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Old 21st May 2018, 10:07 am   #1
Barraman
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Default Testing 3-500Z triodes

I have a pair of Eimac 3-500Z valves, boxed and look new, but is there any way

of testing them, short of installing them in an amplifier?

Richard
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Old 21st May 2018, 10:51 am   #2
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Default Re: Testing 3-500Z triodes

Not really, I'm afraid.

You need the full power supply capability to perform any meaningful test. Any decent amp ought to have sufficiently comprehensive metering to show how well the valves are doing Once you are applying these sorts of voltages you need serious enclosure to be safe, so you might as well have the full amplifier.

3-500Zs have a bit of a reputation for leaks and going gassy.

David
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Old 21st May 2018, 10:58 am   #3
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Default Re: Testing 3-500Z triodes

I knocked together a test rig for some 304TLs once https://www.vintage-radio.net/forum/...ad.php?t=40860. I could measure emission and gm at 1200V but I really didn't want to go much higher than that in HT voltage given that there would have been hundreds of milliamps behind it with substantial energy stored in the psu caps. Unfortunately that rig was disassembled quite a while ago.

Cheers,

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Old 21st May 2018, 5:26 pm   #4
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Default Re: Testing 3-500Z triodes

Thank you.

I have a TL922 which is working well, and which I only use about once a week.

The 2 valves are ''spares'', which I've had for some years, so I think it would

be best to cross the replacement bridge when, and if, I ever need to.

Yes?

Richard
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Old 21st May 2018, 5:50 pm   #5
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Default Re: Testing 3-500Z triodes

Yes indeed!
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Old 22nd May 2018, 6:14 pm   #6
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Default Re: Testing 3-500Z triodes

Apart from death-by-internal-flashover the only way you can really kill these sorts of valves [apart from hitting them with a shovel] is by wearing-out or otherwise damaging the emissive cathode-current.

Any 'valve tester' therefore needs to be able to source/sink an Amp or so of HT! Not something easy to do in a consumer-grade valve-tester.

When I used to do this stuff commercially I would regularly send a batch (that had done 1000/10,000 hours service in an amplifier) back to Eimac and ask them to run tests - then present the test-results to the client to show that "after 10,000 hours the valves are still within original production-specification, so we really don't need to be changing them...".

This was great for me, since I could then sell the returned "Fully-Tested-by-Eimac" valves [along with the test certificates] to various radio-amateurs for their HF/VHF amplifiers.
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Old 23rd May 2018, 9:33 am   #7
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Default Re: Testing 3-500Z triodes

Quote:
Originally Posted by Barraman View Post
Thank you.

The 2 valves are ''spares'', which I've had for some years, so I think it would

be best to cross the replacement bridge when, and if, I ever need to.
I'd be inclined to give the spares a good cooking into a dummy load from time to time anyway - the danger is that they might flash over if you apply all the HT immediately: slowly winding up a variac on the HT transformer, being ready to back off quickly at the first sign of purple might avoid some expensive drama.

Colin.
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Old 30th May 2018, 4:58 pm   #8
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Default Re: Testing 3-500Z triodes

Hi,
Just to follow on from Colin's remarks - power valves may not take too kindly to long periods of no use. The cathode emission can suffer as can the gas seals. Normally they will recover quite quickly when put into service, though as Colin says there can be fireworks if the full heater voltage and full HT voltage are applied without warning.
In a professional environment, it would be normal to rotate the spares with the working set - say once every 6/12 months - so that none have to experience an extended period of no power. There is the added benefit of knowing that the spares will actually work if called upon to do so in a fault repair situation.
As you say that your spares have already been out of service for some years, then installing them in the linear and using a variac to bring the power up slowly is probably as good a test method as you're going to get. Once the power is at 100%, leave the rig on standby for 12 hours or so to make sure the cathodes are really properly heated, then apply CW drive to whatever the rated continuous power output is and leave the rig generating this output power into a dummy load for another 12 hours or so, to ensure them to reach their maximum operating temperature, which should take care of any gassiness.
I realise you were looking for a test method which didn't involve installing them in the linear, but as others have said, at these power levels you need all of the protection afforded by the equipment which was designed for operation of them.
cheers
Peter G8BBZ
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Old 30th May 2018, 5:25 pm   #9
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Default Re: Testing 3-500Z triodes

Quote:
Originally Posted by G6Tanuki View Post
Apart from death-by-internal-flashover the only way you can really kill these sorts of valves [apart from hitting them with a shovel] is by wearing-out or otherwise damaging the emissive cathode-current.
Ahah, but I think it's worth knowing which glass envelope power valves can be run orange-plated or red plated!

