Can't understand why amp shows amplitude/distortion BEFORE the Vol & Tone pots

[aaron]

So, I've built a Dave Hunter 2 Stroke.....
(It's a Champ deviation)
When I hit a note at high volumes and treble turned up, I get a wierd kind of distortion that sounds like someone's playing with cellophane wrapping paper, especially as the note decays.....

So I'm injecting a 440 Hz (A note) signal into the input jack, and everything seems OK on the oscilloscope at pin 2 ot the 12AX7, and at pin 1 as it comes out of the 12AX7.
From there it goes to both the Vol and Tone pots (1M, audio), after passing through a 0.0022uF signal cap.
It is here that I attach the oscilloscope probe---after the cap, before the Vol and Tone pots.

SO THIS IS WHERE MY MIND IS GETTING BLOWN

With the Tone all the way off, as I turn up the Vol, nothing happens (as is to be expected, right? B/c I'm measuring BEFORE the Vol pot.

Yet, at about 90 up till 100 % of the Vol pot turned up, I get a sine wave distortion on the top of the wave (normal kind, where it starts to flatten out).
BUT even though there's distortion, the height of the sign wave doesn't increase with increased volume (which it shouldn't, right? B/c I'm measuring BEFORE the Vol pot. 

1.  So why am I seeing distortion ???

next case:

W/ the oscilloscope in the same place, BEFORE the Vol & Tone pots,


With the Vol pot all the way down, when I increase the Tone pot, I get a significant increase in the sign wave ( it goes from 6.5 V to 9.5 V on the oscilloscope)

2.  Why am I seeing an amplification in signal BEFORE the Tone pot by adjusting the Tone pot?

W/ the oscilloscope in the same place, BEFORE the Vol & Tone pots,

Now starting with Tone at 100% --
(please imagine the dial as a clock, where zero is seven o'clock and max is five o'clock)

3.  As I increase Vol the sign wave distorts is a "furry or fuzzy" manner, from nine till four o'clock, getting worse from 9-4, then it goes away from four to five o'clock!

I"ve triple checked the solder joints, connections, etc.

Aside from the wierd distortion at higher volumes, the amp works.

My mind meltdown is b/c I think that the oscilloscope should be picking up the effects of the Vol and Tone pots AFTER  the pots, not before.....am I wrong?

thanks for any help!

[PRR]

The pots load the stage before depending on their settings.

[sluckey]

Yes, changing the pots changes the loading on the previous stage. A schematic would be very useful toward in depth understanding. Layout ain't very useful.

It's also important to know what the level of your test signal is at V1 pin 2.

[pdf64]

To the preceding stage, the grid-cathode is a reverse biased diode. When the positive instantaneous signal = the bias voltage, the diode becomes forward biased and chops off the top of the wave.

[AmberB]

Just curious, why are you using a .0022 uf cap before the volume and tone controls?  Isn't that supposed to be a .02 uf?  Or am I wrong on this?

[aaron]

The pots load the stage before depending on their settings.

Yes, changing the pots changes the loading on the previous stage. A schematic would be very useful toward in depth understanding. Layout ain't very useful.

It's also important to know what the level of your test signal is at V1 pin 2.

OK, so you're saying what I'm seeing is normal.............
mind blown...........

Sorry, I thought that I had posted the schematic with the layout, I will correct that!

To the preceding stage, the grid-cathode is a reverse biased diode. When the positive instantaneous signal = the bias voltage, the diode becomes forward biased and chops off the top of the wave.

Thanks, but you're way over my head with that explanation!

In summary, I can expect to see "things" "upstream" from  the oscilloscope probe.......
I thank everybody for the clarification!

I thought that by placing the oscilloscope probe at different points along the signal path, I could identify the villian by the method of "clean signal before component X, dirty signal after component X, so component X must be the villian."

So, since I'm looking to find out where is the cause of my strange distortion, what do you suggest?

I've lost my file of the schematic, so here's the link:

https://music-electronics-forum.com/filedata/fetch?id=835448&d=1411751072]

edit... fixed file link

[aaron]

Just curious, why are you using a .0022 uf cap before the volume and tone controls?  Isn't that supposed to be a .02 uf?  Or am I wrong on this?

My bad, you're correct.

[pdf64]

…
Thanks, but you're way over my head with that explanation!

