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I recently went to a local electronics supply shop which has an old Sylvania tube tester from the 60's. It's the type that has about 40 different tube sockets and three selector switches. You look up the tube on a chart which tells you the switch settings and socket to use. There is one VU meter which measures "merit" on a scale of 0 to 120, and there is a pass/fail line in the center at about 60. I don't really know what is the merit that is being measured, is it gain, current, or what. But here is the predicament: i took a bunch of old 12AX7A tubes that i had around, plus one new one for a reference point which is an Electro-Harmonix 12AX7EH. The mystery is that the new one measured the worst merit of all of them, a 30 score which is a failure on the tester. The wide variety of old 12AX7/12AX7A tubes i tested included Mullard, Telefunken, Sylvania, ECG Philips, RCA, and no-name ECC83 (Tesla?). Most of these measured between 70-80, some even went to 90. But the new one that failed appears to work fine in a guitar amp. So i'm wondering if this tube tester gives a result that is valid for all applications, if not then what good is it? I've got a collection of vintage guitar amps, 2 Hiwatt DR103's from 1974, 1968 Fender Vibrolux Reverb, 1969 Fender Super Reverb, plus a new Peavey 5150 Van Halen design which is a monster tone machine. So i'm always concerned about tube quality and a valid test method. Any tube tester experts out there that can explain if a tube tester like this has any use for guitar amps ? In general, guitar amps have a preamp stage that is considered high gain, and intentionally non-linear, producing the wonderful and rich tube overdrive characteristic. This is different from home audio applications, hence this may be the reason for the useless tube tester data.
Follow Ups:
I confirm that the Sylvania is an emission tester, the most basic sort for TV servicemen of yesteryears.The only purpose I can think of is to pre-sort those tubes you get from flea markets or swap meets for a buck a piece. It'll tell you 2 things:
1) Are they safe? (short test)
2) Are they worth a listening test? (emission test)
What it does NOT tell you is HOW the tubes sound, especially for guitars where TONE matters.Your tester may be out of calibration. I've got some SED's, Sovtek's and Sino's that test about the same or better than NOS's.
probably was an emissions tester, which basically just tests whether the cathode is emitting enough electrons. That's OK for rectifier tubes but is pretty useless for anything else. The more useful kind of tester measures mutual conductance, e.g. the old military TV-7 or the Hickok testers. You can tell those because they usually have relatively few sockets, but lots of knobs for setting filament voltage, connecting up the appropriate pins to the proper voltages, etc.However, the best test, as has been pointed out, is putting the tube in the circuit and seeing if the voltages (measured with either a digital multimeter or a VTVM) match the manual specifications. If the voltages are OK, the tube should be fine IN THAT CIRCUIT, regardless of the results on a tube tester, which tests tubes for a "generic" circuit. If the voltages are off, then the tube is NOT OK for that circuit regardless of the tube tester results.
Per my knowledge, all new manufacture 12AX7s except Sunguang measure only 1200 gm. The Shuguang & NOS measure a nominal 1600 gm with some exceeding 1800 gm. The 12AX7EH is low gain right out of the box, but perhaps you have a bad tube at less than 1200 gm or the tester is out of calibration.When I sell Sylvania 12AX7 that are 'hot' tubes around 1800 gm, musicians state 'boy that tube screams compared to my new manufacture tube'. Yes, circuit gain is dependent upon the circuit itself, but a bad tube is bad.
From your comments it looks like the tube tester is checking the gm of the tube. But what you say about the EH tube is opposite of what the New Sensor web site says - "high gain and ultra low noise". Therefore the tube should have a high merit score if the gain is high, since gm (transconductance) is a measure of gain of the device. If my particular tube has low gain/low gm, that may explain the poor test result, but it also does not comply with the New Sensor spec. JimL commented that the tube tester is looking at electron emission, this i imagine would be analogous to measuring current from the device at a particular bias point. So it still is not clear what is going on - are the EH tubes crap, or is the tube tester useless ?
It makes sense of course to test the tube in the circuit that it is being used in, but this is not practical and it is a pain in the neck. For one, it's a pain to get the schematic and layout of some amp designs, and schematics do not always have the voltages and currents indicated. Secondly, it's a double pain to take apart the amp to get access to the circuit, every time i want to test a tube. This is a ridiculous science project that i don't have time for. A tube tester machine should be a valid and painless way to check the quality of a tube. Wishful thinking...
Sorry but that tester is darn near worthless. Sadly there are very few testers that are truly good and give readings that are meaningful to real world conditions. Typically the best tester is your amp. If you know what voltages are supposed to be and what currents are supposed to be you can tell a lot about a tube.Most small signal tubes like a 12ax7 are used with conservative operating conditions and might last many years...even up to a few decades+. It is the output tubes that fade in a couple/few years. A loss of highs and a dull sound is a good clue that the tubes are tired as it inability to set bias properly.
Russ
at doing any more than a go-no go test. If that tester has not been calibrated in 45 years most likely it is not close to being accurate. John
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