but, I could use som thought on another problem.
Here is the amp I'm building. http://213.67.45.70/se_kt88_3.gif
When I firts tested the circuit I noticed that the input stage clipped quite early wich was expected. However, on my inital measurements on just the input stage I got almost 120V PP signal, but when I hooked it up to my KT88 I didn't even get half of that. Then I got an idea. I knew that the 6n1p didn't have any problem driving a 470k load, so I inserted a 470k resistor right after the 0.22uF cap. That way the lowest possible load for the input stage would be 470k. (This is just a crazy test) Now I got my 120vpp again. But instead the KT88 clips almost as early as the 6n1p did before. Clipping only occurs at one side of the signal. I've posted a question similar to this one just a couple of weeks ago, but that time, I wanted to know if it was possible to offset the input stage. This time I wonder if it's posible to offset the KT88 so that it doesn't clip so early. One half of the 6n1p shouldn't be able to drive it into clip, should it???
Here is the amp I'm building. http://213.67.45.70/se_kt88_3.gif
When I firts tested the circuit I noticed that the input stage clipped quite early wich was expected. However, on my inital measurements on just the input stage I got almost 120V PP signal, but when I hooked it up to my KT88 I didn't even get half of that. Then I got an idea. I knew that the 6n1p didn't have any problem driving a 470k load, so I inserted a 470k resistor right after the 0.22uF cap. That way the lowest possible load for the input stage would be 470k. (This is just a crazy test) Now I got my 120vpp again. But instead the KT88 clips almost as early as the 6n1p did before. Clipping only occurs at one side of the signal. I've posted a question similar to this one just a couple of weeks ago, but that time, I wanted to know if it was possible to offset the input stage. This time I wonder if it's posible to offset the KT88 so that it doesn't clip so early. One half of the 6n1p shouldn't be able to drive it into clip, should it???
I have roughly 36V at the cathode. When I had a 470ohm resistor I got 68mA at the plate and 8mA at the screen. This might be a little low, but the next resistor value seem to be 330ohm which I think might be a bit too low. I tried it breifly and I think I got something like 90mA at the plate and 10mA at the screen.
I have hooked up my single channel experimental amp to my home-made 2-way book-shelf speakers. I have only tested it with the speaker standing on my kitchen table i the middle of the room, which makes the sound thin without a lot of bass, but the sound is beyond my wildest expectations. Insanely dynamic and tight.
I think I'll try to make a 2 channel test-amp so I can hear how it sounds in stereo, and in a better room. Since my speakers are only 88dB, I think the output power might just be a tiny bit too low, but again, I'll make a more certain statement when I've listened to it some more.
I think I'll try to make a 2 channel test-amp so I can hear how it sounds in stereo, and in a better room. Since my speakers are only 88dB, I think the output power might just be a tiny bit too low, but again, I'll make a more certain statement when I've listened to it some more.
Good to hear!
Stick to the 470R build the other one...and go listen...then you have 2 tools...your measuring equipment and ears...Trust your ears! But let the test equipment guide you!
Cheers,
Bas
Have a good and SAFE weekend...don't hurry ......take things easy...I don't know if you've done a lot at these voltages...but WATCH out..I still get shocked from time to time....like last week I wanted to feel how warm my CCS were getting!! The heatsink at 320V on them..damn that hurt..
I just put in a 330 resistor at the cathode and measured. However, since I've returned one of the borrowed DVM's I could only measure the current going through the transformer, and NOT at the plate and screen individually. At 400V i got 99,7mA through the transformer. When I measured VDC at the cathode I got 33 now so I guess I just have to build 2 channels and listen to it to figure out which works best.
Lesse...
Cathode voltage subtracts from B+. You do indeed have about 360V.
The reason you get clipping is because the 6N1P can't source enough current into the KT88's grid. When the grid goes positive it starts drawing current like a screen, only since it's closer to the cathode it pulls a stronger current for a lower voltage. This makes the grid like a diode, and the voltage that develops goes across the coupling capacitor and grid leak (the 470k). As the DC grid voltage goes negative, it biases the KT88 to a higher class - AB, then B, then C. This is accompanied by extreme amounts of distortion on the output, since there is no second tube (as in push-pull) to fill in where this one shuts off.
But note that full power occurs when the grid is *just* crossing zero volts (with respect to the cathode). Any more and the grid-leak biasing occurs, and severe distortion ensues. So don't worry about it! It just means the 6N1P stage has much less distortion than the KT88 when it's at maxium output.
Tim
Cathode voltage subtracts from B+. You do indeed have about 360V.
The reason you get clipping is because the 6N1P can't source enough current into the KT88's grid. When the grid goes positive it starts drawing current like a screen, only since it's closer to the cathode it pulls a stronger current for a lower voltage. This makes the grid like a diode, and the voltage that develops goes across the coupling capacitor and grid leak (the 470k). As the DC grid voltage goes negative, it biases the KT88 to a higher class - AB, then B, then C. This is accompanied by extreme amounts of distortion on the output, since there is no second tube (as in push-pull) to fill in where this one shuts off.
But note that full power occurs when the grid is *just* crossing zero volts (with respect to the cathode). Any more and the grid-leak biasing occurs, and severe distortion ensues. So don't worry about it! It just means the 6N1P stage has much less distortion than the KT88 when it's at maxium output.
Tim
I just wanted to test a little theory...
When I began with this design, I assumed that I could just take a Push-Pull amp and split it. I'll explain what I mean.
I looked around quite a lot before i made my design. I found a few PP-KT88-amps that could deliver almost 30-50W/channel, therefore I assumed that if I built a single-ended-amp, I would end up with almost 25W if I just built a big input stage. But from what I understand now, a tube can just produce one side of the signal at a high level. (My english isn't the best, hope you can understand what I mean) Is that why people build PP-amps instead?
When I began with this design, I assumed that I could just take a Push-Pull amp and split it. I'll explain what I mean.
I looked around quite a lot before i made my design. I found a few PP-KT88-amps that could deliver almost 30-50W/channel, therefore I assumed that if I built a single-ended-amp, I would end up with almost 25W if I just built a big input stage. But from what I understand now, a tube can just produce one side of the signal at a high level. (My english isn't the best, hope you can understand what I mean) Is that why people build PP-amps instead?
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