Tubes Asylum

Posted by Joe S on May 08, 1999 at 17:06:29:

In Reply to: Any vintage or new tube tester recommendations. (nt) posted by Jerry P on May 05, 1999 at 13:27:38:

...comment on both of your posts since I've messed about a bit with both a few vintage testers and the George Kaye unit.

1st - Vintage testers. I don’t claim to be an expert here but I've investigated a few units and can share a few experiences and suggestions. Some of these comments will echo some of the existing posted responses so excuse me if it’s a little redundant. The two brands that get the most play and respect are Hickok & B&K. While both are excellent testers they are a bit different in design and execution. The already mentioned SND tube web site (vacuumtubes.com) is a great place to see these guys first hand and get basic descriptions but it seems to boil down to a few basic points.

The things to look for when considering a vintage tester include:

1. Get a transconductance (often called a mutual conductance) type tester. The other, more common type is the emission tester and these simply pass to many questionable, used tubes. As one poster has indicated in a thread below, you can actually fool these if you know what you are doing with a used tube. Transconductance testers stress the tube more and will actually give you the ability to determine whether or not that latest find is genuine NOS with a long life in front of it or a used pull on its last legs.

2. Get a tester that includes its set-up guide or scrolls with the instructions on how to set the tester up for testing any given tube. In testers like the B&Ks the set-up guide is a manual, while on many Hickoks and other old testers its a paper scroll inside the tester itself viewable through a small window and adjusted by a thumbwheel. Also make sure it has it's owners manual as it will document the testing and calibration procedures.

3. Get a tester that will test the types of tubes you use. Check with the seller and see if the set-up guide or scroll lists the tubes you want to test. This is a bigger issue with tubes like 6DJ8s, 6922s & 7308s since these tubes were introduced in the late '50s and some older testers don’t have instructions on how to test them. Its also an issue with things like 300Bs and older (1920s - 1930s) radio tubes in that some newer testers wont test them. Most testers will test the common types - 12AX7s, 12AU7s, 12AT7s, 6550s, EL34s, etc - but check to make sure if in doubt.

4. Either buy from someone who will give you a 30 day warranty / return privilege if it doesn’t work in your hands or buy so cheap you don’t mind taking the risk if it doesn’t. Ideally, get a unit that’s been calibrated. These things do drift, though that's a bigger problem with tube based units. Some, like my B&K are easy enough to calibrate that you can do it yourself if you know what a capacitor is & know how to use a cheap Rat Shack multimeter.

When it comes to specific brands the two that get the most attention are Hickok & B&K. Both made many models and all that I'm aware of are transconductance type testers. Both will give you the info you need.

The Hickoks are tube-based units and in general they quantitate more performance parameters (to what practical purpose I'm not entirely sure) and they are a bit complex to set up & interpret in comparison to the B&Ks. In the Hickoks the 539B, 539C and already mentioned military units get a lot of attention and are tubeaholic favorites. The one thing to remember about these guys is that they are tube units themselves and less stable than the B&Ks and from what I've seen the calibration procedure can be a bit challenging (there is a Hickok tester webs site that lists the calibration procedure that I'll post as a link to if I can find it). All in all they are very highly regarded, if a bit complicated, and someday I'm probably going to buy a 539 B or C on principle due to my inherently tubeaholic nature...And by the way, if you use 300Bs or really old types it is tester of choice since they often come with adapters to handle older tube types.

The later B&Ks are a little different kettle of fish in that they are solid state units that are very stable and easier to set up & use than most testers. The model of choice here seems to be the 747. Its distinguishing characteristic is it's large number of preconfigured tube sockets (over 20!) that allow you to just plug in the tube & test it. It also has programmable sockets for testing odd types but the thing it wont do are things like 300Bs and very old radio tubes - so if that’s your bag look elsewhere. The other thing the B&Ks don’t do is quantitate as many parameters as the Hickoks, but both will sort out the good from the bad. Personally I went for ease of use and stability of the B&K 747 over the complexity of the Hickok. I got it on Ebay and I find it easy to use and very reliable - and while it didn’t really need it very much I was able to recalibrate it myself fairly easily.

While vintage transconductance testers will tell you if the tube in question is new or used and allow you to do rudimentary matching between tubes, the thing they wont do is tell you anything at all about noise or microphonics. This is true of all vintage testers, not just the Hickoks & B&Ks so that’s where something like the George Kaye unit comes in to play...

2nd - the George Kaye Small Signal Tube Checker (now that’s a mouth full!). This unit is very different from vintage tester in that it tests a few parameters that they don’t - namely noise and microphonics. The thing it doesn’t do is a transconductance test so while its great for sorting out line level tubes its not so good at identifying used pulls that are on their last legs.

This tester ONLY tests small signal line level tubes - output tubes need not apply - so it's a bit more specialized than the vintage testers. It’s optimized for 9 pin miniature triodes though it will test some other types with an optional adapter. The thing that makes this guy different are a series of special tests optimized for audio tube assessment along with a meter and a headphone jack that lets you view and listen to the proceedings with your own dynamic headphones. The noise test lets you measure & hear the noise spectra of the tube as well as evaluate its microphonics while tapping the tube itself when installed in the tester.

The unit performs an output test that lets you match tubes for gain and also includes a maximum output level test mode. Unfortunately, the maximum output test is the weakness of the unit in that it will often (20 - 30% of the time) pass marginal used pulls that fail on the B&K, so it really cant reliably sort out used tubes from new ones. The one thing it will do is quickly predict which tubes will work well in audio circuits and allows you to screen & classify tubes for noise & microphonics sensitive applications very quickly (my amp input stage can accept very microphonic tubes with no problem but the preamps output stage doesn't like 'em at all).

So what you end up with here are vintage testers that sort good from bad & used form new, but wont predict noise levels or susceptibility to microphoinics and a new specialized tester that measures noise and microphonics but doesn't reliably identify marginal used tubes. So what's a tubeaholic to do? Pick one or both? (Three guesses what I did and the first two don’t count! - Basically I screen all my tubes on both the B&K and the George Kaye unit).

Now this is where cost comes in. A new George Kaye costs $500 or so the last time I checked, B&K 747s go for $300 +/- $30 on Ebay and the most sought after Hickoks can cost anywhere from $300 to $500 and up, depending on condition. There's a huge range of other cheaper vintage testers that are worth a look as long as you stick with the mutual conductance / transconductance types and get one that really works. So what would I do if I were you?

Well if I used 9 pin miniature triodes and cost is no object, I'd get both a vintage tester and the George Kaye unit.

If I used 9 pin miniature triodes and cost is an object I'd get a vintage unit and test for microphonics and noise by installing the tubes in my system and listening - not too bad if you're not buying many tubes (very time consuming if you are evaluating 20 or 30 tubes though...)

If I used 9 pin miniature tiodes and I only bought from reputable dealers who guaranteed their tubes and tested them on a mutual conductance tester before shipping I'd consider getting the George Kaye and using it to sort out the best tubes.

If I didn't use 9 pin miniature triodes I'd get a vintage tester and skip the George Kaye and evaluate noise in my system as above.

And when it comes to vintage units Id definitely buy a transconductance / mutual conductance type. Emissions testers don’t reliably differentiate good tubes from used / bad ones.

Now pardon me while I go test a few new Siemens CCas that just arrived....

Joe