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Old 27th August 2011, 06:26 PM   #1
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Default Output signal distortion

Hi,
sorry for the double-post from here Tubelab SE checkout shows -300V Bias voltage?

Since the question is a generic, I hope to get a larger audience with potential ideas.
The scope screenshot shows the full 440Hz sine input signal (before pot) at the top, and the speaker output signal at about 3/4 of the volume at the bottom.
Where should I start looking for the cause for the distortion visible on the decaying side of the sine wave? It doesn't sound nice when the input signal is music, but is OK on lower volume.

Any thoughts from folks who have seen something similar and can provide some guidance would be appreciated.

Thanks!
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Old 27th August 2011, 08:14 PM   #2
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My first guess would be some kind of output tube loading issue -- with the transformer, or otherwise -- particularly since the test frequency is midband. What power level does the bottom display represent?

Dave
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Old 27th August 2011, 08:37 PM   #3
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Originally Posted by dcgillespie View Post
My first guess would be some kind of output tube loading issue -- with the transformer, or otherwise -- particularly since the test frequency is midband. What power level does the bottom display represent?

Dave
It starts at about three quarters of the full volume. Output tubes are Chinese 300B (Guigang), OPTs are UBT-3.

I have another set of 5K transformers that I can try, but they look a little skimpier than the UBT-3
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Old 27th August 2011, 09:40 PM   #4
DF96 is offline DF96  England
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Grid current limiting?
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Old 27th August 2011, 09:46 PM   #5
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Had the same in a SE amp I built when the bias was off, mind you I only looked at it around 100Hz..
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Old 27th August 2011, 09:49 PM   #6
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Originally Posted by DF96 View Post
Grid current limiting?
I just did some more testing and the symptoms are similar across the frequency band, although more pronounced at low frequencies, which seems reasonable.
What I did notice - in support of your theory - is that one channel is worse than the other. The 'bad' side's grid resistor current rises with volume much more than the other side. I stopped at 130mA to not artificially limit the tube's life.

Could a bad MOSFET be the cause? I'll go back and check to make sure I put the correct 20K resistor on both sides. If OK, I guess I will replace the MOSFET and see what happens.

Thank you!!!
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Old 28th August 2011, 12:29 AM   #7
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Triple checked all components for correct value, but the problem persists. Swapped output tubes, but the problem didn't follow. It's way more pronounced on the right channel than on the left, although visible there too.

Re-adjusted bias and it remains stable for the lower half of the volume spectrum, after which the right channel rises quickly. At full volume, right channel bias current is almost double of the left channel.

Haven't replaced the MOSFETs yet, because they are a Royal Pain Diaz to get out (heatsinked and bottom-mounted), but that's the last thing to try, I guess.

Music starts to sound pretty awful when played at higher volume.

I noticed that the sine wave distortion seems to affect the falling flank on the signal with lower frequencies (<400Hz), but moves over to the rising flank on higher frequencies (which is when the cats in the neighborhood started whining... ).

Anyway, if anybody has another idea of what may be going on, please let me know.
Cheers,
Stefan
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Old 28th August 2011, 12:38 AM   #8
kmaier is offline kmaier  United States
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Default Diagnosis

Well, you've scoped the amplifier input signal and output signal to the load. You have a linearity problem... so, why haven't you scoped the grid of the output tube to see what it's drive signal looks like? If it's the same, then the output stage is just following input and you back up to the next stage of the circuit. If the grid drive matches the input signal then you've isolated the problem stage. I'd scope each stage input/output to see where it breaks down... why guess?

Regards, KM
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Old 28th August 2011, 01:25 AM   #9
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Originally Posted by kmaier View Post
Well, you've scoped the amplifier input signal and output signal to the load. You have a linearity problem... so, why haven't you scoped the grid of the output tube to see what it's drive signal looks like? If it's the same, then the output stage is just following input and you back up to the next stage of the circuit. If the grid drive matches the input signal then you've isolated the problem stage. I'd scope each stage input/output to see where it breaks down... why guess?

Regards, KM
Thanks for the suggestion, KM. Did I mention that my experience with using the scope in this 'environment' is limited?

I did try to see the output signal coming into the 300B grid, but as soon as I turn on the amp, the bias current rises above 1A and there is really bad static, which can't mean good things, so I pulled the plug. I may just be too you-know-what to use the scope properly, so I'll read up on proper scope settings and usage for this scenario.

Maybe I need to park this for a couple of days and come back with a fresh set of eyes and ideas, who knows.

Thanks again for responding!
StS
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Old 28th August 2011, 02:14 AM   #10
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Whoa, 130ma into the *grid*,and 1A of bias!?

SWAG,but I'd check the MOSFET's,and their associated resistors..perhaps one of the resistors isn't the correct value or something.. We've all mis-read a resistor once or twice.
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