KLUDGE@AGCB1.LARC.NASA.GOV (10/10/90)
That's it. I've had enough of crappy Chinese KT88's which suddenly decide to pull tremendous amounts of current, opening up the cathode resistor and glowing red hot. This has happened three times on my Citation, with three different tubes. Never happened at all with the old Mullard tubes. What's the problem here? It's not as if the driver stages were going into oscillation or anything catastrophic, but it happens repeatedly. --scott I've had this problem many times, watching smoke billow out from the base of the tube socket. In every instance, the solder between the tube lead and the pin was poor, or nonexistant. Resoldering the pins fixed it. -tjk]
mikes@iuvax.cs.indiana.edu (Michael Squires) (10/11/90)
In article <6866@uwm.edu> KLUDGE@AGCB1.LARC.NASA.GOV writes: >That's it. I've had enough of crappy Chinese KT88's which suddenly decide >to pull tremendous amounts of current, opening up the cathode resistor and >glowing red hot. This has happened three times on my Citation, with three This was fairly common ten years ago with the Dyna MKIII and British Gold Lion KT88's. The problem was the tube socket which was either improperly soldered or whose contacts were no longer making good contact. I wound up replacing sockets in my MKIII's with milspec sockets and the problem went away. If a tube is gassy it can also run away in this way; I would inspect the tubes in operation, looking for something other than normal operation. Finally, some tube amps ran the outputs red hot; the Eico HF-89 ran 6CA7's with the plates glowing when properly biased. That's how they got 50 watts output from the tube.... -- Mike Squires (mikes@iuax.cs.indiana.edu) Phn: 812 855 3974 (w) 812 333 6564 (h) mikes@iuvax.cs.indiana.edu 546 N Park Ridge Rd., Bloomington, IN 47408
lstowell@pyrnova.pyramid.com (Lon Stowell) (10/12/90)
The old Dynaco actually had a rather nice blue glow at higher levels... As a matter of course, it is not a bad idea to touch the pins of the tubes themselves with a good soldering iron and GOOD solder. Dip the pin (or paint it) with liquid ROSIN flux, then hit it with a good iron and, ideally, a tiny touch of a good silver-bearing solder..... plopping the pins in Tweak or Cramolin helps, for the same reason it helps phono jacks not gold plated.... The mil-std sockets are a great idea, although if you are mechanically handy you can get a better contact by adjusting the sockets with a pair of fine jeweler's needle-nose.. Idea is to ensure an absolutely tight mechanical and electronic seal...
alp@techbook.com (Alan Peterman) (10/12/90)
In article <6866@uwm.edu> KLUDGE@AGCB1.LARC.NASA.GOV writes: >That's it. I've had enough of crappy Chinese KT88's which suddenly decide >to pull tremendous amounts of current, opening up the cathode resistor and >glowing red hot. This has happened three times on my Citation, with three >different tubes. Never happened at all with the old Mullard tubes. What's >the problem here? It's not as if the driver stages were going into >oscillation or anything catastrophic, but it happens repeatedly. >--scott Also check the blocking capacitors to the grids of the KT-88's. They block the high voltage from the plate of the drivers from the negative bias on the grid. When they start to leak, the grid becomes less negative increasing the current - until meltdown. They tend to leak more when warm, so the amp runs OK for a half an hour (or two hours) and then starts to self destruct. If they haven't been replaced, they are due to fail - replace all four. They're lots cheaper than the tubes! -- Alan Peterman (503) 684-1984 (hm)