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What exactly plate/grid caps do?
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"The point is, you see, that there is no point in driving yourself mad trying to stop yourself going mad. You may just as well give in and save your sanity for later." ~Douglas Adams -
On some tubes - like the 6146B in the first SVTs or a 6K5 in something old - instead of all the internal elements being wired to a pin on the base, sometimes they bring an element out through the top and put a cap there. A wire is run up from the chassis and clipped on.
In the case of the 6146B, a hefty power tube, it is the plate. Same on an old TV sweep tube. The TV tube might see 1500 volts, the 6146 was only good for 600v, but still, that is a lot of voltage to be one or two socket pins away from a grounded heater line or a negative control grid. So the plate cap keeps the high voltages away from everything else. Remember your plate might be running at close to 500 volts, but the signal peaks can be substantial. We don't see plate caps often in audio, but in high power radio transmitters, they are a lot more common. And 3000 volts on a plate is more common as well.
In the case of a grid cap, typically the wire from underneath was shielded, and there would be a metal cover over the clip and top of the tube. In this case, the grid is a far as possible from the AC heater wires and other sources of noise or crosstalk.Education is what you're left with after you have forgotten what you have learned. -
Rad, thanx."The point is, you see, that there is no point in driving yourself mad trying to stop yourself going mad. You may just as well give in and save your sanity for later." ~Douglas AdamsComment
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