Parasitic oscillation problem untraceable

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Bieworm
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Re: Parasitic oscillation problem untraceable

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TriodeLuvr wrote:
Thu 01/07/21 11:07 am
This looks like it might contain strong higher-order harmonics. Is this clipping occurring at the output tubes, or is it in one of the preamp stages? Do the grids of the phase inverter look like this? And what's creating a 1.0000 kHz tone?

Image
It's a 1kHz tone generated by a signal generator.
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Re: Parasitic oscillation problem untraceable

Post by TriodeLuvr »

Bieworm wrote:
Thu 01/07/21 12:18 pm
TriodeLuvr wrote:
Thu 01/07/21 11:07 am
This looks like it might contain strong higher-order harmonics. Is this clipping occurring at the output tubes, or is it in one of the preamp stages? Do the grids of the phase inverter look like this? And what's creating a 1.0000 kHz tone?

Image
It's a 1kHz tone generated by a signal generator.
Gotcha. Am I correct in assuming this sounds fizzy? If so, is it the same tonal quality as the fizz you're hearing when you play?
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Re: Parasitic oscillation problem untraceable

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TriodeLuvr wrote:
Thu 01/07/21 12:27 pm
Bieworm wrote:
Thu 01/07/21 12:18 pm
TriodeLuvr wrote:
Thu 01/07/21 11:07 am
This looks like it might contain strong higher-order harmonics. Is this clipping occurring at the output tubes, or is it in one of the preamp stages? Do the grids of the phase inverter look like this? And what's creating a 1.0000 kHz tone?

Image
It's a 1kHz tone generated by a signal generator.
Gotcha. Am I correct in assuming this sounds fizzy? If so, is it the same tonal quality as the fizz you're hearing when you play?
Don't know. It's hooked to an 8 ohm 50W dummyload. But at those settings I think It's fizzing the crap out of the amp. I don't think my ears could handle 1kHz at the input with the volume of the amp on 6 😃😃😃
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Re: Parasitic oscillation problem untraceable

Post by TriodeLuvr »

What's the amplitude of that signal? There are multiple voltage readings on the display, none of them seem right for the signal to be so clipped. What is the Volts/Div?
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Re: Parasitic oscillation problem untraceable

Post by Bieworm »

JMPGuitars wrote:
Thu 01/07/21 12:00 pm
TriodeLuvr wrote:
Thu 01/07/21 11:07 am
This looks like it might contain strong higher-order harmonics. Is this clipping occurring at the output tubes, or is it in one of the preamp stages? Do the grids of the phase inverter look like this? And what's creating a 1.0000 kHz tone?

Image
For that waveform to be really useful, he needs to drop the amplitude significantly (assuming that's what's driving the amplitude read by the scope). The signal generator amplitude should be 125mA. He should also stretch the time base a little so we can see the rise and fall better, in which case the crossover distortion will likely be more visible (if present). At its current amplitude, he could be introducing problems that aren't relevant to actual circuit voltage levels (assuming his source is set too high).

Bieworm, what's the signal generator amplitude set to? Where is the signal being inserted?
What do you mean? On the volume dial? I have to measure that wit the DMM
I plugged in an unused jack at the input and connected that to the signal generator with a probe. I saw DLabs do it once ..I think?
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Re: Parasitic oscillation problem untraceable

Post by TriodeLuvr »

I'm trying to figure out how large that signal is. I've never seen a digital scope that doesn't display the Volts/Div setting on the screen somewhere.
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Re: Parasitic oscillation problem untraceable

Post by TriodeLuvr »

Just to make the point, one of the readouts on the screen says the voltage delta is 1.96V peak-to-peak. If that's real, the amp is clipping at a very low signal level. If it's a measurement after a 10:1 probe that the scope doesn't know about, it's still low. 19.6V p-p is about 6.9V RMS, or about 6W into 8 ohms. That should be nowhere near output clipping for this amp.

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Re: Parasitic oscillation problem untraceable

Post by dbharris »

When I am scoping something out, I will hook a test lead up in the first channel to the input jack where I am injecting the test signal. Make sure that is at the correct level before doing anything else. If you aren't sure, then plug a guitar in and play while monitoring input voltage on your scope at the input jack. That will tell you what range you need to be in.

Then, you hook up the second test lead to the output or any other part of the circuit you need to test. Before probing very high voltage, make sure your leads can handle that. I bought a 100x lead to use on the plates of the output tubes. You should be able to set your scope channel on the padding level of your particular probes.

If you see oscillation on the output, then I would start at the front end and work your way through the signal path. Note measurements at the grid and then output of that preamp stage to see if it is amplifying or buffering like you would expect. You will be able to see where the oscillation shows itself and you can narrow down your search to that block of the circuit.

Does your scope have an FFT function? You can use that to test the harmonics on the output signal. You just have to set the range of the FFT reading to the appropriate ratio for your 1K signal. On my scope I have to change the test signal to 1.5K to make everything line up cleanly.

Don't give up, just attack this strategically.

-Dan
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Re: Parasitic oscillation problem untraceable

Post by Bieworm »

JMPGuitars wrote:
Thu 01/07/21 12:00 pm
TriodeLuvr wrote:
Thu 01/07/21 11:07 am
This looks like it might contain strong higher-order harmonics. Is this clipping occurring at the output tubes, or is it in one of the preamp stages? Do the grids of the phase inverter look like this? And what's creating a 1.0000 kHz tone?

Image
For that waveform to be really useful, he needs to drop the amplitude significantly (assuming that's what's driving the amplitude read by the scope). The signal generator amplitude should be 125mA. He should also stretch the time base a little so we can see the rise and fall better, in which case the crossover distortion will likely be more visible (if present). At its current amplitude, he could be introducing problems that aren't relevant to actual circuit voltage levels (assuming his source is set too high).

Bieworm, what's the signal generator amplitude set to? Where is the signal being inserted?
Josh... how do I set the amplitude to mA? With the DMM I can only measure VDC on the output of the generator. Measure it with the scope?
I measure at an empty jackplug that's put in the input of the amp. As if it were a guitar..

Here's the voltage chart that corresponds with the schematics I sent today.
20210107_201532.jpg
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Re: Parasitic oscillation problem untraceable

Post by TriodeLuvr »

mA isn't amplitude. I'm pretty sure that was a typo.
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Re: Parasitic oscillation problem untraceable

Post by TriodeLuvr »

Just want to add, the waveform at the output looks like preamp clipping. Output stage clipping usually leads to spikes and other anomalies that aren't so symmetrical in appearance. I think you might be driving the amp too hard with the generator.

Jack
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Re: Parasitic oscillation problem untraceable

Post by Bieworm »

TriodeLuvr wrote:
Thu 01/07/21 2:51 pm
Just want to add, the waveform at the output looks like preamp clipping. Output stage clipping usually leads to spikes and other anomalies that aren't so symmetrical in appearance. I think you might be driving the amp too hard with the generator.

Jack
I'm testing right now with 125mV output of the generator.
Last edited by Bieworm on Thu 01/07/21 4:24 pm, edited 1 time in total.
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Re: Parasitic oscillation problem untraceable

Post by Bieworm »

Input stage 2 OD channel
20210107_222303.jpg
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Last edited by Bieworm on Thu 01/07/21 4:28 pm, edited 2 times in total.
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Re: Parasitic oscillation problem untraceable

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Input grid stage 3 OD
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Last edited by Bieworm on Thu 01/07/21 4:32 pm, edited 4 times in total.
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Re: Parasitic oscillation problem untraceable

Post by Bieworm »

Input grid 6V6's
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