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In Reply to: B&K Dynascan 747 tester tube tester posted by tesla on April 18, 2004 at 01:36:36:
You may not need to do much at all. After exhausting all the tips that Jim and other members offered to no avail, I changed the two light bulbs (#44, I believe). Tester calibrated up right away. Try that first, but you may want to change a few of the caps and such anyway.
You can tell if the bulbs are good because they should both glow dimly and equally. The sockets can be troublesome. Correct bulb for a 747 is a #55.
Is it difficult to get these bulbs. Any where i can order online. How do you check if bulbs are bad and need replacing?
Antique Electronic Supply has the bulbs listed on page 58 of their catalog. They have all those hard to find Hickok bulbs also. Part number for the #55 is P-55 and shows $2.95 for a pack of ten bulbs. Really, the bulbs rarely go bad. They should just glow equally in the sockets--if they do they are good. You would not want a bright bulb and a dim one. There's more trouble with poor connections at the socket, than there is with the bulbs themselves.
It was also mentioned that the caps shouyld be changed. I have the new ones, though not i axial form. The original ones have an arrow pointing to one lead with the negative sign in it. Just never dealt with these caps before, do i presume right that the arrow point to the negative?
I just want to be sure as I did adjust the set as the manaul states but get very low reading with all my Tungsol 6550 only in the middle green range which is questionable, even for the tubes that we bought as new and sure look it. but my svetlana show much better a reading. Any clues or my tubes are bad?
Anything else i can do to be sure the tester is reading reasonably-i got it off ebay.
Modern electrolytic caps have the arrow pointing to the negative, and it's easy to confirm this. The can will be negative and the lead coming from the insulated end of the cap will be positive. Old electroytic caps tended to have the positive end marked--why they changed is anyone's guess.
Tubes that read mid-green on the meter should be fine. Mid-green is where new tubes are supposed to test. Just because the other 6550's test higher does not mean that they will perform any better in the amp or last any longer. Everyone using tube testers needs to remember what they were designed to do: find bad tubes. Bad tubes are those that are likely to cause trouble in tube gear due to inadequate gain, shorts, or grid current. Tube testers were not designed to determine the "goodness" of a new tube, or determine if a tube had ever been used before. Determining "goodness" was a job for lab equipment, and determining if a tube has ever been used comes closer to the crystal ball department.
When I was talking about the can being negative I was talking about the axial caps seen on most older equipment. Modern radial caps may not have the can as part of the cap's circuit.
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