Tube tester question

[glebert]

OK, first off I know that tube testers are limited as to usefulness, especially simpler ones. That being said, I picked up a Mercury 990 for not too much money, which just does basic shorts test and cathode emissions. It seems to work fine for 6L6 and 12AX7 tubes, but the meter doesn't register anything when using 6550 type tubes. I have tried it with GT 6550s and JJ KT88s.

Thoughts?

[Helmholtz]

Thoughts?

Schematic?

[glebert]

Mercury990schematic.pdf
Mercury990schematic.zip

[TomCarlos]

Here is a link to the manual - Mercury 990 Tester

[glebert]

Here is a link to the manual - Mercury 990 Tester

Cool! My manual doesn't have the test and calibration section.

[TomCarlos]

You can download the manual from Elektrotanya... too big to upload here.

https://elektrotanya.com/mercury_990...wnload.html#dl

[nosaj]

OK, first off I know that tube testers are limited as to usefulness, especially simpler ones. That being said, I picked up a Mercury 990 for not too much money, which just does basic shorts test and cathode emissions. It seems to work fine for 6L6 and 12AX7 tubes, but the meter doesn't register anything when using 6550 type tubes. I have tried it with GT 6550s and JJ KT88s.

Thoughts?

Ok what about setting the tester up. Then checking the pins on the tester that should be for the heaters. No heaters no emissions. Ok after looking at the datasheets 6l6 /6550 same pinouts. You could have a dodgy switch connection.
set it to 6l6 and check the voltages on the socket, then set it to 6550 and check the voltages on the socket.

nosaj

[Pedro Vecino]

If you have known parameters for 6CA7 or EL34, use it. At least as a relatively close reference.

[TomCarlos]

According to the manual, the tester has the same settings for both tubes - pages 21 and 47; 7 E 1 8 -

[glebert]

OK, think I got this somewhat figured out. It seems like the 6550 types take considerably longer for the filaments to heat up enough to conduct. Instead of the 1-2 seconds the manual recommends it takes more like 15-20 seconds to get up to full conduction. I just wasn't waiting long enough. Took me a while to figure out because some of my GTs don't conduct no matter what, so don't know if they are dead or this is some other issue.

[Justin Thomas]

Most tube data sheets themselves state an 11-second warmup time for heaters... And my Eico 667 manual says always allow the warmup time spec'd by the tube manufacturers.

Just a different perspective.

EDIT: damn, none of the 4 data sheets I found spec'd a time... But I know that except for some very ancient "quick-heat" PA & battery operated types, 11 seconds seems to be pretty standard. And extra time won't hurt the tubes or tester, just the electric bill...

Justin

[glebert]

Actually in looking I'm not sure where I even got the 1-2 seconds. Maybe from some website. The 6L6 tubes definitely come up much quicker than the 6550s.

[TomCarlos]

Glad you figured that one out!!

BTW, I have an old Sencore Mighty Mite VII. For the most common Preamp and Power tubes, using sockets 1 and 3, I installed "socket savers." These do not interfere with readings and help save the life of the onboard sockets.

Tom

[Enzo]

Only certain tubes have the 11 second controlled warmup. Tubes for TV sets say, where 25 tubes all need to come up at about the same time. Table radios. I recently ran a collection of tubes through my tester, and the 6L6s and 8417s and what not were taking like 40 seconds to come up fully. Tubes with series wired heaters are especially important to heat up in a coordinated manner. Old sets like 50C5, 35W4, and three 12xxx types.

[Helmholtz]

Warmup time also depends on the max heater current the tester can supply.