I'm trying to understand some noise / distortion I'm seeing in a small guitar
25 W combo amp.
The guitar is plugged directly into the amp. No effects.
Simple amp design. No reverb, no tremolo, just one clean channel.
Amp is set a mid volume no dirt.
The problem isn't distortion on all notes/chords, the amp sounds good except
for notes primarily on the high E string.
When I play say a G# note on the high E string, I get a kind of ghosting note that is
off tone. A lower octave G# does not seem to have this distortion. The notes
on the high E string ( higher freq ) seem to have this problem / character.
I've swapped all preamp tubes with new tubes one by one to see if it was a
microphonic tube. Still sounded the same.
I also plugged the amp into a different speaker cabinet so there would be no
vibration in the amp head. Still sounds the same. So it isn't the speaker or
a vibration issue.
I also tried a different guitar, to rule out any pick-up issues.
Anyone have any ideas what might cause this type of distortion ?
Perhaps I need to hook it up to a dummy load and drive it with a signal generator to
see what the output looks like... I'm hoping to isolate this asap. I need the amp
for an upcoming gig.
Tom
25 W combo amp.
The guitar is plugged directly into the amp. No effects.
Simple amp design. No reverb, no tremolo, just one clean channel.
Amp is set a mid volume no dirt.
The problem isn't distortion on all notes/chords, the amp sounds good except
for notes primarily on the high E string.
When I play say a G# note on the high E string, I get a kind of ghosting note that is
off tone. A lower octave G# does not seem to have this distortion. The notes
on the high E string ( higher freq ) seem to have this problem / character.
I've swapped all preamp tubes with new tubes one by one to see if it was a
microphonic tube. Still sounded the same.
I also plugged the amp into a different speaker cabinet so there would be no
vibration in the amp head. Still sounds the same. So it isn't the speaker or
a vibration issue.
I also tried a different guitar, to rule out any pick-up issues.
Anyone have any ideas what might cause this type of distortion ?
Perhaps I need to hook it up to a dummy load and drive it with a signal generator to
see what the output looks like... I'm hoping to isolate this asap. I need the amp
for an upcoming gig.
Tom
The noise you hear is third, fifth and seventh order harmonic distortion. Most Marshall / Ampeh / Fender Valve amps have lots as that sort of distortion, it enables lots of sustain on the higher strings. Although it could be something else entirely of course, without hearing it.
A scope is your friend, specially in these cases.
Feed it the same "problem" frequency and look at the output, anything you can hear you must also see.
Post a screen picture.
A schematic will also help.
Feed it the same "problem" frequency and look at the output, anything you can hear you must also see.
Post a screen picture.
A schematic will also help.
Can you record this (microphone in front of the speaker) on a computer and upload a .wav file? You can use a free recording program such as Audacity, or a simpler one that may come with the computer/OS, just make sure the recording is below clipping. This way we can see what an oscilloscope shows, and also look at the spectrum. Play a few chords as well as the problem and non-problem notes.
With using different speakers and guitars you've eliminated several of the things I was thinking of. About the only thing I can imagine is something related to the output transformer, but I still won't rule out other things.
With using different speakers and guitars you've eliminated several of the things I was thinking of. About the only thing I can imagine is something related to the output transformer, but I still won't rule out other things.
I'm going to try and look at each stage with a scope while driving the amp with my
signal generator. Hopefully I can find it. I'll try and make an mp3 or wav file of the
sound.
I have some matlab / octave scripts I wrote to do fft's so I can take a look at the
spectrum.
I will take a look at the feedback loop from the O.T. since I did add a small cap ( 330pf ) around
the feedback resistor to increase the amount of HF feedback.
I will post the sound file and findings as soon as I can...
Thanks for the ideas !
signal generator. Hopefully I can find it. I'll try and make an mp3 or wav file of the
sound.
I have some matlab / octave scripts I wrote to do fft's so I can take a look at the
spectrum.
I will take a look at the feedback loop from the O.T. since I did add a small cap ( 330pf ) around
the feedback resistor to increase the amount of HF feedback.
I will post the sound file and findings as soon as I can...
Thanks for the ideas !
If you remove your added 330p cap, does that stop the noise? That is the first thing to do, I would think.
As above..
Double check NFB is actually NFB and not PFB, if its a build its easy enough to get the plate wires back to front, and at low feedback levels may not be noticable at the start. Adding the cap would certainly account for the problem becoming noticable. Been there before...
Double check NFB is actually NFB and not PFB, if its a build its easy enough to get the plate wires back to front, and at low feedback levels may not be noticable at the start. Adding the cap would certainly account for the problem becoming noticable. Been there before...
Just hooked it up to my scope and set my signal generator to G3 4th octave 415hz same
tone as I was playing on the guitar.
Waveform looked and sounded fine. I ran a Sine / Triangle and Square wave through with
the same settings as when I had the guitar plugged in. ( See attached )
The strange thing is, with the amp cold, the problem is not really as apparent as yesterday
when the amp had been on for a few hours and was good an hot.
I suppose it might be an issue with one of the power tubes ( 5881 ) . I didn't swap these
since I'd of had to re-bias the amp.
tone as I was playing on the guitar.
Waveform looked and sounded fine. I ran a Sine / Triangle and Square wave through with
the same settings as when I had the guitar plugged in. ( See attached )
The strange thing is, with the amp cold, the problem is not really as apparent as yesterday
when the amp had been on for a few hours and was good an hot.
