Strange S5 K-16LS oscilloscope readings

mattthegamer463 · 5th January 2009, 03:13 AM

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:

And this is the output:

Closer:

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?

DigitalJunkie · 5th January 2009, 10:25 AM

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..

Jeb-D. · 5th January 2009, 06:01 PM

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.

5th January 2009, 03:13 AM	#1
mattthegamer463 diyAudio Member Join Date: Nov 2008	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: And this is the output: Closer: 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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5th January 2009, 10:25 AM	#2
DigitalJunkie diyAudio Member Join Date: Apr 2003 Location: Portland,Oregon Blog Entries: 4	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..

5th January 2009, 06:01 PM	#3
Jeb-D. diyAudio Member Join Date: Nov 2005 Location: SoCal	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.