B&K 747 Tube Tester - Page 1
Forum section: electronics / beginners

author: Wil_Bloodworth
author_rank: Regular Contributor
timestamp: January 04, 2024, 03:54:52 am
content: Happy New Year!I figured I needed a tube tester and couldn't get the EICO 666 or TV-7 that I wanted so I went ahead and got a B&K 747 (not the 747B) and it arrived today! I've have to say, I really like the way this test is laid out as well as how it works. It's a nice looking tool as well which may lead me to purchase more B&K items in the future.Ok, enough with the rigmarole! I popped the hood, took out the screws, and flipped it over to see what was going on. I can tell it has had a few playground brawls that didn't end well but nothing major. I tried to test a few capacitors and they all tested badly but tested them in circuit so I'm assuming that could be the problem.All of the original capacitors are in this and I plan to replace all of them as well as the aging carbon composite resistors. That way, I know it will be reliable for at least the rest of my life.I have a few questions which are all related to tubes and this tester.Question 1:Assuming the tester correctly as was stated by the seller, the manual says,"Do not plug the tube into the socket until the program switches are properly set."If that is the case, then when I am trying to test a tube that has more that one "set" of settings to test... such as a 12S8 (which has 4 separate tests). Does that mean I need to configure the settings for test one, insert the tube, let it warm up, test the tube... then remove the tube from the chasis, reconfigure for test #2, reinsert the tube... and repeat over and over? It just seems a bit time consuming to have to remove and replace the tube over and over every time I have to change the settings.Question 2:I tested a tube's "Grid Emission" and the manual says that if needle moves into the "Grid Emission-Reject" section, the tube is defective. The tube in question tested just fine other than failing the grid emission test. What does failing the grid emission test actually mean? Should I care?Question 3:I ordered a couple of 12SQ7-GT tubes from a reputable dealer and both tubes tested on the "iffy" side. They were listed as NOS and were not cheap; around $22 a piece. I don't know if I should trust the tester but I did test another 12SQ7-GT that I already had and it tested somewhat fine. Should I just recap the machine and then recalibrate it or maybe have someone else test these tubes before I order more from that vendor?Question 4:There was a questionable "fuse" with a resistor inside. This weird resistor/fuse thing was blown and someone had piggy-backed a fuse holder thingy to it... sorry no photo of the contraption... effectively, there is a normal fuse sitting on top of the old resistor-based fuse so that the circuit is bypassing the old one (photo below). It's late or I would post a photo of the entire thing (which I might do tomorrow). Should I be worried about this "bypass" hack?Thank you,- Wil

author: CaptDon
author_rank: Super Contributor
timestamp: January 04, 2024, 01:52:44 pm
content: Not inserting the tube until properly set up will prevent you from blowing up a 6 volt tube if the last one you tested was 12 volts and 12 volts is still being applied to the socket. You can move the remaining settings for multi element tubes after the initial setup for that tube. As for grid emission, generally speaking a grid will have no emission unless it is heated to incandescent as happens sometimes in powerful amplifier tubes in RF work and sometimes when a bias circuit fails. More often grid emission comes when a tube is very gassy and has the tell tale sign of blue glow within the grid elements. Many power tubes will show a blue glow on the inside edge of the glass and this is somewhat normal. As for your purchased tube testing low or lower than one you already own, this seems to indicate your purchased tube may have many hours on it. I think your tester is only an emission tester based on cathode emission and is not a 'dynamic' tester. Cathode emission testers are a bit subjective and the very same tube made by 5 different manufacturers will probably show 5 different 'quality' readings even when new!! All 5 however should be capable of testing in the green 'GOOD' area which guarantees it meets the minimum emission specified for a new tube. Sadly an 'emission' tube tester can also indicate that a tube is good but the same tube can be nearly useless for work as an RF or IF amplifier due to other factors within the tube. As for the fuse bodge, it should be fine. The original solder in fuse was probably a low amperage (.5a or less) GJV style like GJV 1/4. Cheers mate!

author: Wil_Bloodworth
author_rank: Regular Contributor
timestamp: January 04, 2024, 03:07:02 pm
content: Thank you Don!Quote from: CaptDon on January 04, 2024, 01:52:44 pmThe instruction manual specifically refers to the program switches and at this point in the manual, the heater voltage and sensitivity have already been setup. This warning is specifically about changing the program switches.Quote from: CaptDon on January 04, 2024, 01:52:44 pmWell, the writing on the front of the machine begs to disagree. LOLQuote from: CaptDon on January 04, 2024, 01:52:44 pmI went ahead and took it apart again for this photo. Seems a git dodgy but I'm sure this issue is the least of my problems.Cheers!- Wil

author: Wil_Bloodworth
author_rank: Regular Contributor
timestamp: January 04, 2024, 03:15:44 pm
content: And I am apparently wrong... mine is the 747 "B" version.- Wil

author: CaptDon
author_rank: Super Contributor
timestamp: January 04, 2024, 05:02:05 pm
content: I just pulled my old 607 down off the shelf. The setup instructions for it start with POWER OFF. They first say to set the filament voltage and then the pushbuttons for configuration and THEN press the POWER pushbutton which latches on. You have to press it again to power off. Very strange. We left it on all the time and just preset the buttons and then plugged the tube in. It does seem that yours is a dynamic 'gm' transconductance (gain) tester. That is a better tester where the actual gain of the tube is a measurement of its quality. Sadly, my old EICO is only an emission tester. I am not sure about the B&K 607 'DynaJet'. At the T.V. repair shop if we ever had a doubt we just tried a new tube. The old Sylvania G.T.E. chassis (there were several different ones) that used a 6GH8A in the color section were finicky about the tube. They would lose color lock or other weird color problems and a new tube would cure the problem. I don't think I ever tested an improperly functioning 6GH8A tube that actually tested bad. I bet it would show on a curve tracer however.