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I am in need of a 6u5 tube and noticed the 6e5s for sale on ebay. My question is will it work but be more sensitive?
The 6U5 has a 6-pin base, whereas the 6E5S is an octal base. The 6U5G has an octal base, but the pin-out is different from the 6E5, so you can’t use the 6U5G as a plug-in substitute for the 6E5.
You could rewire the base to suit, and it might be OK, but you may need to tweak some components.
Yes, but the 6E5 is more sensitive – it needs about -22 volts to fully close the eye on the 6U5 and only -7 volts on the 6E5. A 50/50 voltage divider on the grid circuit may work to bring the 6E5 sensitivity down (maybe try a couple of 5.6Mohm resistors?). However, I think the 6U5 has remote cutoff characteristics and the 6E5 sharp cut off, giving the 6U5 greater sensitivity at lower (negative) grid voltages and less sensitivity as the (negative) grid voltage increases, ie non-linear response, whereas the 6E5 response is more linear – better-suited to instrumentation applications.
The resistor voltage divider works because when two resistors are connected in series and you connect a voltage to one end of the series pair, and another voltage to the other end, the junction between these resistors will have a voltage somewhere between the two outside voltages. For a 50/50 divider, you make both resistors the same value and the middle voltage will be exactly half-way between the two applied voltages.
Actually, that needs some further information! The theory only works if both resistors have the same current going through them, so it is important that when you are tapping the 50% voltage, you don’t take much current, otherwise one of the resistor pair will be drawing more current than the other one, and that will change the middle voltage. That means that the thing you connect to the junction must have a very high impedance – like most terminals on a tube.
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