Apologies; simply put, the signal level at which the waveform begins to look funny in the test described is where the next stage is being overdriven.
The grid-cathode is a diode, and when the upper half of the signal wave tries to put the grid voltage above the cathode voltage, the diode conducts and clips it off.

See https://www.electronics-tutorials.ws/diode/diode-clipping-circuits.html

Merlin goes through this on p19, section 1.14 grid current clipping, see http://www.valvewizard.co.uk/Common_Gain_Stage.pdf

BTW, kudos on your investigate endeavour, and on asking about the stuff your don’t understand 

[sluckey]

What is the level of your test signal at V1 pin 2?

[astronomicum]

I would suggest you first start by understanding what is happening in your first stage, then move to understanding what you are seeing in the volume/tone section. I am assuming you can adjust the output level of your signal generator and you are running your amp into a dummy load. If so, conduct these tests:

1.   First, note where you have your signal generator and V1A Cathode Bypass Switch set.
2.   Measure the ACRMS at V1A Grid (Pin 2).
3.   Turn the output down on the signal generator.
4.   Turn your Volume and Tone controls to max.
5.   Attach your oscilloscope downstream of V1A Plate (Pin 1) Coupling Cap (or directly on Pin 1 if you have verified your scope can handle the voltage).
6.   Set you Bypass Cap Switch on V1A to the 0.47µF position.
7.   While watching your scope, increase you signal generator output until the signal you are seeing on you scope starts to clip (distort). What you are effectively doing here is seeing what input level is the maximum signal level that will produce a clean signal.
8.   Measure the ACRMS at V1A Pin 2.
9.   Note the Voltage on your oscilloscope (1/2VPP). Multiply by 0.707. This will give you ACRMS.
10.   Divide you Plate ACRMS reading by your Grid ACRMS reading. The quotient is the gain.
11.   Repeat the procedure 3-10 with the Bypass Cap Switch on V1A in the 25µF setting. You should notice that the input signal ACRMS required to begin to produce distortion of V1A is lower for the 25µF Bypass Cap Setting than for the 0.47µF setting.
12.   Compare your test results to where your signal generator and bypass switch were set in step 1.

Now that you know what signal levels will begin to produce distortion, were your original settings going to produce a clean or distorted signal from V1A? Repeat the tests you originally posted, only this time, use one of the two signal generator and bypass switch settings from your testing above.

[aaron]

What is the level of your test signal at V1 pin 2?

I didn't forget you, sluckey, it's just that the scope and amp aren't put together yet (kind of like I need to clear off the kitchen table, but others are there )

[aaron]

I would suggest you first start by understanding what is happening in your first stage, then move to understanding what you are seeing in the volume/tone section. I am assuming you can adjust the output level of your signal generator and you are running your amp into a dummy load. If so, conduct these tests:

1.   First, note where you have your signal generator and V1A Cathode Bypass Switch set.
2.   Measure the ACRMS at V1A Grid (Pin 2).
3.   Turn the output down on the signal generator.
4.   Turn your Volume and Tone controls to max.
5.   Attach your oscilloscope downstream of V1A Plate (Pin 1) Coupling Cap (or directly on Pin 1 if you have verified your scope can handle the voltage).
6.   Set you Bypass Cap Switch on V1A to the 0.47µF position.
7.   While watching your scope, increase you signal generator output until the signal you are seeing on you scope starts to clip (distort). What you are effectively doing here is seeing what input level is the maximum signal level that will produce a clean signal.
8.   Measure the ACRMS at V1A Pin 2.
9.   Note the Voltage on your oscilloscope (1/2VPP). Multiply by 0.707. This will give you ACRMS.
10.   Divide you Plate ACRMS reading by your Grid ACRMS reading. The quotient is the gain.
11.   Repeat the procedure 3-10 with the Bypass Cap Switch on V1A in the 25µF setting. You should notice that the input signal ACRMS required to begin to produce distortion of V1A is lower for the 25µF Bypass Cap Setting than for the 0.47µF setting.
12.   Compare your test results to where your signal generator and bypass switch were set in step 1.

Now that you know what signal levels will begin to produce distortion, were your original settings going to produce a clean or distorted signal from V1A? Repeat the tests you originally posted, only this time, use one of the two signal generator and bypass switch settings from your testing above.

