[Amps] 3-500Z short / fix? Update

Robert woodr90 at gmail.com
Tue Jul 28 21:22:33 EDT 2020

The thought of the battery charger:  the grid wire is smaller than filament
- the grid wire causing problem will melt/fry/disconnect before the filament

Your results may vary.  This is posting of one day of test results, you
might not have same results (but could).  


July 28, 2020 Midland, TX: 

took the repaired 3-500 and inserted into AL82 for test


on energizing amp, as the filaments heated up, short returned..


De-energized, repeated high current injection with battery charger and fixed
again, this time no flash was seen inside tube.


Tube back into the AL82 ( I didn't write it down but think this time the
safety switch was open & no HV)

on energizing amp, as the filaments heated up, short returned..

Typically indicated by reverse reading on Grid current meter.


De-energized and pulled tube, but this time it tested OK with no shorts.

placed back into amp

on energizing amp, as the filaments heated up, short returned..


Pulled 3-500 tube and decided to make use of RF deck setup with 4-400 tubes
in other room

No HV issues as the HV power supply would not be on or even wired in at this

removed one 4-400 tube and inserted the 3-500

connected battery charger battery cables between grid / filament (after
reviewing schematic to insure nothing would get fried)

but left battery charger line side unplugged.


Powered up filaments & Blower

applied line voltage to battery charger

slight quick additional glow seen in tube then gone, filaments still going.



Tube tests good


into the AL82 it goes


no reverse grid current, 3400Vdc showing on HV meter, white hot filaments

wait few minutes while getting rig setup on 20 and lining up knobs on AL82

4 watt drive - all good, short does not return.

50 watts - all good

60 watts - all good - watch tubes, small red glow of plate seems equal
between tubes 

70 watts - all good, similar - the plate glow is equal between tubes - this
setup has MFJ wattmeter showing 1350W out


Appears a Eimac 3-500Z can be saved.  Your mileage (results) may vary.

If initial repair doesn't work, may have to put high current through
filament to grid short while filament is on.

I plan to use the AL82, of which one of the two tubes is the repaired Eimac
3-500 and give it a good RTTY work out, see if tube holds up OK.


CYA disclaimer:   HV can kill, if you are not sure of that, don't remove
cover on amp.. 


73 W5AJ 



Midland, Texas


From: Robert [mailto:woodr90 at gmail.com] 
Sent: Monday, July 27, 2020 7:47 PM
To: 'amps at contesting.com' <amps at contesting.com>
Subject: 3-500Z short / fix?



while repairing Zener diode - pulled two 3-500 Eimac tubes 

one had short (all five pins read zero ohms to any other of the five pins)


took shorted tube to shop for a photo.   sell tube as sample/light bulb.



there is my old, really old car battery charger

tube is bad so why not

place battery charger in 6Vdc position

there are no protective shutdowns in this charger

(multi-tap transformer & one diode) 

one clamp on two filament pins

other clamp on grid pin

let's see if grid will "melt" at area of the short


small glow/flash inside - then battery charger current drops to zero

go to 12vdc, no current 


Measure ohms at tube pins:

filament good

Grids all connected

No short between filament/grid


shake tube - still OK

shake tube while measuring ohms, still OK


so, do put 3kV on the plate and see what happens??


anyone tried this before?

AL-82 test bed





Midland, Texas


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