02-15-2016, 03:35 PM
An interesting thread! 
The most important thing about a "calibration tube" is the instrument that the tube was tested on. Any calibration tube should be tested on a laboratory-quality tester in order to be as sure as possible about the value of the tube. Examples of laboratory type testers would be: Triplett 3444, Weston 686, RCA WT 100A, and Hickok 700. Testers like the Hickok 539 and Precise 111 are very good benchtop testers.
BUT herein lies the problem with calibration tubes: 1. The testing method used by the laboratory tester may not be the same as the method used by your tester; 2. Calibration tubes measured on any of the testers using "bias/english" pots with no associated meter to read what you are dialing in mandate that the operator will place these pots in the exact same place when they are verifying the tester calibration; 3. All tubes perform differently depending on the tester method utilized. Some testers use a 4 or 5 KHz grid signal, while most others use 60 Hz. Grid bias, plate, screen, and even cathode potentials differ depending on tester components and methods used.
Ed's calibration tubes ARE a good idea for his personal tester, as they can indicate if cal is on or off. That is, assuming that the bias/english (shunt) pots are in the exact same place.
It is important to remember that most of us have "bench top" testers. They are accurate to a degree, but this should be considered only with respect to tubes tested on your own tester. On most Hickok testers, the NOS value for your 6L6 is vic 5200uMho, while NOS for Weston and Precise benchtop models is 6000 uMho.
Comparing the results to other testers will often lead to some interesting results. In his book Allan Douglas made some interesting comparative studies of uMho testers and the "Dynamic Conductance" testers like the Eico, Jackson, and Precision models. Although I LIKE these testers, the results proved that different testers will sometimes yield VERY different results.
For those of us who own benchtop uMho testers, a frequent issue is voltage sag when testing high current tubes like 2A3 and 6L6. Testers like the Precise 111 and Weston 978 utilized two power transformers vice one to mitigate this. Another way to lessen this is (with Hickok-circuit type testers) replace the 83 tube (or 5U4 in Precise 111) with a solid state replacement. This frees up ~3A current draw for both examples.
The most important thing about a "calibration tube" is the instrument that the tube was tested on. Any calibration tube should be tested on a laboratory-quality tester in order to be as sure as possible about the value of the tube. Examples of laboratory type testers would be: Triplett 3444, Weston 686, RCA WT 100A, and Hickok 700. Testers like the Hickok 539 and Precise 111 are very good benchtop testers.
BUT herein lies the problem with calibration tubes: 1. The testing method used by the laboratory tester may not be the same as the method used by your tester; 2. Calibration tubes measured on any of the testers using "bias/english" pots with no associated meter to read what you are dialing in mandate that the operator will place these pots in the exact same place when they are verifying the tester calibration; 3. All tubes perform differently depending on the tester method utilized. Some testers use a 4 or 5 KHz grid signal, while most others use 60 Hz. Grid bias, plate, screen, and even cathode potentials differ depending on tester components and methods used.
Ed's calibration tubes ARE a good idea for his personal tester, as they can indicate if cal is on or off. That is, assuming that the bias/english (shunt) pots are in the exact same place.
It is important to remember that most of us have "bench top" testers. They are accurate to a degree, but this should be considered only with respect to tubes tested on your own tester. On most Hickok testers, the NOS value for your 6L6 is vic 5200uMho, while NOS for Weston and Precise benchtop models is 6000 uMho.
Comparing the results to other testers will often lead to some interesting results. In his book Allan Douglas made some interesting comparative studies of uMho testers and the "Dynamic Conductance" testers like the Eico, Jackson, and Precision models. Although I LIKE these testers, the results proved that different testers will sometimes yield VERY different results.
For those of us who own benchtop uMho testers, a frequent issue is voltage sag when testing high current tubes like 2A3 and 6L6. Testers like the Precise 111 and Weston 978 utilized two power transformers vice one to mitigate this. Another way to lessen this is (with Hickok-circuit type testers) replace the 83 tube (or 5U4 in Precise 111) with a solid state replacement. This frees up ~3A current draw for both examples.
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