ECC83/12AX7 Tube test query

[waam68]

Hi
I recently aquired a B&K Dynajet 707 tube tester (allegedly serviced & calibrated). It seems to work well on nearly all the tubes in my old Heathkit amps & Tuners. However I get a short > 1Mohm (deflects the meter) on all the ECC83/12AX7's I try (4 so far). To check a reference tube I plucked a nearly new chinese 12AX7B out of my yaoin phono preamp but that also showed a short >1Mohm. The other tubes are all original mullard yellow label. I'm pretty new to tubes but am wondering if all the 12AX's could be faulty & if the 12AX7B would test the same as the B&K tube chart settings for 12AX7? Heater at 12Volts & sensitivity at 85. Apart from finding someone else with a tester is there any other way to check if these tubes are indeed bad?
Thanks
Mike

[Merlinb]

A short between what and what?

[pmillett]

Originally posted by waam68
I get a short > 1Mohm (deflects the meter)

Unless I misread, this is a good thing. A short > 1Mohm is not a short, it's open. So I suspect the tubes are fine.

Pete

[waam68]

mmm sorry for the delay in getting back on this but...
According to the 707 tester manual for the shorts test in the Jet check section of the tester-
"Shorts or Leakage up to 1 Megohm of value between elements of the tube under test, will cause the shorts lamp to glow. Leakage of more than 1 Megohm will be indicated on the meter....Any tube which causes the shorts lamp to glow, or the meter to read into the "shorts reject" area should be discarded immediately. Do not give further tests to tubes which are shorted".
As I said I'm new to tubes/valves & am refurbing an old Heathkit S99 9w amp and an FM4U tuner. The tuner is done & works very well after I fitted a newmains smoothing can & a couple of resistors but the valves all tested ok (this was my first real audio project & I'm definately a novice). The S99 valves all check out ok on the 707 except the 3 ECC83/12AX7's on the preamp board which I think fail the "shorts" test. I could see the Mullards being shot due to age but am surprised the chinese 12AX7B showed the same reading. The mullards seem pricey so I want to make sure I'm interpreting this correctly.
Mike

[anatech]

Hi Mike,
Try testing for shorts with no tube plugged in. Could be leakage in a socket or wiring.

Old tubes are in no way more likely to develop a leakage path unless the base is dirty (or the tube is gassy) in normal storage or use. It's rare I measure this defect in a signal tube that is clean.

-Chris

[SY]

The other thing is that ECC83/12AX7 tend to show a lot of grid current well before hitting Vgk = 0. If Vgk =-1 in this test, you could easily see a microamp of grid current.

[waam68]

Thanks guys
I will try the suggestions...I put the tubes in the chinese phono stage & they sound smoother than the stock chinese tubes & volume seems the same so I guess they are A ok. I'll try cleaning the pins gently & re test.
Mike

[kevinkr]

Originally posted by SY
The other thing is that ECC83/12AX7 tend to show a lot of grid current well before hitting Vgk = 0. If Vgk =-1 in this test, you could easily see a microamp of grid current.

Keep what SY posted above in mind when retesting. I have encountered the same issue, and invariably the 12AX7A are fine. I usually ignore the short test on the 12AX7A for this reason.

[waam68]

Cheers Kevin
I ran all the other tests & the tubes check out fine so I guess it's a feature of these tubes on this tester.
Mike