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Re: [Amps] Fw: Parts needed for Drake L-7 amp

To: "Torrey L. Mitchell" <torreym@mchsi.com>," AMPS" <amps@contesting.com>
Subject: Re: [Amps] Fw: Parts needed for Drake L-7 amp
From: R.Measures <r@somis.org>
Date: Thu, 20 Nov 2003 04:36:02 -0800
List-post: <mailto:amps@contesting.com>

>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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