I suppose it might be an issue with one of the power tubes ( 5881 ) . I didn't swap these
since I'd of had to re-bias the amp.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
Last edited:
Will do ... I suppose it's a possibility. Usually it's painfully obvious when the NFB is
PFB. But the amp is using a small amount of FB and this is possible...
The NFB loop in place is a single 840 ohm resistor off the 8 ohm
tap. It is fed into the long-tail pair splitter driving the power section. The amount
of feedback was set using a 100 ohm resistor to ground. I added the cap around
the 840 ohm to increase the amount of feedback in the higher freq.
I'll increase the 100 ohm to say 1K and see if it squeals 😉
1) Mmmmmhhhhh , images do not open for me.
2)
I never discard it as a possible culprit.
3) agreeing with Jon , I also suspect
No need to swap wires as Donzo suggests, at high frequencies cascaded phase shifts turn NFB into PFB .
Your amp might not be outright oscillating full time, but do so at some frequencies, specially because real world speakers have crazy impedances (so crazy phase shifts) all by themselves.
Even way more phase accurate SS amps need Zobels , added capacitive poles, sometimes output inductors and in general careful design to avoid instability.
But any instability will clearly show at the scope screen, something similar to:
2)
Cool, did you also try the guitar?
I never discard it as a possible culprit.
3) agreeing with Jon , I also suspect
Amps which include a transformer in the signal path are, by definition, marginally stable, and can take only so much NFB .
No need to swap wires as Donzo suggests, at high frequencies cascaded phase shifts turn NFB into PFB .
Your amp might not be outright oscillating full time, but do so at some frequencies, specially because real world speakers have crazy impedances (so crazy phase shifts) all by themselves.
Even way more phase accurate SS amps need Zobels , added capacitive poles, sometimes output inductors and in general careful design to avoid instability.
But any instability will clearly show at the scope screen, something similar to:
All that for sure, worth a quick check though, I've had numerous amps come to me after they replaced their OT or built something and it's not behaving right sonically, turns out phase of tranny was topsy turvy, I've even done it myself.
My quick confirm its right - pull up a sine wave (not full juice), remove Nfb, does it get bigger or smaller, correct if it get smaller 🙂
My quick confirm its right - pull up a sine wave (not full juice), remove Nfb, does it get bigger or smaller, correct if it get smaller 🙂
If a signal generator dosnt show the same problem as a guitar maybe it's the input? Do you have a active guitar you can try. They have a much lower output impedance.
1) Mmmmmhhhhh , images do not open for me.
2)
Cool, did you also try the guitar?
I never discard it as a possible culprit.
3) agreeing with Jon , I also suspect
Amps which include a transformer in the signal path are, by definition, marginally stable, and can take only so much NFB .
No need to swap wires as Donzo suggests, at high frequencies cascaded phase shifts turn NFB into PFB .
Your amp might not be outright oscillating full time, but do so at some frequencies, specially because real world speakers have crazy impedances (so crazy phase shifts) all by themselves.
Even way more phase accurate SS amps need Zobels , added capacitive poles, sometimes output inductors and in general careful design to avoid instability.
But any instability will clearly show at the scope screen, something similar to:
Try selecting the image and do "open in new window" or "open in new tab" it worked
for me.. Tried putting these on google images...
Trying to post again.. Images on google but hopefully you can see it now.
Not too much ring and I can flatten it out pretty well by adjusting the tone stack
but the settings for this image where how I normally have the tone set.
https://drive.google.com/open?id=0B3i9uKciNaGRc1ByNlVuZHppWEk
https://drive.google.com/file/d/0B3i9uKciNaGRc1ByNlVuZHppWEk/view?usp=sharing
Last edited:
Checked the NFB loop ... The output gets larger / louder with the NFB loop
disconnected. So looks like the NFB is really NFB... 🙂
Don't have an active guitar handy...
I'm using an ES-335 with Seth Lover pickups. I also tried an older L7 with Johhny
smith pickup with similar results.
Played for a few hours today and the problem was not really noticeable. Yesterday
after playing several hours and the amp was hot was when the issue was consistent...
Wanted to revisit this as I spent some time with the amp after the original posting.
I spent and hour or so with my signal generator and scope trying to figure out what
this was. I couldn't find anything.. and to make matters worse the issue didn't seem
to occur unless the amp was on for a long while and was good and hot.
I ended up playing a show with it and it was fine, but I put a fan on the power section
to keep it cool as it was over 30 C that day and the gig was outdoors.
Got around to using the amp again today and it started having lots of noise problems.
One of the power tubes was microphonic. I had given it a good tap or two before and
it seemed fine but now it was noisy. Swapped the power tubes for a new set and it sounds
great. I believe the issue was most likely related to one of the power tubes having a
noise issue when it got to temperature etc.
In the end I did learn some nice things about the amp looking at the freq response and analysing
the spectrum peaks (harmonic content) from each stage etc.
I spent and hour or so with my signal generator and scope trying to figure out what
this was. I couldn't find anything.. and to make matters worse the issue didn't seem
to occur unless the amp was on for a long while and was good and hot.
I ended up playing a show with it and it was fine, but I put a fan on the power section
to keep it cool as it was over 30 C that day and the gig was outdoors.
Got around to using the amp again today and it started having lots of noise problems.
One of the power tubes was microphonic. I had given it a good tap or two before and
it seemed fine but now it was noisy. Swapped the power tubes for a new set and it sounds
great. I believe the issue was most likely related to one of the power tubes having a
noise issue when it got to temperature etc.
In the end I did learn some nice things about the amp looking at the freq response and analysing
the spectrum peaks (harmonic content) from each stage etc.
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