Strange S5 K-16LS oscilloscope readings - diyAudio
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Old 5th January 2009, 03:13 AM   #1
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Default Strange S5 K-16LS oscilloscope readings

I put my newly built S5 K-16LS kit amp on my oscilloscope today and ran some signals through it using my computer (no function generator available) and got a strange output at the relatively lower frequencies.

This is the 1khz input signal:

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And this is the output:

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Closer:

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I'm not sure whats going on, it seems like a vibration within the sine wave or something. I'm no expert on signal analysis or anything but if anyone has a clue whats going on could they offer some insight?
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Old 5th January 2009, 10:25 AM   #2
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It appears to be some kind of oscillation/HF instability,it's kind of odd that it only affects one half of the waveform/one conduction cycle-you can see it starts just below zero-crossing,and continues about 3/4 up the positive going peak...
Maybe one tube is unstable..or the grid-stopper resistor needs to be inspected/replaced/added (I dunno if this amp has stoppers or not?) or perhaps some kind of compensation/NFB issue.

Just a couple tired thoughts..
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Old 5th January 2009, 06:01 PM   #3
Jeb-D. is offline Jeb-D.  United States
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I've had various oscillation problems with my k-502 which is similar to the k-16LS. Though, I have never seen it look quite like that from oscillation. 1st make sure that your speaker terminals aren't slightly shorting to the chassis. If you are using the supplied terminals installed on a conductive chassis, this could very well be the case. If you can actually see the wave fluttering, that's what it looks like when an output tube starts to draw excessive grid current before the grid even goes positive.

If it is not a visible fluttering, but a continuous appearance, shrink the size of your scopes time devisions, you may be able to see the high frequency wave that is modulating your 1kHz tone.

Check to see if it occurs on both channels. If only 1 Ch, you can swap tubes between channels until the problem starts to occur on the other channel instead. If that happens, you have just found a problematic tube. Replace it.

If the problem does not change channels with a tube rotation, or occurs on both channels. First try removing de-soldering one led of the feedback resistor and capacitor. You will now be running open-loop. Does the problem still occur? If so, track which stage it starts at. I actually had a strange encounter, where my coupling cap choice was causing the concertina phase splitter to oscillate. I'm not sure if it was due to excessive leakage or what. But I switched out the 250V caps with 400V caps and it went away. If the problem stops when feedback is removed, the compensation cap in the feedback path may need some adjustment.

The K-502 does not use grid stoppers anywhere. If that is the case with yours, you may have to add some. But, first check if/where oscillation is starting. You need to run open loop to track it down. In closed loop, the output effects the input, which makes it difficult to find the origin.
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Old 7th July 2013, 07:11 AM   #4
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Transformers...... Transformers....... Transformers
You guys don't want to see my readings below 100 Hz
No reply from S5 after see them.....
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Old 7th July 2013, 01:59 PM   #5
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Not strange at all. This is a typical waveform of a marginally (un)stable amplifier (aka poor stability margin) going into spurious oscillations during some portions of the driving signal. This subject has been well covered in old litterature (see below) and usually affect poorly designed NFB amps. There are many different reasons for an amp to become unstable and there's no single cure, your scope will be your best friend to help debug this. We can mention as possible causes: poor stability margin due to badly implemented NFB (nyquist stability criterion not met), non original or poor quality ouput transformer, poor circuit layout, lack of HF rise compensation and/or uncompensed excessive phase shift,unbalanced output stage and/or highly unmatched tubes, power supply decoupling issues, etc,etc... The first thing to try is to open the NFB loop to check if the amp is stable without it and then add grid stoppers. The trouble with marginally stable amps is that they break into spurious oscillations only when some particular conditions are met and may look stable the remaining of time. This means you will need to check the amp with various loads (resistive, capacitive, and both) and at various output power levels before declaring it as unconditionally stable.
To get a better idea about the subject I strongly suggest reading the following article below by the great N.H Crowhurst (from the Sept.1955 issue of Radio & TV News) and/or download his book (High Fidelity Circuit Design) which is a real classic for the more technically involved. (some math required). Chapters 2 & 3 are especially dealing with this subject.

Book download link:

http://www.tubebooks.org/Books/Atwoo...t%20Design.pdf
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Last edited by Tubologic; 7th July 2013 at 02:15 PM.
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