singlendpentode

the way to test these tubes is take the measured mmhos reading from the tube tester, and compare it to the spec for that tube from a tube database in other words if a 12au7 is spec'd at 2200 mmhos, then that is the 100% NOS new reading should be the tester itself has to also be calibrated to that 2200 spec when it measures a new tube likewise, if the tester is set up to test a 100% NOS new tube to a 3100 mmhos spec, then the technician needs to know that going in, and judge accordingly. the tube tester shootout had testers calibrated to a variety of scales, 2200 mmhos, 3100 mmhos, and % scales and roughly so as many showed well over the 100% number. the only way to sort it out and actually use those testers, is put in a known brand new 100% tube of the type to be tested, and test it remember that reading on the gauge as the 100% point, then compare all subsequent tubes of the same part # to that spec. if a vintage tester is calibrated and doesn't show a known NOS new 100% tube as being 100% good then it's still out of calibration, or there is a compromise or defect in its basic design- whereby it checks some tubes correctly, but checks other tubes incorrectly, showing good tubes as bad tubes, or vice versa. I have 4 testers and often run a tube through 3 or all 4 of them, looking for a correlation usually when it a tube shows better on the higher end tester, it also shows better on the console store emissions tester too. but I have had known NEW tubes show 90% on the emissions tester and 50-60% on a high end dynamic or mutual conductance tester meaning the high end tester scale is off, OR it's a design flaw. so that 60% point is in reality, the 90% point for that tube in that high end tester. there was a lot of BS-ing going on with these testers throughout the years...and there still is. in reality, the best tester is the one with minimal settings, tests tubes quickly, and has a complete tube roll chart or manual, and never or seldom needs calibration- and that is an emissions tester. an expensive high end space-age tube tester, that is quirky, defective and can't be calibrated, or is calibrated to an unknown spec, is nothing more than a high priced door stop. if a tube tests bad on a decent low-end design emissions tester, that ties all the elements together and tests like a diode, expert techs will agree the tube is actually bad, 90% of the time. I'll take it further by saying 99% of the time, the tube is bad, if the emissions tube says it is. so at that point, we have to ask ourselves- what sense does it make to go further into a $1000 tester, when the $50 emissions tester has already made the call correctly.

one more thing, if you take a look at the testing results for the 539 Hickok, it lists a reject point of 1950 mmhos if the 12au7 was being used in a circuit with a 250 volt plate voltage, it would have 2200 mmhos at max for a new tube spec no way would 1950 be a reject tube. 1950/2200 = 88% tube which is a new tube in many cases, or only slightly used but 1950/3100 is 63%, which would be the reject point using 3100 mmhos as the 100% new tube reading it's obvious that all those testers, even within the Hickok family itself, were not all calibrated on the mmhos scale to the same 100% reading it appears some were calibrated to 2200 as the 100% reading for the 12au7, while others were calibrated to 3100 as the 100% top reading for the 12au7 in this case, the 539 reading 3200 mmhos would be a 3% deviation, from 3100. 100 over. this is something they never touched on in the original test, how to decipher the readings. because the tube spec changes based on plate voltage obviously the RCA lab tester used to spec the 12au7 at the beginning of the test, was testing the tube in relation to a 250 plate voltage circuit. at that point, to get all the testers to tie in, and the tests to be valid, a 2200 max reading scale would have to be used. for ALL the testers otherwise, the results have to be extrapolated using math per above. from a 2200 scale max to a 3100 scale max. Hickok Model 539 No. 1 3200/3200 RP=1950 No. 2 3300/3350 RP=1950 Hickok Model 539B No. 1 2800/2900 RP=1950 No. 2 2800/2900 RP=1950 No. 3 3350/3350 RP=1950 No. 4 2600/2650 RP=1950 No. 5 3100/3100 RP=1950 No. 6 3600/3600 RP=1950 Hickok Model 539C No. 1 3150/3175 RP=1950 No. 2 3250/3350 RP=1950

EXACTLY- great deduction- this is what I think- being trained in electronics, mathematics, and analytical statistics this is an old post, but well worth reviving. hello all, I was compelled to reply here. the tube tester shootout started with a 12au7 tube that was lab tested on a high-end RCA lab tube tester the readings were 2400/2480 on each side of the tube if you read the results of the shootout carefully, you'll find that the high end testers were off by a mile. some of those high-end TOTL 539 Hickoks were off 30-35% the ones that really kicked arse and took names, were the Hickok 752, Heathkit TT-1, Hickock 6000, and a distant 4th place was the Hickok 800 they tested a SINGLE Hickok 752 and it nailed it at 2400/2400 that is only 3% off on the 2nd value, and dead nuts on the first one the TT-1 also hit it at 2400/2400, but they tested 3 other Heathkit TT-1 machines, and they were off by a farther margin to test only ONE 752 and have it nail it like that, statistically says that is a very good machine in other words, if you were an insurance company, you'd bet on the 752 as being the lowest risk, most reliable tester many of those other so-called high end testers were over 3000-3400 mmhos, on a tube that really was only 2400 mmhos a huge error there's more- depending on which plate voltage is used, the 12au7 is either rated at 2200 mmhos, or 3100 mmhos http://www.nj7p.org/Tubes/SQL/Tube_query.php?Type=12au7 Class A Amplifier Source ........................................ RCA RC-15 - 1947 Plate Voltage ................................. 100 V Grid No. 1 Voltage ............................ 0 V Amplification Factor .......................... 19.5 Plate Resistance (approx) ..................... 6.25K Ω Transconductance .............................. 3100 µ Plate Current ................................. 11.8 mA Class A Amplifier Source ........................................ RCA RC-15 - 1947 Plate Voltage ................................. 250 V Grid No. 1 Voltage ............................ -8.5 V Amplification Factor .......................... 17 Plate Resistance (approx) ..................... 7.7K Ω Transconductance .............................. 2200 µ Plate Current ................................. 10.5 mA now, funny how the lab RCA measured 2200/2280 mmhos- the exact spec for a 250v plate voltage on that tube and how the other high end testers, were over 3100 mmhos, that seems to match the 100v plate voltage spec for that tube so the question is, when they are taking the reading on the RCA lab tester, which spec are they shooting for ? because it seems like most of those high end testers were calibrated for the 100v/3100mmhos spec while the few that really hit it dead nuts on, were calibrated for the 250v/2200 mmhos spec if the high end testers were calibrated to check a new 12AU7 spec'd at 2200 mmhos, then those testers are a real POS, because they are off by 35% and the Hickok 752 leaves them all in the dust, with the Heathkit TT-1 a close second by a hair going by the tester shootout readings, yes, a Hickok 752, whips a Hickok 539....and quite badly too... the 752 is DEAD NUTS ON, it doesn't get any better than that- measured 2400/2400, against a lab RCA tester reading of 2400/2480 that is no deviation on the first reading, and only a 3% deviation on the second reading