I destroyed a wonderful Svetlana 572B - when I didn't know better, around 15 years ago. It was in an RF power application - one of my early Tesla coils.

It survived a flashover, but I accidentally red-plated it and then then envelope started to kink and collapse. I think I was just too dumfounded to kick the switch as mayhem unfolded in front of me.

It was probably all over in a minute.

I'm much, much, wiser and vastly more experienced now and always have belt and braces overcurrent protection circuitry.

I have also studied, practiced and learned experientially about the efficiencies associated with different classes of amplifier and so can build in appropriate margin for anode dissipation.
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Old 30th May 2018, 5:37 pm   #10
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Default Re: Testing 3-500Z triodes

Quote:
Originally Posted by G8BBZ View Post
In a professional environment, it would be normal to rotate the spares with the working set - say once every 6/12 months - so that none have to experience an extended period of no power.
Not in my world: I designed valve-based 'stuff' such that it would run continually with no need for any maintenance or "tube-swapping". Some of my "stuff" regularly clocked 40,000 hours of service with zero downtime.
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Old 31st May 2018, 9:42 am   #11
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Default Re: Testing 3-500Z triodes

Not sure about the TL922 but if doing the variac thing, be sure to connect directly to the HT transformer bypassing any soft-start circuitry. Obviously run the heater transformer as normal. Two soft-starts in series can lead to other dramas in the PSU!
Also have a handful of HT fuses handy. If it does flash over before reaching the rated HT, run it for a few hours just below the flash-over voltage and it might recover, though you might have to repeat several times, and it might never recover quite enough. Given the price of these bottles, it's always worth a shot..
Cheers,

Colin.
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Old 1st Jun 2018, 7:23 am   #12
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Default Re: Testing 3-500Z triodes

Thank you for all the advice.

I am still rather nervous of this amplifier, especially when the meter is showing 3.2 kV, and wouldn't dream of running it with the lid off, especially with my level of experience!

So I'll leave the 2 valves in their boxes for now!

It may be possible in the future to have them professionally tested.

Best wishes,

Richard
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Old 1st Jun 2018, 8:33 am   #13
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Default Re: Testing 3-500Z triodes

Quote:
It may be possible in the future to have them professionally tested.
Back in the day, that meant sending them back to Eimac.

Nowadays?

David
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Old 2nd Jun 2018, 7:56 pm   #14
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Default Re: Testing 3-500Z triodes

You shouldn't need to run it with the really nippy voltage parts exposed (the interlocks shouldn't let you) - and your lack of experience will make you careful!

Be sure to let the HT discharge before opening the lid and use an earthed chicken-stick to ground the anode supply before putting your hands in there to change the bottles.

Colin mm1aps
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Old 2nd Jun 2018, 8:23 pm   #15
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Default Re: Testing 3-500Z triodes

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Originally Posted by Radio Wrangler View Post
Back in the day, that meant sending them back to Eimac.

Nowadays?

David
Eimac are still there - though now part of "Communications & Power Industries" (CPI).

And they still offer a "service report" analysis of used valves.

And they still have their reference documents online: their "Care and feeding of Power Grid Tubes" in hardcopy was one of my go-to documents when building juicy amplifiers.

See http://www.cpii.com/library.cfm/9
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Old 2nd Jun 2018, 8:55 pm   #16
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Default Re: Testing 3-500Z triodes

Interesting.

Wondered what had happened to them in all the shenanigans at the end of Varian as one group.

Still have a copy of 'Care and feeding..." on my bookshelf. The best guide to beef thermionics up to UHF yet written.

David
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