I don't have a signal generator.........
I'm using a smartphone to input the signal...........
I started out with a smartphone and a probe to generates a signal, made a slight movement of my hand while injecting the tone on the 12AX7, where the pins are nice and close to each other, and destroyed my only 1 yr old smartphone
So now, I use cables to plug into the input jack

BTW, re:
"1.   First, note where you have your signal generator and V1A Cathode Bypass Switch set."
I got the same distortion and the before the Vol/Tone pots results regardless of the switch setting.

I will do as you instructed.

[aaron]

…
Thanks, but you're way over my head with that explanation!

Apologies; simply put, the signal level at which the waveform begins to look funny in the test described is where the next stage is being overdriven.
The grid-cathode is a diode, and when the upper half of the signal wave tries to put the grid voltage above the cathode voltage, the diode conducts and clips it off.

See https://www.electronics-tutorials.ws/diode/diode-clipping-circuits.html

Merlin goes through this on p19, section 1.14 grid current clipping, see http://www.valvewizard.co.uk/Common_Gain_Stage.pdf

BTW, kudos on your investigate endeavour, and on asking about the stuff your don’t understand 

Thanks!
and I'll read the links 

[sluckey]

So I'm injecting a 440 Hz (A note) signal into the input jack, and everything seems OK on the oscilloscope at pin 2 ot the 12AX7

All I want to know is what voltage level is that 440Hz signal at pin 2? 200mV_pp is a good level to put into a guitar amp. If your test signal is 1V or 10V that's too high and will cause problems. Can you adjust the level of the signal from your cell phone? Don't rely on the phone app to tell you what level you have. Let your scope do the measuring.

Which scope do you have?

[astronomicum]

I don't have a signal generator.........

If interested, I believe you can do do the same testing using your smart phone. Substitute "smart phone" for "signal generator" and "volume" for "output". I also use my smartphone for testing but with music rather than a tone signal . Going to try using it in place of the signal generator next time I am at the bench.

[jamaio]

I don't have a signal generator.........

If interested, I believe you can do do the same testing using your smart phone. Substitute "smart phone" for "signal generator" and "volume" for "output". I also use my smartphone for testing but with music rather than a tone signal . Going to try using it in place of the signal generator next time I am at the bench.

+1

Connect the output of you headphone jack on your phone to your scope and measure the output with your scope. Use the volume to get to an acceptable level to input to the amp.

The first time I injected a signal into my amp I did not consider the level of the signal generator. The signal was distorted all through the signal path because the level was too hot. When I turned up the volume my dummy load started smoking!!! My signal generator defaults to 5v when its turned on. The output from a guitar is very low, depending on volume and tone levels and how hard you strum, it's about -30 dB or .005v. Now when I inject a tone at .005v I can see a perfect sign wave all through the signal path that increases at each gain stage.

Hope this helps!!

[aaron]

So I'm injecting a 440 Hz (A note) signal into the input jack, and everything seems OK on the oscilloscope at pin 2 ot the 12AX7

All I want to know is what voltage level is that 440Hz signal at pin 2? 200mV_pp is a good level to put into a guitar amp. If your test signal is 1V or 10V that's too high and will cause problems. Can you adjust the level of the signal from your cell phone? Don't rely on the phone app to tell you what level you have. Let your scope do the measuring.

Which scope do you have?

OK, I'm finally back in front of the amp & scope, so I'm setting the signal to 200mV for all subsequent measurements.

[aaron]

I would suggest you first start by understanding what is happening in your first stage, then move to understanding what you are seeing in the volume/tone section. I am assuming you can adjust the output level of your signal generator and you are running your amp into a dummy load. If so, conduct these tests:

1.   First, note where you have your signal generator and V1A Cathode Bypass Switch set.
2.   Measure the ACRMS at V1A Grid (Pin 2).
3.   Turn the output down on the signal generator.
4.   Turn your Volume and Tone controls to max.
5.   Attach your oscilloscope downstream of V1A Plate (Pin 1) Coupling Cap (or directly on Pin 1 if you have verified your scope can handle the voltage).
6.   Set you Bypass Cap Switch on V1A to the 0.47µF position.
7.   While watching your scope, increase you signal generator output until the signal you are seeing on you scope starts to clip (distort). What you are effectively doing here is seeing what input level is the maximum signal level that will produce a clean signal.
8.   Measure the ACRMS at V1A Pin 2.
9.   Note the Voltage on your oscilloscope (1/2VPP). Multiply by 0.707. This will give you ACRMS.
10.   Divide you Plate ACRMS reading by your Grid ACRMS reading. The quotient is the gain.
11.   Repeat the procedure 3-10 with the Bypass Cap Switch on V1A in the 25µF setting. You should notice that the input signal ACRMS required to begin to produce distortion of V1A is lower for the 25µF Bypass Cap Setting than for the 0.47µF setting.
12.   Compare your test results to where your signal generator and bypass switch were set in step 1.

