Single ended 6v6 oscilliscope trace question

[jackplug1]

Here is a photograph of an oscilliscope trace connected to the speaker output.
There is a 440hz sine wave input of around 75mv and it's fully dimed and measures 7 watt at the output into 8.5 ohms resistance dummy load
I expected it to be a square wave but there are those peaks, is this normal?
I cannot find any visual reference to this online

[shooter]

did you calibrate your probe 1st?
when you decrease volume, do the spikes go away, or become more "rounded"?
when scoping inductive circuits you do see the "kick" (an AC signal going through an inductor)  inductors don't like current to change, so it takes a little kick in the pants to get it through

[jackplug1]

The spikes decrease as the volume decreases as it gradually rounds off and goes back to a sine wave a low volume

[shooter]

is your probe adjustable, small screw on the probe?
the scope itself usually has a squarewave cal point, hook the probe there and adjust screw for squarewave.
the probe is cap coupled, add in the OT (Inductor) and it's pretty hard not to get the overshoot.


i like to tweak the PA drive signal so I don't get to squarewave until about 9 on the volume dial since most SE's sound pretty raw full square.

[jackplug1]

I think the probe maybe defective - I need to investigate this further
Thank you
- I will look into this tomorrow and report back

[jackplug1]

I tried a different probe as i couldn't calibrate the other one for some reason. The spikes were less pronounced but still there.
Does this look acceptable, bearing in mind the inaccuracy caused by inductance.
The amplifier does not sound very good too overdriven, this might be a limitation of a single ended stage.
I don't like to use a full on overdriven setting anyway just breakup.

[shooter]

here's a couple scope shots from my last build  (It's PP 6V6 cathode biased)  Bottom trace is speaker, top trace is input.
you are driving the PA section real hard, my suggestion is to tame down the output of the pre so you have a waveform closer to my "max" pic.
that will still get you some breakup.  I shoot for that to happen when my amp VOL is set for about 9.


SE and self biased PP don't have any problems, sound-wise amplifying distorted signals as long as the PA section isn't "generating" the distortion.  So pedals in front get's you a better sound than abusing the PA


What volume setting gets you closer to a signal somewhere between my clean and max pic

[tubeswell]

The amplifier does not sound very good too overdriven, this might be a limitation of a single ended stage.

Your ears are the best judge. SE has a mix of even order and odd order harmonics which may not sound musically pleasant. The type of speaker you use can disguise this unpleasantness. An inefficient low power speaker is better IMO. Also, I prefer smaller OT iron for SE amps, so that you’re closer to the verge of OT saturation when overdriving the output stage. YMMV however.

[PRR]

Try different frequencies. 100Hz, 400Hz, 1500Hz, 5000Hz.

The spike suggests better treble than bass. A response sweep is a good cross-check.

How does the spike change with level?

[jackplug1]

I think there maybe be something odd going on in the preamp stages which is contributing to this
Here is a photograph of the scope trace with the preamp volume full but the mv lowish
It appears oddly asymmetrical with a peak at the top at 4.669 Watts

[shooter]

time to post a schematic, or make/model of amp.


depending on where you measured that, the scope could be interacting with the signal.

[tubeswell]

Yes schematic time. The preamp distortion looks slewy.

[sluckey]

I would like to see a scope pic of the signal as it comes directly out of the sig gen. I'd also like to see a hi-rez pic that shows the entire front of your scope. I want to be able to read the labels by all the knobs and switches.

[jackplug1]

Here are the requested images

[jackplug1]

Here are the requested images

[jackplug1]

Here are the requested images

[sluckey]

Here are the requested images

Is the scope displaying the signal directly from your sig gen? That's not even close to a sine wave. The amp is probably working perfectly.

[jackplug1]

The scope is displaying the signal from the speaker output of the amplifier with a dummy load.
The output from the signal generator is a perfect sine wave.

[jackplug1]

I have noticed that I made a mistake and omitted the 5.6k grid resistor on the 6v6gt
However I'm still getting the strange scope traces at higher levels
I tested at the grid of the 6v6 and it was less extreme.
I did notice that with the master nearly full and the volume at half the the trace was square at the top but rounded at the bottom
(when testing at the grid of the 6v6)

[tubeswell]

I did notice that with the master nearly full and the volume at half the the trace was square at the top but rounded at the bottom
(when testing at the grid of the 6v6)

No surprise with that amount of gain from the preamp.


You need a grid stopper on the 6V6 to delay the onset of grid current limiting when the signal swing is large and bassy. Put the 5k6 in, or go higher (you can go up to 47k on an output tretrode/pentode without rolling off HF).

[jackplug1]

I still don't understand why the waveform is that shape, shouldn't it be square on the bottom as well as the top or in other words symmetrical?
(As in the picture of the scope shots provided by shooter)
Would increasing the 6v6 grid bias resistor to 20k help
Maybe I should add a 220k grid stopper at vb2.

https://www.aikenamps.com/index.php/grid-resistors-why-are-they-used

[tubeswell]

I still don't understand why the waveform is that shape, shouldn't it be square on the bottom as well as the top or in other words symmetrical?
(As in the picture of the scope shots provided by shooter)
Would increasing the 6v6 grid bias resistor to 20k help
Maybe I should add a 220k grid stopper at vb2.

https://www.aikenamps.com/index.php/grid-resistors-why-are-they-used

Depends where the clipping is occurring- what does the scope show for the preceding stage’s plate on the same amp settings?

[shooter]

the schematic looks like one of my experiments 


read this on DCCF circuits;
https://www.valvewizard.co.uk/dccf.html


you have the vol output going straight to the grid of the PA driver, with a 470k to ground
ADD a 10k from the pot wiper/470k junction to the grid. 

[jackplug1]

I wonder if that 470k should be 220k to reduce the gain at that point?

[PRR]

I still don't understand why the waveform is that shape, shouldn't it be square on the bottom as well as the top or in other words symmetrical?

A single-ended amplifier past clipping is never exact symmetrical, and often way far out. (Shooter posted a push-pull amp.)

At the edge of clipping, idle bias affects symmetry. In heavy clipping it gets complicated. That's why I suggested trying different frequency and level, see a trend.

[jackplug1]

The first preamp tube has gone microphonic so I have ordered some more.
That might make a difference

[jackplug1]

Everything is fine now - thanks