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diyAudio Member
Join Date: Jul 2005
Location: West Virginia panhandle
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Quote:
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The JJ EL509 claims a 700 volt rating on the screen grid. If this is true it will be useless for screen driven applications.......Can you explain me why?
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There are a few of us here that have played with screen driven amps. Notice that the common element among all of the successful designs are they all use sweep tubes. My experience is that the lower the maximum screen voltage rating, the easier it is to drive.
The screen grid has almost as much control over the current flow in a pentode as the control grid. Like the control grid it is possible to wind the grid wires with a fine or coarse pitch, and the spacing between G1 and G2 can be varied. A fine pitched G2 winding will exert more control over the current flow as will a close spacing between the grids. Both of these things will also cause G2 to intercept more electrons and therefore draw more current. Placing G2 in the shadow of G1 is another tube makers trick to reduce G2 current.
Most of the typical audio tubes like the 6L6GC, EL34, KT88 and so forth have G2 optimized for operation at voltages from 350 to 500 volts. This implies that G2 will have some control over the tube current, but a large change in G2 voltage is needed for a big change in tube current. Attempting to screen drive a KT88 requires about 500 volts Peak to Peak of drive voltage. The linearity under these conditions sucks too. I have tried it.
Most sweep tubes have a maximum G2 rating in the 175 to 250 volt range. A small change in G2 voltage can make a big change in plate current. A 6BQ6 or a 6AV5 can be driven to 80 watts in P-P with about 150 volts Peak to Peak of drive and the linearity is very good.
Further investigating of the JJ specs leads me to believe the 700 volt G2 spec is not real. Look at the spec for the original EL509. The 700 volt G2 spec is for a non conducting tube. The maximum voltage spec for G2 in operation is 250 volts. I don't know what the real spec for the JJ tube is. If the tube can operate with 700 volts on G2 then it will take more drive voltage than you can make. If that spec was blindly copied from the original with some info lost in translation (which is what it looks like) the JJ tube may actually work. I have not tried one, and the JJ EL509 does have a reputation for blowing up so it may actually be a real sweep tube.
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EL509 (JJ) or PL519?
