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I tried the archives and a couple of other places but didn't see a specific answer to this.Since the 5751 has less gain than the 12AX7 why are the test settings the same when using the Hickok 600A.
I think the answer might be that the gain doesn't come into play when testing. Or that when we test - it's actually using various parameters to get to the result number and those parameters are the same for 5751 and 12AX7.
I know we don't like to get hung up on test results - but when someone buys or sells a tube the results are the only indication of the tube's condition as well as a fair price. Of course it could be questionable if the pins look like a pretzel too.
Thanx for any replies. Fran
Follow Ups:
I checked two more high end Hickok testers. Both the 539C and the KS Cardmatic use the same data and reject points for these two tubes. The Triplett 3444 and the Heath TT1A use a slightly different set-up for 5751 than for the 12AX7. They have higher reject points for the 12AX7, and also a different bias setting. Hickok just seemed to ignore the differences between the two tubes. This might have been an oversight--we will probably never know. But--Hickok used the same bias setting to test both tubes--where Heath and Triplett used a higher bias for the 5751. So--is is possible that the expected reading would be the same.
Hi Chris and thanks for the info. I'm loving the 600A I got from you over a year ago. I didn't want to bug you privately though so posted my question here. It works great and always gives repeat readings. Although you calibrated it over a year ago - it probably only has about 10 hours use.I have some 5751 that all test in the 950 - 1200 range. Some are supposed to be NOS and others used only a few hundred hours. I found that lowering the bias from 12 (as the chart shows) to 10 - I get readings that are 150 higher making them 1100 - 1350 which is more in line with 1250.
I'd never fudge settings to deceive anyone - and like I mentioned earlier not at all intimate as to how a tester really works, but it just seems to me that since the AX7 has 30% more gain than the 5751 - some setting on the tester should be different.
I read some of the Hickok history you and others posted here and it's very interesting.
I wonder when building the first testers if they took new tubes off the assembly line and tested them for electrical parameters. Then put them in the tester socket and did what they had to do to get the scale to read 1250. Record those settings and then move on to the next tube.
Thanx again - Fran
The key to your post is that only some of the tubes test low. If others test normally--then I suspect that the set up data is correct--and works--at least for most tubes. OR--this could be another one of those funky tubes like the 6DJ8--where an individual bias setting needs to be determined for each tester. The base grid signal used (2.5 volts RMS) on the common Hickoks like 600A and 533A is pretty high for a tube like the 12AX7. This is something that Alan Douglas covers in his book on tube testers. All testers have their limitations.The best thing to do would be to test those tubes on another tester like a 539, a TT1, or a Triplett that has a lower base grid signal. That way you could get "another opinion".
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