Parasitic oscillation problem untraceable

Forum to discuss amp circuits besides the 18 Watters and their ilk.

Moderators: JMPGuitars, CurtissRobin, colossal, zaphod_phil, Daviedawg, Graydon

Post Reply
User avatar
Bieworm
Verbose Moderator
Verbose Moderator
Posts: 2287
Joined: Mon 02/10/20 8:24 am
Location: Belgium

Re: Parasitic oscillation problem untraceable

Post by Bieworm »

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.
0 x
"THIS should be played at high volume..preferably in a residential area"

User avatar
TriodeLuvr
Frequent poster
Frequent poster
Posts: 616
Joined: Tue 10/16/18 11:19 am

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?
0 x

User avatar
Bieworm
Verbose Moderator
Verbose Moderator
Posts: 2287
Joined: Mon 02/10/20 8:24 am
Location: Belgium

Re: Parasitic oscillation problem untraceable

Post by Bieworm »

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 😃😃😃
0 x
"THIS should be played at high volume..preferably in a residential area"

User avatar
TriodeLuvr
Frequent poster
Frequent poster
Posts: 616
Joined: Tue 10/16/18 11:19 am

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?
0 x

User avatar
Bieworm
Verbose Moderator
Verbose Moderator
Posts: 2287
Joined: Mon 02/10/20 8:24 am
Location: Belgium

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?
0 x
"THIS should be played at high volume..preferably in a residential area"

User avatar
TriodeLuvr
Frequent poster
Frequent poster
Posts: 616
Joined: Tue 10/16/18 11:19 am

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.
1 x

User avatar
TriodeLuvr
Frequent poster
Frequent poster
Posts: 616
Joined: Tue 10/16/18 11:19 am

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.

Jack
1 x

dbharris
Newbie
Newbie
Posts: 44
Joined: Thu 09/17/20 2:56 pm

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
1 x

User avatar
Bieworm
Verbose Moderator
Verbose Moderator
Posts: 2287
Joined: Mon 02/10/20 8:24 am
Location: Belgium

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
You do not have the required permissions to view the files attached to this post.
0 x
"THIS should be played at high volume..preferably in a residential area"

User avatar
TriodeLuvr
Frequent poster
Frequent poster
Posts: 616
Joined: Tue 10/16/18 11:19 am

Re: Parasitic oscillation problem untraceable

Post by TriodeLuvr »

mA isn't amplitude. I'm pretty sure that was a typo.
1 x

User avatar
TriodeLuvr
Frequent poster
Frequent poster
Posts: 616
Joined: Tue 10/16/18 11:19 am

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
1 x

User avatar
Bieworm
Verbose Moderator
Verbose Moderator
Posts: 2287
Joined: Mon 02/10/20 8:24 am
Location: Belgium

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.
0 x
"THIS should be played at high volume..preferably in a residential area"

User avatar
Bieworm
Verbose Moderator
Verbose Moderator
Posts: 2287
Joined: Mon 02/10/20 8:24 am
Location: Belgium

Re: Parasitic oscillation problem untraceable

Post by Bieworm »

Input stage 2 OD channel
20210107_222303.jpg
You do not have the required permissions to view the files attached to this post.
Last edited by Bieworm on Thu 01/07/21 4:28 pm, edited 2 times in total.
0 x
"THIS should be played at high volume..preferably in a residential area"

User avatar
Bieworm
Verbose Moderator
Verbose Moderator
Posts: 2287
Joined: Mon 02/10/20 8:24 am
Location: Belgium

Re: Parasitic oscillation problem untraceable

Post by Bieworm »

Input grid stage 3 OD
20210107_223126.jpg
You do not have the required permissions to view the files attached to this post.
Last edited by Bieworm on Thu 01/07/21 4:32 pm, edited 4 times in total.
0 x
"THIS should be played at high volume..preferably in a residential area"

User avatar
Bieworm
Verbose Moderator
Verbose Moderator
Posts: 2287
Joined: Mon 02/10/20 8:24 am
Location: Belgium

Re: Parasitic oscillation problem untraceable

Post by Bieworm »

Input grid 6V6's
20210107_221811.jpg
You do not have the required permissions to view the files attached to this post.
0 x
"THIS should be played at high volume..preferably in a residential area"

User avatar
Bieworm
Verbose Moderator
Verbose Moderator
Posts: 2287
Joined: Mon 02/10/20 8:24 am
Location: Belgium

Re: Parasitic oscillation problem untraceable

Post by Bieworm »

I can't seem to zoom in on the PI input grid wave...
20210107_224833.jpg
You do not have the required permissions to view the files attached to this post.
0 x
"THIS should be played at high volume..preferably in a residential area"

User avatar
Bieworm
Verbose Moderator
Verbose Moderator
Posts: 2287
Joined: Mon 02/10/20 8:24 am
Location: Belgium

Re: Parasitic oscillation problem untraceable

Post by Bieworm »

Now I'm going to bed... good night
1 x
"THIS should be played at high volume..preferably in a residential area"

User avatar
JMPGuitars
Super Duper Admin
Super Duper Admin
Posts: 3955
Joined: Tue 09/18/12 8:00 pm
Location: South Central, MA
Contact:

Re: Parasitic oscillation problem untraceable

Post by JMPGuitars »

TriodeLuvr wrote:
Thu 01/07/21 2:44 pm
mA isn't amplitude. I'm pretty sure that was a typo.
LOL, I dunno what you're talking about, it says mV on my post. ;) ;) :rol:
1 x
'I installed a skylight in my apartment yesterday... The people who live above me are furious.' - Steven Wright

Modern Ground Schemes
Soldering Technique
B+ Voltage Reduction
Amplifier Tools & Parts Info


Web Design: DolceVittoria.com
Guitars / Amps / Effects: JMPGuitars.com
(anti)Social: Facebook · Instagram
Items for Sale

User avatar
JMPGuitars
Super Duper Admin
Super Duper Admin
Posts: 3955
Joined: Tue 09/18/12 8:00 pm
Location: South Central, MA
Contact:

Re: Parasitic oscillation problem untraceable

Post by JMPGuitars »

Bieworm wrote:
Thu 01/07/21 4:50 pm
Now I'm going to bed... good night
And when you wake up, make the amp match my circled changes:
Preamp.jpg
PowerAmp.jpg

Note the tube changes I listed on the attached voltage chart. Take new voltages after making the changes.

Thanks,
Josh
You do not have the required permissions to view the files attached to this post.
1 x
'I installed a skylight in my apartment yesterday... The people who live above me are furious.' - Steven Wright

Modern Ground Schemes
Soldering Technique
B+ Voltage Reduction
Amplifier Tools & Parts Info


Web Design: DolceVittoria.com
Guitars / Amps / Effects: JMPGuitars.com
(anti)Social: Facebook · Instagram
Items for Sale

User avatar
crgfrench
Frequent poster
Frequent poster
Posts: 827
Joined: Fri 04/27/18 3:02 am

Re: Parasitic oscillation problem untraceable

Post by crgfrench »

Just curious, what does swapping the triode pin connections in V1 & V2 accomplish?
0 x

Post Reply