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Old 6th June 2014, 05:13 AM   #1
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Default Scope pattern

Hey Anyone,

I built Pete Millet's uniamp using 2A3 outputs. The voltages are all good but it is really distorted? My scope is a goner so I borrowed an old Simpson . The Simpson uses banana plugs for the inputs and I don't have any probes or leads for it so I just used speaker wire from the output. And when taking a digital picture of the scope you can't see the pattern at all. Maybe it will work better in daylight? Anyway, I drew a crude sketch of the pattern at the output and have never seen this pattern before. Does anyone have any ideas? I plan to make up a set of leads tomorrow to find out where the distortion is occuring but in the mean time any ideas?
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Old 6th June 2014, 08:56 AM   #2
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Is the feedback, if any, the correct phase? What signal are you feeding it, looks like square wave and what frequency?
Looks like a partially integrated square wave.
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Old 6th June 2014, 10:51 AM   #3
Matt BH is offline Matt BH  United Kingdom
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What cathode resistor value are you using for your 2A3's?

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Old 6th June 2014, 12:05 PM   #4
DF96 is offline DF96  England
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Looks like bass boost, combined with a little overshoot.
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Old 6th June 2014, 01:52 PM   #5
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Thanks guys, there is no feedback. The signal is a square wave at 1k. cathode resistor is a ten ohm, the amp uses fixed bias. The outputs are set at 40ma. And yes the low frequencies seem to be more distrotred.
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Last edited by mr2racer; 6th June 2014 at 01:55 PM.
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Old 6th June 2014, 03:25 PM   #6
Matt BH is offline Matt BH  United Kingdom
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Ah ok, fixed bias and square wave. It just looked like way under biased and cutting off one valve either side with the notches which I guess are around zero crossing.

Anyway a sine wave would tell more for now.

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Old 7th June 2014, 03:00 AM   #7
hooman is offline hooman  Iran
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can check sine wave ?
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Old 9th June 2014, 04:40 AM   #8
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I checked the amp with a sine wave and found it to be clipping the bottom of the wave form. Then I traced it to a problem with the way I wired the input transformer. Thanks for the help!

Kevin
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Old 9th June 2014, 08:26 AM   #9
disco is offline disco  Netherlands
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Yep,, thought so
It's a classic 'too low bandwith' sign, according to Valley & Wallman.
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