Eico 666 tube tester plate, screen and grid voltages too high by 15% - Page 1
Forum section: electronics / beginners

author: cincin
author_rank: Regular Contributor
timestamp: May 25, 2025, 07:03:24 pm
content: I'm cleaning up and calibrating an old Eico 666 tube tester. While going through the post-build tests from the building instructions, it asks me to verify the voltages of the various secondaries (see image) from the transformer (15V, 45V, 90V, 180V). Voltages that are to be used on the plate, grid and screen. These voltages are all about 15% above what they should be on my unit (17.5V, 52.5V, 104V, 208V). And this is AFTER calibrating the LINE CAL, LEAK CAL and Line Adj, so they shouldn't be off by up to 25% at this point, as suggested by the building instructions.I know nothing about tubes or tube testers, yet.My questions are:Is this normal?Is this within specs?Is this because of the lack of load?Does this indicate a fault with the transformer?Thanks!

author: TimFox
author_rank: Super Contributor
timestamp: May 25, 2025, 09:34:43 pm
content: The attached text reminds you of the +/- 25% tolerance on those voltages, so +15% is within spec.

author: Andy Watson
author_rank: Super Contributor
timestamp: May 25, 2025, 11:31:41 pm
content: What sort of meter are you using to check the voltages? Something to keep in mind is that when these devices were made, the meters of the time would have been analogue with typical impedances of around 2000per volt (or less). Whereas most modern DVMs are 10M. Not sure it would account for 15%, but it can make a difference.

author: cincin
author_rank: Regular Contributor
timestamp: May 25, 2025, 11:42:14 pm
content: Quote from: TimFox on May 25, 2025, 09:34:43 pmThe 25% tolerance sited is for measurements before the Line Cal, Leak Cal and Line Adjust have been adjusted. My unit is calibrated and the Line Adjust is set correctly, so at this point the tolerance should be much smaller than 25%. I'm wondering if 15% is too much, I don't know enough about tubes to tell.Quote from: edavid on May 25, 2025, 10:45:47 pmMy line voltage is... normal. I don't think it makes a difference considering that's what the Line Adjust does: tweaks your line voltage so that the unit's test voltages are within specs. My >15% measurements were all done with the unit calibrated and the Line Adjust set properly (which is lower than 130V). I can only imagine that the voltages will be even higher if I set the Line Adjust to 130V. Also, the filament voltage secondaries are correct, so R1 (see the schematics) the Line Adjust pot, is probably also correct.Quote from: Andy Watson on May 25, 2025, 11:31:41 pmI'm using a Fluke 115. I don't think I have an old meter around to check.Thanks for the help!

author: cincin
author_rank: Regular Contributor
timestamp: May 26, 2025, 02:38:36 pm
content: Would someone who knows more than me about tubes and tube testing confirm whether these voltages are fine?Thanks!

author: TimFox
author_rank: Super Contributor
timestamp: May 26, 2025, 03:55:07 pm
content: Slight correction: typical passive VOMs (e.g. Simpson 260) were 20,000/V, or 50 uA.

author: Andy Watson
author_rank: Super Contributor
timestamp: May 26, 2025, 05:24:10 pm
content: Quote from: TimFox on May 26, 2025, 03:55:07 pmOn D.C.; A.C. sensitivity was as low as 1000per V to 5000per V for the later models.

author: TimFox
author_rank: Super Contributor
timestamp: May 26, 2025, 05:29:10 pm
content: Quote from: Andy Watson on May 26, 2025, 05:24:10 pmYes, I forgot about the AC mode.

author: cincin
author_rank: Regular Contributor
timestamp: May 27, 2025, 06:44:16 pm
content: So we're all clear on vintage multimeter impedance, but I still have no idea or reference to tell whether my transformer has gone wonky, or if this is all very normal

author: CaptDon
author_rank: Super Contributor
timestamp: May 27, 2025, 08:00:44 pm
content: I would expect those particular voltages to run a bit high with no load. If the filament voltages are within reasonable tolerance then just go with it.

author: cincin
author_rank: Regular Contributor
timestamp: May 27, 2025, 08:03:49 pm
content: Thank you!The filament voltages are spot on so go with it I will.