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Old 29th August 2003, 02:00 AM   #1
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Default tube matching fixture

Has anyone built a low budget tube matching fixture? I would like to find a link or circuit for a low dollar tube matching fixture so I can match the 10) KT90 and (10) EL34 tubes I have. I don't feel like dropping hundreds of dollars for a good tube tester. I just need something cheap and effective.

Any ideas?


Joe
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Old 29th August 2003, 05:49 AM   #2
SHiFTY is offline SHiFTY  New Zealand
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Ideas? Find someone with a tube tester and be friendly
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Old 29th August 2003, 05:56 AM   #3
Colt45 is offline Colt45  Serbia
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a tube tester isnt going to match them anyways, unless it does current and gm.. my hickok just does gm.. and a lot of testers dont even do that, they just diode-strap the tube and test for emission.

edit: but yeah, a homemade current testing deal would be piece of cake to make..

take tube, place 1k resistor on the plate, 1k on the screen, ground the cathode (throw in a 10om here if you want)

apply B+ so that Va is roughly what it would be in the amp. set Vg to whatever it will be biased at in the amp.

measure current across the 1k resistors.. switch tubes, measure, on and on.. compare results


i guess you could cathode bias it instead if you want.. just throw in the proper size Rk, and ground g1
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Old 29th August 2003, 06:28 AM   #4
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Or, slap them in a fixed-bias SE amp and measure away

Tim
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Old 30th August 2003, 10:46 PM   #5
316a is offline 316a  England
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Default Re: tube matching fixture

Quote:
Originally posted by burnedfingers
Has anyone built a low budget tube matching fixture? I would like to find a link or circuit for a low dollar tube matching fixture so I can match the 10) KT90 and (10) EL34 tubes I have. I don't feel like dropping hundreds of dollars for a good tube tester. I just need something cheap and effective.

Any ideas?


Joe
Hello ,
With power tubes it would be far more advisable to burn the tubes in first before measuring . No point in matching tubes that will drift after just a few hours of use . A quick and easy burn-in rig (I built one for bulk testing Russian EL84 for a customer) would be to parallel 4 octal sockets , tie grid to cathode , cathode to 0V ,g2 to anode then run the anodes from full wave rectified but unsmoothed DC , preferably with a standby switch . You will need to work out the dissipation limits and base your HT voltage upon this , I would suppose a 120V secondary transformer should do the trick (guessing) with EL34 / KT90 . ***Be careful if considering building one of these not to kill all your tubes due to incorrect wiring*** As Tim suggests a simple SE type output stage can be used to match individually for idle current but this will not provide measurement for transconductance . After burn-in , finding someone with a decent tube tester would be the best idea . Luckily I have an AVO which can measure both gm and current A luxury as a (semi-professional) tube dealer I can't do without

316a
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Old 30th August 2003, 11:20 PM   #6
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Hi,

Quote:
With power tubes it would be far more advisable to burn the tubes in first before measuring . No point in matching tubes that will drift after just a few hours of use .
While I agree with the above, I would recommend extending the same policy to small signal tubes.

This is especially valid for current production Russian tubes ( I never bothered with anything Chinese) which benefit from a burn-in period.

After which serious matching can begin.
BTW, you can use a tester as a burn-in device.
Building your own burn-in rigs for non-commercial purposes isn't worth the outlay IMHO as it will only leave you with astronomical electricity bills.


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Old 30th August 2003, 11:42 PM   #7
316a is offline 316a  England
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Quote:
Originally posted by fdegrove
Hi,



While I agree with the above, I would recommend extending the same policy to small signal tubes.

This is especially valid for current production Russian tubes ( I never bothered with anything Chinese) which benefit from a burn-in period.

Cheers,
Funny you say that ! I'm about to build a similar bulk tester for testing 16 12AX7's at a time . These current production valves seems to die after a few days or last and last , well not as long as a Mullard ! Chinese tubes are like Chinese food : satisfying at first but wear off after half an hour then you need another one . I can't understand why these Chinese mesh globe pretend 300B thingies are so popular . I doubt the things would have a lifespan anywhere near a real WE300B which cost about double but last a whole lot longer

316a
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Old 1st September 2003, 03:51 AM   #8
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Default website for homemade tube matcher

Here is a website for a home made tube matcher:

http://home.comcast.net/~priceamp/

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Old 1st September 2003, 12:01 PM   #9
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Barry

Thanks for that nice article and schematic of the homemade tube matcher. This really hits the nail on the head. Also, thanks to the others who have made some very usefull comments.

Joe
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