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Old 30th April 2013, 10:15 AM   #1
mr2racer is offline mr2racer  United States
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Hey All,

I'm working on an RH84. And as I increase input the amp clips the bottom of the sine wave. In fact even before it clips the bottom of the sine wave seems rounder and more blunt than the top. (I haven't reversed the polarity of the input from the signal generator to eliminate that yet. Just thought of it.)

The more I drive it the squarer the bottom gets. But it never seems to effect the top of the waveform. Can anyone tell me is this an SE effect or an RH effect? Or is there a problem with this individual amp?

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Old 30th April 2013, 10:41 AM   #2
DF96 is offline DF96  England
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All amps will clip. SE amps are much more likely to clip asymmetrically, as you have seen. Before they clip they will usually round off the waveform on one peak. This is what even-order distortion looks like in the time domain.
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Old 30th April 2013, 10:49 AM   #3
SemperFi is offline SemperFi  Wake Island
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Properly biased the output should clip on positive and negative peaks at about the same amplitude. Try to increase the bias current.
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Old 30th April 2013, 11:06 AM   #4
Vincent77 is offline Vincent77  Europe
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Check where this clipping appears:
- signal generator/first stage grid?
- 1st stage anode - maybe the signal generator has a DC offset on the output or wrong 1st stage biasing;
- output (check power stage bias current as SemperFi said).
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Old 30th April 2013, 11:23 AM   #5
Vinylsavor is offline Vinylsavor  Germany
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As has been mentioned: It is good practice to bias the output tube such that it clips as symmetrically as possible. With most SE amps one side of the wave will start to clip soft by just getting a bit rounded. That is the output tube driven into cut off. The other side usually clips harder, that is the output tube driven into positive region. Most drivers cannot deliver power to push the output tube into grid current. Of course it depends on the amp circuit.

Here is an example of measurements of such an amp as described above:

VinylSavor: Low Cost Single Ended 6CB5A Amplifier - Part 5

Best regards

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