Doug's recent comments about Tube Checkers is a good reminder about some rudimentary tests that should go along with restoring an amplified phonograph. He wrote:

"Tube testers do not put tubes under actual operating conditions. A tube may test good but if you leave it in the socket for 10 minutes or so the grid leakage may become apparent. A tube tester is a useful tool. I always test all the tubes in the sets that I work on as a starting point but they still get tapped with a plastic screw driver handle and rocked in the socket while they are running. Sometimes sparks fly inside the tube envelope. I've had 6CG7s test just fine but one of the triode sections causes a buzz in only one of the channels on a stereo amp. I’ve solved many problems just by swapping out a suspected bad tube with another and yet the offending tube tests fine. Keep in mind that the tube tester is not the final word.

Anyone that tries to sell a pair of tubes as "Matched" because they both test 105.2% on their tube tester doesn't have a clue. Anyone who believes him is a chump.".

I had the benefit of an "old timer" friend who gave me some good lessons about Tube Checkers. I remember running down to the local hardware store or drug store to check tubes when I was a kid; more often than not, tubes came up Bad. Had I know then....

But I now trust my trusty Sencore TC154 Mighty Mite (actually two of them because I don't completely trust either). I wish there was some kind of detailed manual or training document that goes along with the TC154 because it's amazing what you can test or "check". The TC 154 Service Manual is helpful, but is there more? Learning to use this Tube Checker is also a great exercise in learning about tubes including basic tests such as Cathode Emissions, Grid Leakage, Shorts, Life testing and more. It's easy, but not that easy. As Doug notes, grid leakage (a crucial test) may not be detected unless you leave it on for 10 minutes. I was glad to see that rocking a tube and taping a tube with a plastic screwdriver handle is done by a pro.
So much to learn from filament warm up time, filament voltage sensitivity, "temporary rejuvenation?" and more.

How bout it gents, who's using a Tube Checker on phono amplifiers? Comments, suggestions?

Jack

I always test all the tubes on my Heathkit tube checker, but sometimes a bad one won't test bad. Like Doug said, it is a good starting point.

I think the drug store tube checkers were set to test tubes bad so the store could sell more new tubes.

I have a nice Heathkit TC-1. When working on any player, guitar amp or radio I pull the tubes to clean and check them. It is always good to see if there is any obvious issues from the get go like shorts etc but a tube tester is far from the final word. It is just another tool to help me out. I have had bad tubes check good and and vice versa. I also check the tube voltages in the amp and working (yet another tip learned from DVC). Tube testers are great but as you mentioned not a fail safe diagnosis.

As far as the comment about DVC rapping on his tubes with a screwdriver handle, I recall one of my first times up at Doug's. I had just recapped my very first amp one of the RCA stereo/mono SHP-8 amps. I had an issue with one channel and Doug was helping me out. He checked my tubes for me (I had no tester at the time) and was stunned to see how hard he was rapping on the tubes and wrenching them around in the sockets! Then he took the chassis and pounded it on the table several times to remove any loose solder bits. I was stunned, here I had been handling this thing with kid gloves and he is beating it like a two-bit-ho... Another lesson learned that day, these things are more robust then we sometimes think and in order to get a reaction sometimes you need to put it through the paces so to speak.

From what I can learn, a Dynamic mutual Conductance tester comes the closest to actual operating conditions for the tube.

Hickok made several models that tested tubes that way, so that's what I trust the most. The meter reads in micromhos, which is the closest to the true character of the tube.

I rather imagine that, if two tubes checked the same, or nearly so on such a tester, they could be considered to be "matched". I use a Hickok 539C tester, and I feel good about the readings I get on it.

Hickock, and, Eico, both, were good testers. I use my Hickock 600A for tube testing.
I agree tube testers are NOT the final word...
I also lightly tap each tube while the set is playing to look for noisy tubes, intermittent shorts, etc......
I agree with Phono Jack as well on his comments.
Bill Cahill

Same here, The old guys passed tricks of the trade on to the go-fers that only time and experience can teach. The guys at the local radio shop allowed me to use their Sencor TC-142 on the front counter and directed me away from the tube seller at Town Drugs. They later hired me and what an education, the things they don't teach in high school electronics. I now use a Sencor TC-142 and use little tricks like dropping the filament voltage one increment to determine how the emission holds up. Good tubes hold their emission while weaker ones can totally shut down.


Where's my 5lb hand sledge...er... I mean percussion wrench?


Hey Doug,
I've never used one but I've always heard that they are the best testers. There was a time when a matched set only referred to output tubes and they were curve traced and came in a double box with the documentation. They had to be hand checked and selected. Something like 2 out of 30 some odd tubes would qualify as matched, coming off the same assembly line and they were almost never sequential. There was no such thing as 4 matched tubes you had to buy 2 matched sets. If you were to ask for a matched set of rectifier tubes or 12AX7s, you would have been laughed at, even by an audiophile.

Here's mine, two of my favorite tools:


SENCORE MIGHTY MITE TC154 Tube Checker

Now where did I put that "old timer"? They are becoming so difficult to find, yet they are so valuable. And ya can't get 'em on eBay.
Oh no, am I becoming an old timer? I'm too young for this. Happy New Year to all the old timers.

Jack

I always use a tube tester (Dynamic Mutual-Conductance type) just to make sure there are no shorts or grid emission in the tube, and to get an idea of the performance level. But as many have posted, that will not guarantee good performance.

I use two different tube testers -- a 1970s tester that covers newer type tubes, and a 1950s-vintage one that covers the old "fat-pin" tubes. Most tubes are covered by both, and sometimes the two testers do not agree!

Before I wrap up any radio or amplifier I always measure the tube voltages, particularly the output stage, since it carries high current and if not operating properly could destroy other components. Low-level stages operating at low current I'm not as concerned about.

I think the single most critical measurement in an amplifier is the cathode voltage of a cathode-biased output stage. This voltage represents the grid bias, and directly indicates the plate (and screen) current flowing through the tube. This voltage absolutely MUST be correct, and if it is not, the set goes no further until repaired.

-David