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Old 3rd May 2009, 03:11 AM   #1
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Default 1U5 amplifier design, Newbie

I making an amplifier using a 1U5 battery operated tube. Being a Newbie I'm a little baffled. Looking at my test circuit #1, I am getting about -0.5V dc on grid 1. If change R1 to 10K the voltage drops to about -0.05V or so and the voltage on the plate drops to about 30V. Where is this voltage coming from and how do I deal with it? I want to keep the input impedance high so I can use a small decoupling capacitor C1.
Also, can I make the tube behave more like a triode by using circuit#2 or circuit #3? Thanks for your help.
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Old 3rd May 2009, 06:06 AM   #2
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Grid leakage would be my first guess. A mere 5 microamps will give you the .5 volts across the 100k. Have you tried more than just 1 of the tubes in the same circuit to see if they all have this behavior?
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Old 3rd May 2009, 03:22 PM   #3
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Yes I have tried a 1U5 and 2 1U4's (very similar tube, in this circuit)
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Old 3rd May 2009, 03:32 PM   #4
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More info- I have designed tube circuits using 12ax7's and 6sn7's but this is my first try at a battery powered tube. I am guessing that my problems are from the direct heated cathode or maybe the fact that this is a pentode. Anyone seen example circuits using these tubes?
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