5E3: Terrible Sounding Distortion

[JohnTheConqueror]

A 1V p-p input signal is extremely large for a guitar. A typical humbucker guitar has a signal around 150 mVAC when played hard. I expect your pre-amp is being overdriven during your oscilloscope test. That waveshape is probably originating there and accumulating throughout the rest of the amp. I wouldn't assume that all of that is occurring in the power amp section.

Ok, I'll go back and test it again with a smaller input. Thank you.

[GregS]

No, not at all...but the type of distortion I'm getting sounds terrible. I was just wondering if it was normal for the output to look like that, or if that shape was indicative of crossover and/or blocking distortion.

Crossover distortion appears as a slight offset in the wave shape as it transitions from positive to negative and vice versa - kind of like a notch. Don't know how to describe it better with words. I don't see that with your oscilloscope trace but I'm a little confused as to why part of your waveform is missing in the trace.

I have seen waveforms similar to yours on my scope when I was working on a design. At the time I was trying to troubleshoot a fizzy sounding distortion generated in my pre-amp so perhaps that type of waveform is indicative of that. When the pre-amp wasn't overdriven though I did not see a waveshape like that. It was a normal sine wave up to the point where the power amp started clipping and the tops of the waves started to simply square off.

[teemuk]

Are you sure there is actually anything wrong in the amp? In my experience, those old Fender amp circuits sound nice when clean or slightly breaking up (which is at pretty much the low end of the volume dial, @ circa 3 - 5) but when cranked the tone is about as "phenomenal" as the tone of an average fuzzbox. Buzzy, farty, mushy, no note separation at all, etc. The tone of those amps can even be far beyond horrible if you happen to fit in some crummy speaker that doesn't filter out most of the harsh stuff.

As is, the Deluxe is pretty much an iconic design and shouldn't need any improvements and modifications ...BUT old Fender amps aren't actually famous for their great hi-gain tones so... Perhaps you're expecting too much from that amp design; something that can never be fulfilled in reality?

[puts on the fame suit...]

Yes, that terribly clipped oscilloscope wave is what you'd expect from driving that amp with 1Vpeak input signal. Try about ten times lower value as an input. If the circuit works right you should be seeing it producing about clean sine wave at the rate power, after that it will beging to clip quite harshly. The clipping should first take place in the power tubes, but if overdriven enough also the preamp stages including the phase inverter will begin to clip.

[J M Fahey]

Agree and add: 1Vpp isn't even *that* high.
It would correspond to a sinewave around 300 mV ... and guitar pickups don't provide much of those either.
A strong humbucker and/or thick strings and/or a heavy hand can produce 1Vpp easily, specially considering it will *not* be a sinewave.
Such signal would not register much in a voltmeter, which tends to display average values (or RMS in the best case) but typical clipping appears when peak voltage reaches ground or rails .
The scope pictures show an overdriven amp, and not *that* much either, maybe 6 to 10 dB.
Overdrive any stage by 20dB (10x the expected signal) and then you'll see all kinds of real ugly artifacts, such as blocking, extreme duty cycle modulation, "inverse peaking" and many others.
And some of them even sound "good", at least for thrashers.
So in a nutshell I would not worry too much about your amp, don't see it too far from expected.
One doubt: you said "terrible sounding distortion" because you *heard* it or inspired by the scope waveform?
Can you please post some MP3?
Maybe one man's poison is another man's medicine.
Thanks a lot.

[nosetocar]

Hello. I am new to the forum. Sorry if not well understood. My English is google translator.
I have this same problem with my 5E3. When the volume is low sounds perfect, but when the volume exceeds 3 a terrible distortion appears as if the speaker was broken, and that seems to affect only the low notes. I checked the voltages around the circuit and everything is correct. Not to do.
Fix the problem? Any ideas where to look for the problem? My knowledge of electronics are low.

[J M Fahey]

Please post some MP3 showing the Terrible Distortion.
Start with a clean sound for reference, play at different volumes, different pickups, etc.

[nosetocar]

OK J M Fahey.

[lowell]

My first thought here is cold solder joints on grounding points. I just fixed a DIY AC30 with a similar problem. It had really bad harmonics and a squeaky trash sound to it. My initial thought was to visibly inspect grounds. I could see that the grounding points for all the pots looked cold. The solder joints didn't look smooth. Resoldered all your pot grounds, jack grounds, and don't forget all grid leak grounds too.