The nice thing about the 8950 and the M-2057 are that these tubes give you a pair of 1 amp fusible cathode links on pins 2 and 6. You get two chances to destroy each of those tubes.We call this the "blown fuse" syndrome.
The 8950 tube seems to be susceptible to this sort of failure.
This one was.
Learned to eyeball that thin link from pin 3 to the cathode at the center of the tube before even bothering to put it into the tube tester.
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Super scout info needed pls
- Thread starter Cable Guy
- Start date
The glitch resistor makes a huge difference. I back that up with a cathode fuse when a single stage has a bunch of sleep tubes. One extra step when you're dealing with six tubes in a final like this, is to balance them out with one ohm non-inductive resistors feeding each cathode. The fuse handles a combined overload affecting all tubes. The 1 ohm and glitch resistors handle internal tube arcs affecting individual tubes.
Your idea to link the two cathode pins together is exactly what we do when one fails. I don't like to do this in advance, because it will allow twice as much current to flow through each tube and provides no second chance in the event of a severe overload. That's something you do after you've lost one of the two fusible links. Unfortunately, I am only aware of the 8950 and M2057, having double cathode pins and fusible links. The exception to that was National brand 20LF6's. They were the only brand of that model tube to double link the cathode and unfortunately, some amplifiers used the extra pin on the socket as a binding point and you would get fireworks plugging this brand in. The eagle 525 comes to mind as one amplifier that destroyed a batch of new National 20LF6s.
The thing to look for is the plates on the Plate Tune control. The "twin peaks" rule is that in one 360-degree rotation of the knob you should see TWO positions with a peak on the meter. They might be close together, or 180 degrees apart. What counts is seeing two distinct peak positions of the Plate Tune knob.
And if you see only one peak, it's not really a peak. A glance behind the front panel will reveal that the control's plates are either full apart of fully meshed together. The control reached its end of travel before reaching a proper resonant peak.
I'll skip the detailed discussion for now, but making sure the Plate Tune control reaches a proper resonant peak is the best way to prevent a repeat performance from the next set of final tubes. If it won't the tank coil connected to the Tune capacitor will need to be adjusted so the Tune control reaches a peak before running out of travel.
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And if you see only one peak, it's not really a peak. A glance behind the front panel will reveal that the control's plates are either full apart of fully meshed together. The control reached its end of travel before reaching a proper resonant peak.
I'll skip the detailed discussion for now, but making sure the Plate Tune control reaches a proper resonant peak is the best way to prevent a repeat performance from the next set of final tubes. If it won't the tank coil connected to the Tune capacitor will need to be adjusted so the Tune control reaches a peak before running out of travel.
73