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Old 13th April 2005, 10:30 PM   #1
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Default Tube Headphone Amp Troubleshooting Help

Background: Newbie with basic understanding of electronics from books and I completed the ELLA kit without any troubles. This is my first scratch build. I have posted this same request to the Headwize forum but thought some experts here could diagnose what I am hoping is a basic electronics problem.

I just finished the 6N1P project from Bruce Bender and using a standard Sony portable disc player and Sennheiser 600s I am getting a lot of distortion at higher levels. I think distortion is the correct description - basically sounds like static or white noise when the volume gets too loud. If I keep the volumes pretty low (below usable) all of the music sounds fine, just very faint. If I increase to an appropriate listening level it starts to distort portions of the song and is completely unlistenable at high volumes.

I followed Bruce's design exactly except I did not use the illuminated power switch (just a basic black rocker switch) and I didn't have a 44K resistor for R2 so I used 47K.

Two additional pieces of evidence that might help diagnose.

One, the volume pot (Alps Blue Velvet 100K) on the amp has virtually no impact on the audible output. Only changing the volume on the disc player makes a difference. I'm pretty sure it is connected correctly because I can measure a range of resistance from the input jack to V1 input for each channel between about 0 and 99K as I turn the shaft.

Second, my voltage readings on B+ are higher than what Bruce published. Instead of 348VDC I am getting closer to 380VDC. Could this be cause the distortion? Likewise, I'm getting higher voltage at the other points he measured - 187 instead of 172 at the intersection of R2 and V1, etc.

Click the image to open in full size.

If the cause is high B+ voltage, what is the best way to bring the voltage down? Just increase the resistor values on the supply? If so, can someone help me approximate the new resistor values?

Click the image to open in full size.

Finally, any thoughts as to why mine is so much higher than his? I used the exact same parts (with the two exceptions noted above) and my outlet is right at 120VAC. I'm showing 288VAC when I measure across the two red wires from the Hammond 269AX instead of the 250VAC spec.

I appreciate any help.
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Old 13th April 2005, 11:55 PM   #2
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Distortion sounds rough, not static-ey.

If the pot is indeed connected right (instead of input to grid, try measuring grid to ground), it should work exactly as well (in fact exactly the same) as the CD player's output control.

If it is connected right, your circuit might be oscillating. That would be supported by the unusual behavior of static(??).

The slightly higher voltage is just fine. Anything within 10% is fine, matter of fact.

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Old 13th April 2005, 11:58 PM   #3
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Yup. Oscillation. And probably a miswired pot- I'd suspect that its ground lead is floating.
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Old 14th April 2005, 05:30 PM   #4
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Thanks for the input. I learned that the issue with the volume pot is I did not have the ground connected. I will correct that tonight.

Is it possible this is the cause of the distortion?

Perhaps static is not the best description of the sound. At medium outputs it's more like the sound you get if you try to speak with piece of paper across your mouth - you hear the paper vibrating more than the actual voice. At higher levels it eventually becomes more like a harsh popping and snapping sounds. Almost like the sound of burning wood.
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Old 15th April 2005, 02:49 PM   #5
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I have the volume pot straightened out but wanted to provide one more piece of evidence that I just stumbled upon last night:

If I have the amp running with a CD playing and turn the amp off for a second or two, and then turn it back on (keeping the CD going) the music sounds perfect for a few seconds. Excellent sound level and no distortion. Then, over a period of about 5-10 seconds (without touching the volume), the level drops gradually and starts to distort gradually, eventually ending at the state I described in my initial post. I can repeat continually with the same result.

I was hoping this might point to a particular problem.

Thanks again.
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Old 15th April 2005, 03:28 PM   #6
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What is the voltage across the two cathode resistors, on each channel?

Quote:
Almost like the sound of burning wood
Sounds like something is not soldered properly...cathode resistors or so.....take the schematic...write the pinout on the schematic and check everything again. If everything checks out ok..reheat all the solder joints.
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Old 15th April 2005, 03:47 PM   #7
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At R3 I get 3.08 on one channel and 3.21 on the other. At R5 I get 6.52 (same channel as 3.08) and 5.88 (same channel as 3.21).

Just to confirm, I measured the voltage by placing the black lead on the ground point and touched the red lead to the various points. I think that is the correct way but the phrase "across" always makes me think one lead on each side.
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Old 15th April 2005, 03:54 PM   #8
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Quote:
At R3 I get 3.08 on one channel and 3.21 on the other. At R5 I get 6.52 (same channel as 3.08) and 5.88 (same channel as 3.21).
Mmm that is way out of spec! The other high voltages are not a problem...but there is definately something amiss here.

Quote:
measured the voltage by placing the black lead on the ground point and touched the red lead to the various points
Correct!

BLACK LEAD : To ground
RED LEAD : Yep...as long as the various points are the right ones....that would be pin 3 and pin8

Did you use one 6N1P for both channels in the V1 spot?


If so 2 R3's connected to first valve ...for example left channel ..PIN 3...right channel PIN8

Second valve is seperate per channel...so you'd be measuring to 2 pin 8 or two pin 3's .. depending on which half was connected to earth..
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Old 15th April 2005, 04:09 PM   #9
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Yes, I used one 6N1P for V1. And yes, those are the spots I am touching.

Does the voltage from the power supply effect the cathode voltage? In other words, are my R3 and R5 high because my PS voltage is high?

Thanks for helping. It is much appreciated.
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Old 15th April 2005, 04:21 PM   #10
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Quote:
R3 and R5 high because my PS voltage is high?
Only to a very small extent...not like this..R3 should be 1V and R5 should be 2VDC..in other words there is far to much current running through them..that explains the crackling sound...and why it sounds ok when you just turn it on...the valves are not warm yet..and are not pulling their full current....

R U shure that you've chosen the right values for R3 and R5?

R3 = 750 ohm and R5 = 220 ohm?

What I don't understand is why your B+ would be so high? With the higher current your tubes are drawing the drop over the b+ resistors would be greater as well....something fishy going on here.
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