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

R.Measures 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 
suppressor resistors?

>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 

> 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.   

**  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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