Now that you know what signal levels will begin to produce distortion, were your original settings going to produce a clean or distorted signal from V1A? Repeat the tests you originally posted, only this time, use one of the two signal generator and bypass switch settings from your testing above.

[aaron]

I would suggest you first start by understanding what is happening in your first stage, then move to understanding what you are seeing in the volume/tone section. I am assuming you can adjust the output level of your signal generator and you are running your amp into a dummy load. yesIf so, conduct these tests:
NOTE: I AM READING THE VOLTAGE BY COUNTING SQUARES ON THE SCOPE, IT'S TOO OLD TO HAVE IT DISPLAYED AUTOMATICALLY
1.   First, note where you have your signal generator and V1A Cathode Bypass Switch set.
Unless noted, it will be without any bypass caps switched on.
2.   Measure the ACRMS at V1A Grid (Pin 2).  Not sure here.....I've set the output to be 200mV per Sluckey's instructions.....i.e., from the bottom to top of the sine wave is 200mV  AND the switch is in the position that goes to R6, and neither of the caps (C9/C8)
So, 1/2 x 200mV = 100mV x 0.707 = 70.7............I hope that this is right......
3.   Turn the output down on the signal generator.  I turned it down to about 18mV
4.   Turn your Volume and Tone controls to max.
Before turning up the Vol/Tone, i.e., Vol & Tone = zero, I measured about 325mV after the coupling cap of pin 1 output with neither bypass caps selected
5.   Attach your oscilloscope downstream of V1A Plate (Pin 1) Coupling Cap (or directly on Pin 1 if you have verified your scope can handle the voltage).
Done
6.   Set you Bypass Cap Switch on V1A to the 0.47µF position.
7.   While watching your scope, increase you signal generator output until the signal you are seeing on you scope starts to clip (distort). What you are effectively doing here is seeing what input level is the maximum signal level that will produce a clean signal.
I'm sorry, but I can't follow your directions......With VolTone maxed out, I get crazy distortion of the entire wave.....With the Vol @max, as I increase the Tone it doesn't seem to distort....then once I get past 4 o'clock it suddenly hits a land mine a I get a caterpillar sine wave, whose measurement is 2 Volts
8.   Measure the ACRMS at V1A Pin 2.
9.   Note the Voltage on your oscilloscope (1/2VPP). Multiply by 0.707. This will give you ACRMS.2 volts (2000 mV) x 1/2 = 1000 mV x .707 = 707
10.   Divide you Plate ACRMS reading by your Grid ACRMS reading. The quotient is the gain. (707/70.7 = 10 ?????)
11.   Repeat the procedure 3-10 with the Bypass Cap Switch on V1A in the 25µF setting. You should notice that the input signal ACRMS required to begin to produce distortion of V1A is lower for the 25µF Bypass Cap Setting than for the 0.47µF setting.  It does the same thing at the same point on the Tone control....once it gets past 4 o'clock, the sign wave jumps off the scope with crazy distortion
12.   Compare your test results to where your signal generator and bypass switch were set in step 1.

Now that you know what signal levels will begin to produce distortion, were your original settings going to produce a clean or distorted signal from V1A? Repeat the tests you originally posted, only this time, use one of the two signal generator and bypass switch settings from your testing above.

So, I'm sorry, but the test didn't work out as expected....

Starting out with Tone maxed out, once the Vol goes past 8 o'clock I get the same bad results.....

I'm posting some pics of before and after the wild distortion...

As always, thanks for everybody's help, and sorry if I'm screwing up something/not following directions properly

I'm going to have to figure out how to downsize my pics, everthing's too big.....more coming soon!

Now I've got problems resizing the images.  Please be patient, unless you can understand what I've written so far

[pdf64]

I think the layout is dreadful, ie speaker circuit in extremely close proximity to the input grid.
And a remotely mounted input grid stopper.

