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

[Jim]

What is the date code on the Eimac 3-500 you are trying to save?


Thanks
73
Jim W7RY

On 7/28/2020 8:22 PM, Robert wrote:
> 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
> does.
>
> 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
> time.
>
> 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.
>
> De-energize
>
>   
>
> Tube tests good
>
>   
>
> into the AL82 it goes
>
> Energize
>
> no reverse grid current, 3400Vdc showing on HV meter, white hot filaments
> glowing
>
> 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
>
>   
>
> Robert
>
> 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.
>
>   
>
> Hummm
>
> 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
>
>   
>
> W5AJ
>
>   
>
> Robert
>
> Midland, Texas
>
>   
>
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