1U5 amplifier design, Newbie

Speaker Dude · 3rd May 2009, 04:11 AM

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.

jerluwoo · 3rd May 2009, 07:06 AM

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?

Speaker Dude · 3rd May 2009, 04:22 PM

Yes I have tried a 1U5 and 2 1U4's (very similar tube, in this circuit)

Speaker Dude · 3rd May 2009, 04:32 PM

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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3rd May 2009, 07:06 AM	#2
jerluwoo diyAudio Member Join Date: Feb 2006 Location: Ashland,Ky U.S	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?

3rd May 2009, 04:22 PM	#3
Speaker Dude diyAudio Member Join Date: Oct 2005	Yes I have tried a 1U5 and 2 1U4's (very similar tube, in this circuit)

3rd May 2009, 04:32 PM	#4
Speaker Dude diyAudio Member Join Date: Oct 2005	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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