Frequency doubling

[35L6]

Perplexing issue here . My recent build resembles a Marshal 2204 . I'm getting an odd frequency doubling . Traced it with a scope and it seams to be coming from the tone stack ( Marshal values ) . It presents as a duplicate trace offset by about 90 degrees . It is intermittent and can be triggered by any of the tone controls . Small changes is the controls can produce a small change in shift within a limited range . You can get the single trace to appear in either phase position . I did not measure the phase shift it just looks like a quarter way between the original peaks . I disconnected the volume control with no improvement . Any ideas ?

[Helmholtz]

Real frequency doubling would mean everything sounding one octave higher. Is that what you're hearing?
Would be very unlikely.
Not getting a single trace on the scope is typically a trigger stability issue, i.e. not real.
Are you familiar with manually adjusting the scope trigger?
A real signal produces only one trace at a time.

Please show scope pics and schematic.

[35L6]

It sounds like frequency doubling . Looks like a duplicate trace . The amplitude setting on the scope affects it . Goes away if I turn the amplitude up or down but with different tone control settings I can get it on a different amplitude settings . I'll post schematic and pictures latter when I have more time . I hope it's not the scope because then I'll be back to the drawing board looking for the noise !

[35L6]

OK , time base cleared that right up . Is that normal behavior for a scope or is my scope just old ? I was having trouble with the photos . My computer is also old , but not as old as my scope . The trace just doubled and I saw twice the peaks and thought I found it . I didn't understand why the signals didn't sum . I guess they were switching back and forth too fast to see . Now I'm back to troubleshooting . It sounds like frequency doubling but not in a good way and maybe a little ring moddy . Feeding it a sign wave wasn't helpful .That leaves us with circuit analysis . It does it on high notes , high E string on the twelfth fret and there abouts . I think it has to do with treble peaking and high gain , perhaps some IMD . What does slew rate distortion sound like ? I think I attached the schematic . If it doesn't post I'll try again . I took out the treble peaking cap after the cold clipper because I thought it was too bright .Now I'm thinking of putting it back in and taking out the 470p before the gain control in case that cold clipper is doing something funny with it .

[Helmholtz]

Can't help without scope pics or sound samples.

Double trace on scope has nothing to do with frequency. It's typically a trigger problem (requiring adjustment of scope settings), also might indicate a low frequency (hum) modulation.
Frequency only shows on horizontal axis.

If your problem is intermittent, consider contacts, connections, solder joints.

[The Dude]

......Double trace on scope has nothing to do with frequency. It's typically a trigger problem (requiring adjustment of scope settings),......

^^^^That. It doesn't mean your scope is bad. It just requires adjustment. Can you HEAR a problem? Is there an audible oscillation or frequency doubling? If not, I strongly suspect it's just a scope triggering issue.

[eschertron]

have you tried 'chop-sticking' the lead dress to see if you can change or reduce the effect?

[35L6]

Yes , that scope thing was a red herring and kept me busy all day when I could have been making progress . It was triggering and was cured with a small twist of the knob . I was seeing what I wanted to see and not what was in front of me . There is only one way to get experience . The triggering thing was intermittent . the problem shows up pretty consistently on high notes with gain . It's like a note on top of what you are playing . I'm going to see if my son can help me post an audio clip . I have strongly suspected lead dress and chop sticked and moved wires with no success . Now I'm thinking of making a signal probe so I can listen to the signal at different points throughout the circuit through my practice amp . We'll see how that goes .

[galaxiex]

Apologies if I missed it, but what are you using to generate a test signal?
Guitar or a signal generator?

I had a funny/weird "double frequency" once on an amp I was working on, and it turned out to be my guitar....

[Helmholtz]

Now, scoping a guitar signal is a different thing and means quite a challenge for the trigger, as the signal contains a number of frequencies with changing phase and amplitude.
In other words, a guitar signal is not really periodic, rather changing constantly.
Scoping something like that often requires a storage scope (DSO).

Preamp distortion (especially a cold clipper) indeed ADDS the doubled frequency as well as higher harmonics.
Also with more than one note sounding at the same time (or with power supply ripple) - you get intermodulation distortion (IM), adding difference and sum frequencies.

As long as there's still a fundamental, I woudn't speak of frequency doubling even though the distorted signal contains some doubled frequency.
But if your system (including speaker) has some strong treble resonance, higher notes might sound like frequency doubled indeed.

[mozz]

If using a strat, be sure the pickups aren't too close to the strings, that causes odd sounding notes.

[Greg_L]

The 2204 preamp is sensitive to oscillation. That could be what you're hearing on those high notes.

[35L6]

[35L6]

[Helmholtz]

Oh well, to interpret a scope pic, it needs to be large and clear enough.
Also need to know:
- What was input signal (source, frequency, amplitude)?
- At which circuit point was the probe connected?
- What were scope sensitivity and time base settings?

From far I'd say, looks like a square wave (maybe caused by strong symmetrical clipping) with following treble boost and bad/double triggering.