[AMPS] New member and Drake L7 questions

[measures]

>
>Hello from Spain!
>
>As title says, I am a new (subscribed for 10 minutes ;-) ) List member so I
>think I should introduce myself. I am Jose Gavila, from Valencia (Spain). I
>hold two callsigns, EB5AGV and EC5AAU. I am a 32 years old Telecomm
>Engineer and enjoy working with radio gear, mostly tubed (look at my WEB
>page for samples if you wish)
>
>Well, just last week I had the chance to buy a nice Drake L7, along its
>power supply (L7PS) and manual. As price was really low (around $425), 

Excellent price. 

>I could not check it before buying and it had sat for a long time unused, I
>suspected of some trouble with it. And yes, I found a problem. Famous IRC
>0.82 Ohm resistor was vaporized.
>
Finding the 0.82-ohm resistor vaporized, I would measure the resistance 
of the suppressor resistors. (one end must be temporarily unsoldered).  
If you find that one or both have increased in resistance more than c. 
50%, an intermittent parasitic oscillation may have occurred while the 
0.82-ohm resistor was vaporizing. 

>I have looked for old List messages about L7 and have found lots of info
>about changing it to a 10 Ohm / 10W glass enameled resistor. 

good idea.   However, a second such series resistor is sometimes needed 
if one resistor is damaged by the ''screwdriver test''.  .  

>In my resistor
>junkbox I have only square 'sand' 5W resistors... Is there any trouble
>using them?. I could put two in series, so have 20 Ohm / 10W... is this a
>good arrangement?

Square resistors tend to blow apart and make schrapnel during a glitch.  
Round, glass coated resistors are better able to withstand glitch 
service.  
>
>As I have told, the amp has been resting unused for perhaps 10-15 years. I
>have not powered it yet, due to the failure I found. But it seems tubes
>should be 'cooked' for some time... could you suggest a practical procedure
>with the L7?
>
Little or no gettering takes place with metal anodes unless the tube is 
very hot.  With graphite anodes, the anodes apparently outgas when heated 
to yellow heat.  If you have bad glass/metal seals, no amount of cooking 
will fix the problem.  I prefer to high-pot tubes before testing them in 
an amplifier.  // I currently use an Amperex tube that was manufactured 
in 1967.  The leakage current is c. 10uA.  

>Another thing I would like is to know which mods are 'standard' to protect
>meters and so on.
>
Adding 3A diodes across meter movements is probably a good idea.  

cheers, Jose.  

-  Rich..., 805.386.3734, www.vcnet.com/measures.  
end


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