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Old 25th December 2008, 09:28 PM   #1
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Default Converting tube tester to Mutual Conductance

I have a couple simple tube testers (knight KG-600 and EMC 205) and would like to know if there is a way to convert them to Mutual conductance testers.

Correct me if I am wrong, but it should not be too hard to add a HV regulator to the HV windings. Then one would add a triode and an oscilator to drive the triode (possibly with terminals for adding an external frequency generator.) The tube would need a negative bias and a way to compare input swing to output swing.

Any thoughts?
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Old 25th December 2008, 11:29 PM   #2
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I think it would be a lot more practical to convert some mutual funds into a already manufactured and calibrated mutual conductance tester.
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Old 26th December 2008, 01:57 AM   #3
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Anyone who actually knows what they're talking about care to chime in?

I don't have money for an insanely priced one from e-pray land.
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Old 26th December 2008, 03:53 AM   #4
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That's a bit rude. You asked for people's thoughts. I gave you mine. I did not recommend ebay but you lashed out venemously as though I had. Yes Ebay is pricey and full of get rich wannabees but fortunately not the only source for a quality vintage transconductance tube tester. Anyone who knows what they are talking about will verify this.

Rots of ruck to you sir. Anyone who wants their head torn off want to help this gentleman?
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Old 26th December 2008, 04:33 AM   #5
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sorry if I seemed a bit rude, but I don't like the " eh you don't have it? go buy it!" kind of mentality. After all, this IS a DIY forum.

There aren't any places around here unless I get lucky and find one in a thrift. The ones I have are special to me for other reasons.

With the amount of vintage testers dwindling, I figured it might be a good discussion on how to make one yourself for simple testing purposes. Maybe one designed to work specifically for certain tubes etc. You can usually find the cheap dynamic or cathode conductance type testers for relatively cheap, but to get a MC tester you have to pay upwards of $100 easily.

There are also very very few NEW testers on the market, so it might be nice to be able to build one of your own.
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Old 26th December 2008, 05:16 AM   #6
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OK, fair enough, but actually I don't consider $100 to buy a MC tube tester much money and my own time is certainly worth much more than that to purchase common items that are actually available pre made. You'll likely spend $100 in gas trying to acquire all the parts you will need. Where is the savings gonna come from? I tend to spend my increasingly precious time building either unusual stuff, or prohibitively expensive which I need that I could not acquire by any other path than DIY.

If you had said you were thinking of making a DIY tube curve tracer, THAT would have met with my instant and hearty approval since that is something that is rare and even if you do find one ready made, it will be priced outa reach for many of us. Me included.
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Old 26th December 2008, 12:09 PM   #7
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As one that has thrown good money away in the attempt to save a few bucks I would have to recommend ebay as a way to secure the tester you are looking for. You will most certainly end up spending far more to purchase the parts necessary to convert the tester you have now. This is DIY I know but sometimes it is more practical to buy that to remake.
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Old 26th December 2008, 12:21 PM   #8
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Quote:
Originally posted by rcavictim
If you had said you were thinking of making a DIY tube curve tracer, THAT would have met with my instant and hearty approval
Have you seen this one? Old link, but still interesting:
http://www.dissident-audio.com/Traceur/Page.html

FWIW, I'd agree it makes more sense in this case to just buy something that already does what you want it to do. Even if that means dropping $400 on eBay.

If that isn't an option, go download the schematic for any one of the known and respected mutual conductance tester. Clone what is already designed and proven.
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Old 26th December 2008, 12:22 PM   #9
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It strikes me that the advantage of using an existing tube tester is if you want the gm of a lot of different sorts of tubes with a lot of different sorts of bases. For most diy purposes, there's only a few types of tubes that one might want to characterize (most usually for sorting purposes). In that case, it seems simpler to me to set up a test jig. It takes about an hour a maybe $10 worth of parts to do, assuming you have some basic test gear like a calibrated scope, a power supply, and a signal generator. Mutual conductance testers are one-point devices anyway, and it adds little to jig complication to have the capability of getting gm at several points, especially if you have a variable HV supply.
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Old 26th December 2008, 03:48 PM   #10
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Interesting question, although I would lean toward buying a Hickok or Hickok clone on Ebay.

If you were to convert something that you have, you would still spend considerable time tabulating test voltages, etc for each tube that you want to test.

I recently picked up a clean Hickok 600a on Ebay for $200, and it works just fine and tests every tube I have thrown at it. Good working 600a/6000a/800a's go for $150-$250 or so on Ebay, and there are lots of resources online for calibrating them and fixing them.

If you want a tube tester project, you could also consider picking up a non-functioning MC tester on Ebay for cheap and repairing it. Non-functioning testers or units in unkown condition (many folks have no idea if their units work or not, and do not have the tubes or knowledge/motivation to test the tester) sell for sub-$100 on ebay, some for less than $50 or so. If you pick up a dead Hikcok, keep in mind that if the bias pot is toast, you'll be out of luck or you'll have to find another organ donor, since the pot is no longer made and has a crazy taper.

In my case, it made more sense to pay market rate for a fully functional tester, so that I could spend my time building amps, but if you want to tester project, a fixer may be the way to go.

Functioning Jackson 648 series MC testers can be found for around $100 or so also.

You could always sell your emission testers to partially fund a MC tester.
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