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In Reply to: RE: Weird tube GM readings on eBay posted by JSTRM on May 02, 2010 at 20:30:43
All of her readings seemed abnormally high, and she advertised two tubes at 10,000umhos and 10,200umhos on her Hickok 6000. When I tested these two tubes on my recently calibrated TV-7, they measured 41% and 49%. These tubes were definitely not close no matter what tester is used.
Another eBay seller using a Hickok 6000, achieved micromho readings much closer to my readings for the 7591 tube(see link below).
A very famous TV-7 repair specialist has told me that he has never measured a NOS 7591 tube(any brand)higher than 74% on the TV-7 which equates to 9250umhos. The eBay seller in question has on one occasion advertised 7591 tubes at 12,500umhos.
Follow Ups:
I have measured 7591A EH tubes on my TV-7B (calibrated and in tip top working order) over 10,000 umhos on occasion. I can't recall any old stock measuring that high though.
TV-7s do not read in percent either - they use an arbitrary meter scale that can be converted to umhos by multiplying the meter reading by the conversion factor. On the "D" range where you test 7591s it's meter reading x 125.
the EH7591? I was thinking of using the chart setting for the 6L6 tube. If I use the normal chart setting of 10, then my EH7591 only read 28 on the scale.
Sorry about using the percent when talking about the TV-7 meter. I wasn't sure if the TV-7 meter numbers had units, so I guessed that it might be a percent figure. Anyway, I do know about the conversion to micromhos, or I wouldn't be quoting them in my posts. I keep the handy conversion table in my TV-7 flip chart. Bill.
Bill,
HS 6 3450 is the switch setting, the bias is set on 10 on the "D" range. Of course it uses 6.3 volts on the heater.
I just now pulled a couple untested EH 7591As at random and they read in the 72-75 area (about 9000 - 9250 umhos) with just a very brief warm up. I've had a few that go over 10K as I mentioned before.
If you only read 28 on your TV-7 you either have a bad tube or a tester problem I'm afraid.
A third possibility (bad tube or bad tester) is a bad (sketchy) test condition manual. Besides my USM118 cardmatic, I often test (especially DHT's like my 205D's) on the TV7-D/U because the USM isn't DHT-friendly, as Alan Douglas points out. When I recently (after many months absence) checked cal on my 6L6 reference, which should, according to Dan Nelson read 43, it only read about 28 with the "latest" bias pot setting of 40. Wasted a bunch of time until I fully retraced my steps (seldom have any 6L6's to test)and went back to Nolan Ryan's extensive spread sheet, that points out Rev L specifies 23, Rev M specifies 40 and he questions the validity of the latter. In fact, with a bias of 23, my Nelson-calibrated TV7 reads 43 for the Nelson-calibrated reference 6L6. So 3rd possibility is that the version of the settings manual(or roll chart for other testers) may be obsolete or erroneous for that particular tube. JMO, YMMV.
You mention using the exact chart settings for testing the EH7591, but in a previous post, you mentioned this:
Honestly, I think I just got mixed up or something - it looks like I was describing the EH KT-88 in the prior post!
Sorry, my bad!
....OUCH!
6L6 tube on my calibrated TV-7, and it tested right on the money. The 6L6 was sold to me by Sonny at the Radiola Guy, and it has a value of 42 units on the TV-7.
I re-tested my new EH7591, and I have a correction to make also. They measured 45 units on the TV-7. When I raised the bias to 23(the setting for the 6L6 tube), the EH7591 tested at 32 units. I guess that makes sense actually. So, I am not sure what to do to make the EH7591 measure like a normal, new 7591 tube.
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