|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
In an earlier post about the TV7 tube tester, someone asked about the origin of Hickok roll chart data. The question was also raised about whether the various voltage drops were accounted for in the Hickok data.I spoke with Wendell Hall today about this subject. Wendell has worked on Hickok equipment for many decades, and is the best historical source I know. Wendell was absolutely certain that the voltage drops were accounted for in the data, and that it was not correct to reset a TV7 to line with plate current flowing--saying this would throw the readings off. Wendell wondered how a TV7 COULD be reset to line with plate current flowing--and so do I--unless a separate AC voltmeter was used to monitor the primary of the power transformer. As to the origin of the roll chart data--even Wendell did not know. He told me what I already knew about the secretive nature of the Hickok company. They just did not share much information, and anyone who knew about how Hickok arrived at their data is probably long gone.
Follow Ups:
You wrote "Wendell wondered how a TV7 COULD be reset to line with plate current flowing--and so do I--unless a separate AC voltmeter was used to monitor the primary of the power transformer."It's easy: On a TV7-B/U pressing S3 (mut cond) puts the tube into test mode with plate current. While keeping S3 depressed just push S1 (line adj), It appears to transfer meter over to line measurement circuit w/o upsetting anything in the mut cond circuit. By independent measurement, all element voltages and currents on tube remain unchanged when the line adj button is depressed simultaneous with mut cond. button. I've observed same arrangement on a number of tube testers inc 752/A, 53?, 800 and on Weston 981A. What I find interesting about this is if it was (is) such a "forbidden" mode of operation why didn't they add the mechanical interlock to prevent it? The more I dig into these things the more mysterious they become.
Although the case is definitely building for not adjusting the line voltage during test, the evidence is circumstantial and indirect. There's lots of dos and don'ts in the manuals but nothing about not adusting the line voltage during test. Again, you'd think that if it was important it would have warranted a specific mention like so many other non recommended set-ups.
I think I've figured out how Hickok accounted for the filament sag in some cases: They state that the test button should be depressed only long enough to note the reading: any longer may damage the tube. Don't know about the damage but if under test conditions for only a second or two, indirectly heated cathodes probably won't cool off much. Wonder what happens with directly heated cathodes?
You mentioned the Weston 981 which does not work like a Hickok. With the Weston (and the Heath TT1) plate current begins to flow as soon as the shorts switch is set to the test position. The Gm test button just applies the signal to the grid of the tube, it does not control the plate current. The line adjust on the Heath and the Weston is made WITH plate current flowing. This is also true with the Triplett 3444. In some ways I like the Hickok system better, as high voltage does not hit the tube until you depress a button. This is safer for the operator, and the tester. If you plug a shorted tube into a Weston/Triplett 3444/TT1A, you can toast something in the tester long before you press the Gm test button.I don't think Hickok, or the military had to tell people all the things they were NOT supposed to do in the manuals. That is asking way too much and would make manuals much longer.
i bet if you adjust line voltage during test, a lot of tube suddenly test "as new".... maybe we can call it the ebay adjust.
This post is made possible by the generous support of people like you and our sponsors: