not here, leave a
message and I will phone you back on my nickle a minute/
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