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Tube testers....a necessity?

ricohman

ricohman

Adventure Rider
Now that I have all these 8417 tubes for my Dynaco MK VI amps, I feel the need to test them to see if they are indeed matched quads.
I have read that transconductance and emissions are two methods but few testers can replicate the load a tube sees in the amp.
Are any testers in the budget of hobbyists suitable for this?
Is a tester a necessity?
I can now visualize all those electrons travelling from the anode to the cathode through all the grids of these pentode tubes.
 
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IMHO, the main benefit of most tube testers are knowing that the tube you are about to place in your amplifier actually works, and has no leaks or shorts. Only the most crazy-expensive testers truly tell you of your tubes are "matched", although you can get a decent idea how close they are with a decent transconductance tester.
 
Laugh me off AK?

Tubes dissapate heat. Stronger tubes draw more current, thus dissipate more heat than weaker tubes. I tried a test by measuring the glass enclosure temperature of over 30 NOS tubes from same manufacture. I found a direct correlation in-between tube operating temperature at idle (no audio out) and my transconductance type tube tester results. Perhaps a low cost temperature meter can be used to match power tubes & a simple tube tester for testing tube operation plus no shorts.

I do know a quality tube tester will set one back at least $300. A hand held temperature meter about $60. Actually $25 right now on Amazon.
 
Wouldn't you have the same result setting the bias on 1 tube and swapping tubes without adjusting,and measuring cathode current to match tubes?
 
s-petersen said:
Wouldn't you have the same result setting the bias on 1 tube and swapping tubes without adjusting,and measuring cathode current to match tubes?
Click to expand...

Yes and bad news for matching tubes IMO if different bias is applied to PP circuits. In other words the tubes would be set up with different operating points as viewed on the tubes load lines. If all tubes have the same bias I see a possibility with temperature readings for matching on new tubes of same lot & manufacture. Also filament operating voltage must be the same or extremely close.
 
In my brief experience, I've found that my (emissions) tester is great for quickly checking to see if a tube is good or bad but practically useless for matching them. Case in point: I recently purchased a lot of 12 Russian 6P14Ps which all test almost identically good for emissions (I was really pleased). However, in my Scott amp the current draw varies widely - from .09mA to .31mA at the same bias voltage. That convinced me to install individual bias controls for each tube! I was able to put together a couple of nicely matched sets but had to do it in-amp.
 
Handy to have? Yes. Necessary? No. I have had several. Sold all of them but my B&K 707 and Hickok TV-3. A lot of stuff spewed about matching, yada, yada. The best tests for a tester are gas and shorts just to make sure you have a tube that won't damage your amp. The rest will be amp dependent. You can sometimes do a tap test with a tester for microphonics, kind of hit and miss. That seems to be the biggest pre issue.
 
In vacuum tube transmitters when you are running two or more tubes bias adjustments are not just for best sound without them it goes from the tubes not lasting very long, to cataclysmic failure. You can get matched tubes, or you can match them yourself but it is never good enough all it does it get you in the ball park with a tube set that is with in the range of the biasing circuitry to handle. I am not sure how useful it would for an tube type audio amp but most all tube type HF transmitters have switch a position for the front panel meter to set it up for bias adjustments. This is very nice to have especially now because some of the transmitting tubes are getting expensive and you want to get the most out of one you can, and I see some of the audiophile grade tubes are no cheaper.
 
Skylab said:
IMHO, the main benefit of most tube testers are knowing that the tube you are about to place in your amplifier actually works, and has no leaks or shorts. Only the most crazy-expensive testers truly tell you of your tubes are "matched", although you can get a decent idea how close they are with a decent transconductance tester.
Click to expand...

Good post..


I go by the same here..
 
That is all I ever do with mine, make sure the tube will at least work. I usually have several tubes to try out and which ever one works the best is it.
 
The temperature test is actually quite brilliant!

What amazes me about expensive tube testers is how little is actually in them. When originally made the price reflected the complexity of socket and switches, not any sophisticated circuits. If faced with the prospect of spending $300 for a vintage tester, I'd more likely just build one for the limited number of tubes I need to test, say power tubes and 12A?7s.

The Eico 666 (Satan's Choice) and 667 use circuits very similar to Hickok and usually sell for far less. They can't duplicate an amps circuitry either, but seem to do a decent job of matching.
 
A few months ago I stuck a pair of NOS GE 6L6GC's in a guitar amp. Straight out of the boxes, perfect lettering, no wear on the pins, in other words they certainly looked great. So I didn't bother dropping them into one of my tube testers....hey they're new tubes right? I did hook my Fluke up to the OT to measure current to set bias.

Fired up the amp and watched as current climbed through the roof. Pulled the tubes and yep, one tube was bad from the get-go. Should've tested it first!
 
Wow, that's rare. My success with NOS tubes has been near 100%. Can't say the same for new production. FWIW, when tubes match with the 667 and I put them in a guitar amp using adapters for bias, they always seem to match there too. That's just bias, so large signal performance is unknown unless measured.
 
sony6060 said:
Yes and bad news for matching tubes IMO if different bias is applied to PP circuits. In other words the tubes would be set up with different operating points as viewed on the tubes load lines.
Click to expand...

I always thought that might be the case. I never looked into it deeper, but somehow I didn't think that setting each tube's bias voltage to get some particular idle current was a real work-around for having matched pairs.
 
I know there is some difference,as the Citation II with the bias adjusted and balanced,the AC balance changes with different output tubes.
 
My MK VI Dynaco's require 4 matched tubes. I was also wondering if there is any place that one could mail the tubes in for testing. But after reading about testing it seems that there are many different methods and each vendor may rate tubes differently. Some factories also pull the "better" testing tubes for manufacturers under contract. I would guess the factory has the best shot at matching tubes during the production run.
If you buy tubes from the same lot, is this better for matching than tubes made years apart?
 
Find someone local with a good transconductance tester and at least get your tubes tested for the obvious issues and then see if you can at least get close to determining which ones naturally group together, so you then can start with the right groups/quads in the amps.
 
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