Due to that, the amp may be oscillating at higher volume / tone settings.

[astronomicum]

I have just completed the test I posted on the amp I am working on. I used the tone generator on my smart phone and it worked well  I saw the same thing you did however. If the first stage does not distort, then you will send the amp into overdrive distortion. I was monitoring the amp output at the same time and saw this. I should have run this test myself before posting it. My bad folks 

[HotBluePlates]

Starting out with Tone maxed out, once the Vol goes past 8 o'clock I get the same bad results.....

I'm posting some pics of before and after the wild distortion...

Depends on what you are describing as "distortion."  The pics will greatly help us (a YouTube video would be even better).

I have a sneaking suspicion you're seeing the waveshape change because of the capacitance introduced at the Tone control.  That shifts the relative phase of Voltage/Current, and will fool people into thinking something crazy is happening.

[aaron]

I think the layout is dreadful, ie speaker circuit in extremely close proximity to the input grid.

Very close!

And a remotely mounted input grid stopper.

What do you suggest that I do??

Due to that, the amp may be oscillating at higher volume / tone settings.

Starting out with Tone maxed out, once the Vol goes past 8 o'clock I get the same bad results.....

I'm posting some pics of before and after the wild distortion...

Depends on what you are describing as "distortion."  The pics will greatly help us (a YouTube video would be even better).

I have a sneaking suspicion you're seeing the waveshape change because of the capacitance introduced at the Tone control.  That shifts the relative phase of Voltage/Current, and will fool people into thinking something crazy is happening.

Good idea re: youtube!  I've never uploaded one before!

[aaron]

Due to frustration and lack of time I won't be posting again till Sunday 13th....

I want to explain something.

I live in an apt and I can't play the amp full blast where the wierdness starts, except for a minute here and there.

So, I've been using an oscilloscope and a 8 ohm 100W load resistor to try to reproduce the problem at zero volume.

I took a (literally) minute today to play loud with my guitar.

The strange sound/distortion/whatever it is occurs when I hit the string hard and the amp is cranked.
It happens on all 3 of the capacitor bypass (or none) switch positions.

I will try to  attach the scope probes to the oscilloscope and see if I can catch the amp doing it's "distortion"

[pdf64]

A screening plate, electrically connected to the chassis and mounted between the 2 circuit areas, might be a good way forward.

[aaron]

I want to thank everybody for their help!

By the grace of G-d, I was able to find the problem.

It was parasitic oscillation. (pdf64 hinted at this!)

These two videos helped a lot, b/c they showed exactly what I was getting on my scope!

D-Lab's solution didn't help me b/c I already had screen resistors, but he described the symptom exactly>>>w/Vol all the way up, when you turn up the treble, the sine wave goes nuts.

In the Uncle Doug video, he also show the same problem sign wave, but his solution was moving two adjacent wires about a half an inch apart and it solved the problem.

In my case, I started chopsticking parts and then wires.  I noticed that one of the two primary wires of the output transformer made the crazy sign wave smaller when I pushed it towards the chassis.  In the end, I elevated the wire above the rest of the circuit, and made sure it was perpendicular to the shielded signal wires from the input jack and the volume pot.

It's frustrating b/c you can build an amp correctly, i.e., according to the layout and the schematic, but still have problems.

I hope this helps someone else!

[Soulfetish]

PDFs given good explanations as to what’s happening, and why the waveform appears that way.

If I could try and simplify what you see on the schematic; you can visualize that with your volume pot turned all the may up, even if you measuring “before” the pot, you may as well be measuring at the tube grid.
It would be the same as if it were a fixed grid leak resistor you commonly see in input stages, and other gain stages.
Maybe that helps

[aaron]

PDFs given good explanations as to what’s happening, and why the waveform appears that way.

I'm sorry, but I have no idea what you're saying......
If I could try and simplify what you see on the schematic; you can visualize that with your volume pot turned all the may up, even if you measuring “before” the pot, you may as well be measuring at the tube grid.

Do you mean that by turning up the vol pot all the way, it's as if you're taking it out of the circuit? 
If so, what would that do?
It would be the same as if it were a fixed grid leak resistor you commonly see in input stages, and other gain stages.
Maybe that helps

I'm sorry, but I have no idea what you're saying......

[Soulfetish]

Really? Then Nevermind