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Ray di Tutto" wrote:
One point you mentioned is very interesting. You wrote that when a tube gets gassy, plate current goes up to the point the plate can turn red hot. I'm no specialist and i would have thought that the gas molecules in the tube would be an obstacle for the electrons so plate current would decrease. Vy 73 de HB9SLV Sorry to be so late getting back to you on this. I had forgotten I left a thread open over here on .boatanchors... What really happens is the gas molecules (atoms) become ionized due to the high plate voltage accelerating the plate- current electrons to energies above their ionization potential (typically 10 - 15 electron volts). When this happens the now-ionized gas becomes a very good conductor (almost a dead short) and the control grid is totally ineffective in limiting the cathode-to-plate current flow. The only thing that can halt the process is the removal of the plate voltage. Unfortunately, there is usually enough energy left in the conduction electrons striking the plate to increase its power dissipation disastrously. This process is actually used to good effect in mercury- vapor rectifiers and thyratrons because that "dead-short" effect means the losses through the conducting tube are low. Both of these devices "fire" (ionize) at some forward voltage and then stay in conduction until the AC driving them reverses polarity. Jim Bromley, K7JEB Glendale, AZ |
Usually they don't fail unless abused i.e. not tuned properly, causing
excessive plate dissapation. Internal elements occasionally fail and short, such as grid to cathode, or screen to plate, any of these can cause symptoms as you've described. Since a new set of finals seems to have cured the problem, I wouldn't worry about the exact mode of failure, just treat the new tubes with care and they should last for years. Gary Hildebrand WA7KKP Angel Vilaseca wrote in message . .. Hi I was recently given a TS 820 and I noticed it behaved strangely: -output power was only 50 W CW instead of 100. -one of the tubes plate (the final stage of the TS820 uses two 6146B tetrodes) glowed red after a few seconds of xmitting. - on the 40 meter band, when tuning for max RF out while watching the built-in dial, you could reach 100 W output and even beyond, then the rig fuse would blow. (I did this only once, before taking out a screwdriver and opening the rig, realizing that something was really wrong!) I ordered a new set of 6146Bs and while waiting for them to arrive I checked all components and voltages in the final stage ( not an easy task when one knows how the 820 PA is built!). All were OK. Replacing the 6146Bs with the new ones cured the problem. Now I should be happy, since my rig is working again right? wrong! I would like to understand what was going on when running with one of the tubes bad. -OK, putting 50 watts out seems only logical if one of the tubes is bad, but -why was one of the tubes plate glowing red? -Was it the good or the bad tube? Which one of the two can I keep as a spare? -And why dit it glow red? Could it be that the grid could not stop the electron flow and the power dissipated in the red hot plate was DC power? Was it that RF from the good tube went into the bad, dissipating into heat? A more general question: how do the 6146B tubes fail tipically? Do they fail quickly or progressively? Is it because the grid melts? what about the screen? what about cathode emission? what about secondary emission? I read that this was a common problem in the 4CX250B, another tetrode. What is the best parameter to monitor to see if the final 6146B tubes start getting tired? Retrospectively I realize that i operated the rig for quite some time with only one of the final tubes working! *blush* Could this have caused some damage to the circuit? or spewed out a "dirty" signal? 73 de HB9SLV |
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