Help with Eico 635 tube tester readings

[guitar-rocker]

Well after purchasing a basic tube tester over a year ago, a Eico635, pulled it out from under the bench and went through my tube caddy testing tube stock on hand. All tubes on hand are old stock, various common types, and all had previous test results from sellers on them.

The tester has no tubes, pots, or caps. With the good / bad gauge (0-100) on zero, it consistently tested about a score of 23 less than previously tested tube marked values. On the rectifier tubes it matched pretty close. Since there are no physical adjustments that can enhance calibration such as a trim pot, is anyone aware of any other adjustments for closer readings? Thanks. Terry

[guitar-rocker]

I know tube tester readings are like comparing apples and oranges, but here is an example on 4 each 12ax7 Sylvania's that I picked up a short time ago. The seller stated that his reading came from a recently calibrated 533-A Hickok. His readings are first, then mine today:

His 1325/1550 -----mine 76/78

His 1550/1100 -----mine 77/68

His 1175/1510 -----mine 72/78

His 1275/ 1300 -----mine 78/78

[guitar-rocker]

These were some 6AQ5's that I got through Marty, and I'm not sure what his tester is/was. There was only one number marked per box:

His 91 ----- mine 70

His 90 ----- mine 57

His 96 ----- mine 74

His 100 ----- mine 73

[guitar-rocker]

And these were some 5U4's that I bought from a guy two years ago, unknown tester:

His 108 / 106 ----- mine 54 /53

His 108 /106 ----- mine 53/52

His 114 / 114 ----- mine 52 / 50

His 106 / 106 ----- mine 53 / 50

[xtian]

These type of testers are not calibrated, not even close. I mean, "23" WHAT? They're only good for BASIC tests of functionality. A tube that measures poor strength on my similar tester sounds perfectly good in an amp.

[Stevem]

Since this type of tester does do the GM output of a tube and only checks the tubes Cathode emission they are really only good for a go no go test, or testing Recto's!
With this type of tester a good check is to back down on the heater voltage to test for the life that is left in the tube, comparing this with a known new tube means more and is worth keeping track of .
When I really want to know how good a tube is I simply stick it in my fender champ and measure the drive voltage with my voltmeter set for AC volts at the output tube with a set input drive level to the amp.

[guitar-rocker]

I did replace C1 this morning, a .01uF, and replaced all the carbon comps with metal films. I think the tester is as good as it's going to get for a cheaper tester. Thanks for the replys.

[boots]

I don't necessarily trust tube testers. I have seen tubes test low for emission that sound fantastic in a Champ. I suppose the emission is an indicator of how much life is left in the tube, but it would be a shame to trash a great sounding tube based on an emission reading.

My approach is to spend an afternoon popping tubes into a champ and compare them under similar drive conditions. You can pretty easily spot the ones that sound weak or noisy.

Works for me. YMMV.

[tweedeluxe]

Old topic, but I found it while searching the same question for a customer. There is a misprint on the schematic. R7 should be 47k, not 4.7k. That will correct the issue, for anyone interested.