A friend asked me to make a small and cheap tubes amplifier to get closer to the world of tubes. I thought up and built this project using components already in my possession. The amp sounds good but when I turn the volume up over 1/4 of its drive it starts to distort, regardless of the strength of the incoming signal ... what's wrong?
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If you want to run the EL84 as a pentode, you need to put feedback around the amp. This will reduce distortion and output impedance to levels that make the amp useful.
If you don't want to implement feedback, you can triode strap the EL84 and that will work nicely with the rest of what you have here. You won't get much power because of the 7K load, but you'll have a more useful amp.
If you don't want to implement feedback, you can triode strap the EL84 and that will work nicely with the rest of what you have here. You won't get much power because of the 7K load, but you'll have a more useful amp.
From the Philips EL84 tube characteristics sheet
Your operating point will produce about 4 watts at 10% distortion with 9% at 3rd harmonic.
How much power do you need?
Steve
Your operating point will produce about 4 watts at 10% distortion with 9% at 3rd harmonic.
How much power do you need?
Steve
they are definitely not good data ... but the problem is not the harmonic distortion, but that the amplification reaches its maximum already at 1/4 of the volume.
In triode mode the 6AU6 has a gain of 36.
At the chosen OP 5 volts will drive the EL84 to full power. That means an input in the range of 200 millivolts. What is the input level at 1/4 volume on your pot?
Per audiowize's suggestion you could implement NFB to reduce the gain and improve the distortion.
At the chosen OP 5 volts will drive the EL84 to full power. That means an input in the range of 200 millivolts. What is the input level at 1/4 volume on your pot?
Per audiowize's suggestion you could implement NFB to reduce the gain and improve the distortion.
If you end up needing more gain to be able to implement feedback in the amp, the 6AU6 can be wired as a pentode.
I don't think so because Va in the schematic is too low for that.
Ohm’s Law learns us that Rdc of the primary is 500 Ohm (pretty high).
I doubt that you will reach your 237V after the bridge rectifier (but who knows).
So the two EL84’s take 34 mA each, so 68 mA for both. Kirchhoff’s Law learns us that the rest of your amplifier would have to be taking the rest of the 100 mA, which amounts to 32 mA.
But the two 6AU6’s are not taking 16 mA each; they are taking 3 mA each in the schematic (I didn’t check if they realy would with the values for Rk, Rl, etc.).
So to your question in post #1 what is wrong with this amplifier: Your measurements to begin with...
Edit: I have to correct myself on the calculation of Rdc of the primary. It will be less than 500 Ohm because I calculated it with the G2 current, which ofcourse is not flowing trough the primary. I stand behind the rest of my posting though.
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If your pot is linear type, and you don't have any log pot available please connect 15k resistor from middle pin of your pot to ground.
If the transformer is from a standard tube radio, the usable power is only 2-3W, and less than 1W at 100 Hz. But: You said there is distortion regardless of the signal strenght. This could be a issue of the 6AU6 stage. Have you tried to use it in pentode mode with the datasheet values? It might not sound the best, but it's a good place to start. I attach the GE datasheet table. Check the values Ebb=180V.
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pcan
He said the amp sounded good until he turned the volume pot past 1/4. If it takes less than 200 mv to get to full power that will be achieved between 1/4 and 1/2 turn on a linear pot.
I asked how much power he needs/expected.
Going to pentode mode on the 6AU6 will increase the gain even more.
Steve
He said the amp sounded good until he turned the volume pot past 1/4. If it takes less than 200 mv to get to full power that will be achieved between 1/4 and 1/2 turn on a linear pot.
I asked how much power he needs/expected.
Going to pentode mode on the 6AU6 will increase the gain even more.
Steve