Guard Your Tubes!

MylesBAstor

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425135_101505731076&.jpg
 

cjfrbw

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Got a closet full that I need to inventory and sell some day. I imagine they are worth more than I paid just a few years ago, the tube market is nuts with hoarding.
 

MylesBAstor

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Got a closet full that I need to inventory and sell some day. I imagine they are worth more than I paid just a few years ago, the tube market is nuts with hoarding.

Your next pair of Alexandria XLF sits in your closet :)
 

RogerD

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Wasatch

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Never thought about it, maybe I should buy some extra one's.
 

Mark (Basspig) Weiss

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This reminds me that I need to find a good Tektronix curve tracer and build an adapter plus screen & filament supplies to test my tubes. Matched pairs of some of the tubes I have in my inventory run hundreds of dollars. Curve tracer is the best way to evaluate a tube because it tests it over the whole operating range, not just at low current, as with a tube tester.
 

MylesBAstor

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Apr 20, 2010
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This reminds me that I need to find a good Tektronix curve tracer and build an adapter plus screen & filament supplies to test my tubes. Matched pairs of some of the tubes I have in my inventory run hundreds of dollars. Curve tracer is the best way to evaluate a tube because it tests it over the whole operating range, not just at low current, as with a tube tester.

I think this tube tester is as good as the Tektronix (and maybe better), easier to operate and will give you printouts of your tests too:

http://www.amplitrex.com/about.html

You may be able to do a search and find someone who will match the tubes for you for a couple of dollars with the Amplitrex.

Since there were only probably about 5000 Curve Tracers made (and who knows how many survived), good luck finding one. I only know of two or three around the area.

Another tube tester that runs tubes at real operating points but not as sophisticated is the www.maximatcher.com.
 

Mark (Basspig) Weiss

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That's a nice tube tester with many important features, but I can get a curve tracer for about half or less that cost, used on eBay, with patience. I just missed an auction for one at $750 last week. :(
The curve tracer can show how well tubes match at 12 different levels of bias and plate current at a time, which is advantageous when matching pairs for sale, which I intend to do later this spring. I've got quite the collection of 12AT7s, 12AX7, 12AU7s, 2A3s, etc. that I plan on selling soon. Their value becomes enormously increased once they're proven and any matched pairs are found and identified.
 

Mark (Basspig) Weiss

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Update: I'm now using a Tek 576 Curve Tracer and in the process of building some more permanent tube test fixtures to cover the various tube socket/pin configurations.
The 576 is fantastic. I've already begun selling my first batch of tubes (Western Electric 415A dual triodes) on eBay. What I've been doing is measuring triode A and B, capturing screen photos and producing a report for each tube I sell. I can measure transconductance, plate resistance and other parameters, depending on how I configure the 576's controls.
Next, I'm configuring a 7-pin fixture for testing some WE pentodes. Pentodes can have amplification factors in the thousands. It's amazing that we do not see more of them used in phono preamps, especially of the moving coil variety.
 

JackD201

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Mark, if you've got some matched 417As please drop me a PM :)
 

microstrip

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Buying (or selling) NOS or other tubes is not a risk-less affair. Even if you test them with the best tube testers, the only thing that you can be sure is that it will not be a case of DOA. But you can not be sure you are getting a new tube - it can be at the end of the useful life and you only know of it afterwards.

We should remember that most tube testers make measurements at voltages and currents much lower than those used in typical hifi equipment . Although I have a good tube tester - Hicock 600 - I have built two custom testers. One for the 6550 /KT88/KT120 types and another for double triodes such as 12AX7, 6DJ8 and 6H30. In these testers I reproduce the exact currents and voltages used by my equipment. It is the only way of proper matching tubes and checking for their characteristics, such as noise and distortion.

In this irrational hobby I sometimes make some rational decisions - I have tube equipment but try to avoid pieces that use NOS or not currently in production tubes. Otherwise it would be a source of permanent distress ...
 

MylesBAstor

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I thought the Curve Tracer did measure at "real" voltages unlike the Hickok's of the day.
 

microstrip

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I thought the Curve Tracer did measure at "real" voltages unlike the Hickok's of the day.

Myles,

Which Curve Tracer are you referring ? May be some can work with real voltages, but some of them are really expensive. And unless you are in the business of tubes, you will be paying mostly for the flexibility of testing hundreds of types of tubes, and you only need to test a few types.
 

MylesBAstor

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Apr 20, 2010
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Myles,

Which Curve Tracer are you referring ? May be some can work with real voltages, but some of them are really expensive. And unless you are in the business of tubes, you will be paying mostly for the flexibility of testing hundreds of types of tubes, and you only need to test a few types.

I thought Mark had a Tek Tronix curve tracer. And yes the Tek Tronix cost a small fortune when new. I think something in the neighborhood of 15k.
 

Mark (Basspig) Weiss

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Aug 3, 2010
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I thought Mark had a Tek Tronix curve tracer. And yes the Tek Tronix cost a small fortune when new. I think something in the neighborhood of 15k.

I have the Tek 576, which is capable of plate voltages of 1500 volts. Believe me, it CAN destroy a tube if the setup is not carefully done. You must be careful not to exceed the tube's maximum ratings.

With a Curve Tracer, you can determine Plate Resistance, Transconductance and generate the family of curves seen in tube manuals. The way I check a tube is by referring to the RCA tube manual, where I look up the transconductance in µmhos, then set up the test with a low value of series resistance and plate voltage no higher than maximum ratings. Then I set up the step generator to produce 7 steps of 1 volt each on the grid, typically from -1 to -7 volts. The plate voltage sweeps from 0-350 in a typical triode test and then I read the distance between steps at the maximum plate voltage. This is the transconductance. On the 576, it reads in milli-mhos, so multiply by 1K for µmhos. Then, using a much higher value of series resistance, typical in an audio circuit, I read the change in plate voltage and divide it by the change in grid #1 voltage. This is the amplification factor. Usually, I nail it right on the dot, because when I go check the manual, it's exactly what I calculated.

The curve tracer can test tubes beyond normal operating circumstances. Want to see how a 12AT7 performs with 950 volts on the plate? I have one running nice curves right now in the shop. I wish I could upload some scope photos and attach them here so you can see what the curves look like. But if you open a tube manual, you can find them. For a 12AX7, you will refer to 6AV6 for the curves.

Absolutely powerful tool when you know how to use it. Very expensive, and hard to find these days. Another thing that a lot of folks may not realize... the curve tracer is great for troubleshooting in-circuit. I simply attach test probes to base and collector pins where the fixture plugs in and set to a low voltage range, then I can check components in the circuit. It's like a Huntron Tracker on steroids.
 

Mark (Basspig) Weiss

Well-Known Member
Aug 3, 2010
682
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New Milford, CT
www.basspig.com
Mark, if you've got some matched 417As please drop me a PM :)

I'll look. I've got 415A's and 6211's, 12AT7's, 12AX7's and 12AU7's and a bunch of 2C51's that I'm in the process of testing and readying for sale.
 

JackD201

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Thanks Mark :)
 

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