Treble distortion

[Paul P]

My amp (brown Fender preamps and parallel 6V6's) distorts on high notes
in a way that I find a bit unpleasant and am wondering if this is normal for
my sort of tube amp or not. A single note on the high E string sounds ok
and looks ok on the 'scope. If I play a harmonic at the 12th fret I get a
pretty perfect sine wave. However, if I play two notes at the same time,
like a double stop, I get a raspy, gritty sort of distortion that jumbles the
two notes up and doesn't sound good to my ears. This is more apparent
with volume and treble close to max but it's present to a lesser degree at
lower volumes. Sounds a lot like solid-state clipping.

I've read that you can't get acoustic clarity out of a guitar amp but is this
the reason ? This is my first amp after playing acoustic for the last few
years so I'm used to clean notes and this may just be normal. The mid and
low end tone of my amp is very nice even when distorted at max volume.

I'd like to know if I should just knock down the high end to reduce the
grittyness or if I might have a more serious problem that I should look into.

Paul P

[Enzo]

Please play this amp through another speaker cab, and disconnect the speaker in it. Does it still make this noise through the other speaker? You may be experiencing "cone cry" in your speaker.

Otherwise...

How old is this amp? Scope the B+ nodes with this signal condition - all the nodes - and see if this waveform shows up on the B+ at all. Is there excess ripple on any node? Parallel or not, if the amp is single ended, is the plate supply off the first filter? Or do you have a pi filter in front of the plate? Ripple can modulate the signal. A weak filter cap or decoupling cap can allow signal onto the B+ rail.

The amp may have a nice sounding bottom, but it still may have further response at the high end. Scope the signal through the amp and see if in fact you ARE clipping somewhere.

You could be having beat notes if you are operating in less linear parts of the tube curves.

Does your amp have NFB?


Guitar amps are not designed to be clean reproducers of sound, they are primary producers of sound, and as such are part of your instrument. Tube amp CAN be very clean and accurate. Hifi nuts swear by them. But guitar amps are designed to have a tone and character all their own. That is why some guys like Marshalls and some guys like Fenders. If guitar amps all sounded clear and reproduced sound accurately, they would be like PA amps, and all sound alike, so it wouldn't matter what brand you played through. Likewise speakers. Unlike PA speakers, guitar speakers are not made to be flat and wide range. Again, that is why some guys like Celestion somethings and others like JBL somethings.

[PRNDL]

This may be intermodulation distortion, which is distortion caused when two signals are mixed together, which adds non-harmonic signals. The cause is usually non-linear gain and/or out of phase signals.

In your amp, it could be caused by the parallel 6V6's, especially if one is bad or mis-matched.

Enzo has some good ideas about having a non-linear operating point or a problem in the negative feedback loop.

[Paul P]

I think I've found my problem and it wasn't even the amp but the bridge on
my daughter's MIM Fender strat. There is some awful ringing coming from
the saddles on most of the strings but mainly the high ones. I can see this
on the scope which shows up as little ripples along the main wave. On one
note it's not as obvious but mix several little ripples together and you get a
mess. Generating a harmonic dampened the string enough to prevent the
ripple.

I still appreciate your replies because they caused me to go on a very
enlightening journey along B+ (which looks pretty good) and in the airspace
around my preamps with the tip of my scope (set on the lowest setting) as
a sniffer. I found a few really strong transmitters like the plate resistors
(the V1a plate being always at maximum because it's before the volume
control) and the plate leads. One especially strong radiator is the 250pf
tone cap. I initially throught it was the cause of my problems because it's
right next to my .047 preamp output coupling cap and completely covers it
with signal. If I redo my preamp I'm going to move these two far apart.

I was also surprised to see my test signal show up on ground, even right at
the mains chassis bolt. Faint, but still visible and it changed with the input
signal frequency.

Paul P

[Earl]

I'm not sure why, but this reminded me of an occassion where I was brought a 'dead' amp. The guy said that he checked the fuse.

I put in a new pilot light, and sent him on his way .. LOL

[Enzo]

Well, it's an illuminating story....

[wrathfuldeity]

I think I've found my problem and it wasn't even the amp but the bridge on
my daughter's MIM Fender strat. There is some awful ringing coming from
the saddles on most of the strings but mainly the high ones.
Paul P

Cheap easy fix...unwanted noise from saddle in a knockoff 335, and after setting the intonation, used a couple of drops of finger nail polish applied with syringe to temporarily cement them...no noise and improved tone.