scope help

[trem]

I have never seen am amp through the eyes of a scope. My recent build is basically a BF Fender. It seems to have "normal" voltage readings, but suffers from a real lack of headroom. With any volume or any pick attack, the amp really distorts in a bad way.
The scope seems to show normal sine until I get to the speaker jack. That kind of looks like four, in phase traces, that vary in amplitude.
I am not sure where to look for a fix.
I shorted the plate to the heater on one of the power tubes while checking plate voltage, I did not blow a fuse, but maybe I zapped the OT.? It is brand new from Edcor. I hope I did not damage that.
Anyway.....what do you guys think of the sine wave at the out jack.?
Thank You

Pic 1. Power tube plates (both) 190 Volts

Pic 2. Speaker jack 30 Volts

[eschertron]

The speaker trace sure does look messy.

Can you fill in some background info?
1 what is the sine frequency? The speaker signal looks like its riding on some other wave. If that's the case, zoom out and see if you can calculate its frequency. That might be relevant. The "junk" on the speaker signal might be noise, it might be periodic. Zooming out can help see the trend.
2 is the scope ground connected to chassis ground on both traces? It will help to have a common reference (literally) when doing the measurements.
3 just thought of this one. Do you have NFB on this amp? is there a connection to the OT secondary that could be jacking it around?
4 Vin/Vout = 6.3 --- what's the load impedance? must be extremely high?

[trem]

No experience with a scope but I will do the best I can:

1. Frequency is 1 kHz

2. Not sure what you mean. Both traces.?
I am using just one probe, and the ground is at the tremolo footswich.

3. There is no NFB

4. Using an 8 Ohm dummy load.

Thanks for your help

[olddawg]

For starters, connect both scope probes to chassis ground. Do not use long leads. To me it looks like you have your 1khz modulating 60hz. Check your B+ for ripple.

[trem]

I may not understand fully.....
...I am only using one probe. I am not using the ground clip on the probe, I am using the ground connection on the front of the scope panel.
Thanks

[Enzo]

OK, on the first picture, you have a clean sine wave. The sweep rate is such that you have roughly one cycle per division on the screen. Now look at the output picture. I see roughly one cycle per TWO divisions. Did you speed up the sweep to make the second image? If you want to compare, make all readings at the same sweep, just adjust the vertical for fit.

But try this, slow it down even further and see if your four waveforms don't resolve into one waveform of relative clean sine wave, but it slowly moves up and down like a road through the hills. I agree that it looks like your output is riding a lower freq wave like 60Hz.

Both traces - the two you show pictures of.

Ah, wait, is your speaker jack grounded to the chassis? And when you say the scope is grounded to the trem jack, do you mean at the chassis? Ground the scope to the chassis. If your speaker jack is not grounded, then the output lacks a reference to ground and so I wouldn't doubt 60Hz would get involved.

Please put readings in context. For example 190v on the plate is a very low reading if you mean DC, but maybe as AC signal. SO always use units. And are we talking RMS or are we talking peak to peak. 30v p-p is more like 10v RMS, and that across 8 ohms means more like 14 watts.


Show us the schematic of exactly what you built, or tried to build. We don't know whether it is a Twin Reverb or a Champ.

[trem]

Hey Enzo -
Give me a few hours to figure this all out.
But in the mean time...
I had to change the knobs so the sine would show on the screen.
I will try what you suggest.

I am using a Cliff Jack.
It is not grounded to the chassis. The switching works to short the OT to itself if there is no speaker cable plugged in, But again, the speaker jack is not grounded to the chassis.

The numbers I posted were:
190 VAC peak to peak
30 VAC peak to peak

Will post a schematic ASAIC....in a few hours.

I have the Trem turned off just to confirm that.

The scope is connected to the Trem jack, which goes to chassis ground. Not sure I could get the Scope roach clip down into the chassis, so I used the Trem Jack to make the connection.

Thank You

[Enzo]

There is your problem, the output is not grounded, so when you ground your scope to the amp, it is NOT grounded to the speaker lines, so your waveform is being modulated by the ambient 60Hz. Clip your scope directly across the speaker leads, or ground the scope to the chassis and ground the speaker common. At least while scoping. The amp and speaker themselves don't care if you ground the output.

When you scope the output without a ground reference, you are only getting an induced voltage on the scope. The output is between the hot and return on the speaker jack, but connecting the scope only to the hot, with the scope ground connected to something else(the chassis) there is not a complete circuit with the scope. Let me try it this way. Suppose you have a 9v battery and you want to measure its voltage. You'd put your meter on the two terminals and measure. But if you stuck the red probe on the + terminal of the battery and then grounded the black probe to something, you leave the battery negative unconnected and so your meter can't tell you the voltage. That is what you are doing.

I usually just clip to the edge of the chassis, which is just sheet metal. If that jack is mounted on the chassis, that is OK too. Sometimes I clip to a chassis bolt. For most things it doesn't matter where. The ground clip on the probe is important in certain situations, but otherwise we can be flexible.

If your scope is plugged into a grounded outlet, and so is your amp, then they are already grounded together through the power line, but never rely on that.

[potatofarmer]

I am using a Cliff Jack.
It is not grounded to the chassis. The switching works to short the OT to itself if there is no speaker cable plugged in, But again, the speaker jack is not grounded to the chassis.

Is the OT secondary connected to circuit ground at some point though? You said the amp is "basically a BF Fender" so I'm assuming NFB is involved.

[trem]

Enzo -
Thank you.
Will give this a shot and report back.

Paul P -
BF Fender, but no I did not (i may give it a shot) use NFB.
The OT secondary is not connected to chassis ground.

[trem]

There is your problem, the output is not grounded, so when you ground your scope to the amp, it is NOT grounded to the speaker lines, so your waveform is being modulated by the ambient 60Hz. Clip your scope directly across the speaker leads, or ground the scope to the chassis and ground the speaker common. At least while scoping. The amp and speaker themselves don't care if you ground the output.

When you scope the output without a ground reference, you are only getting an induced voltage on the scope. The output is between the hot and return on the speaker jack, but connecting the scope only to the hot, with the scope ground connected to something else(the chassis) there is not a complete circuit with the scope. Let me try it this way. Suppose you have a 9v battery and you want to measure its voltage. You'd put your meter on the two terminals and measure. But if you stuck the red probe on the + terminal of the battery and then grounded the black probe to something, you leave the battery negative unconnected and so your meter can't tell you the voltage. That is what you are doing.

I usually just clip to the edge of the chassis, which is just sheet metal. If that jack is mounted on the chassis, that is OK too. Sometimes I clip to a chassis bolt. For most things it doesn't matter where. The ground clip on the probe is important in certain situations, but otherwise we can be flexible.

If your scope is plugged into a grounded outlet, and so is your amp, then they are already grounded together through the power line, but never rely on that.

Yeah OK -
So when I put the scope across the speaker jack, I get a much more normal sign wave. Thank You
I hate to ask, but now, when I connect the scope to any point I did before, the trace is real unstable. When I adjust the frequency knob to a position that I can read the amplitude, the whole trace moves across the screen from left to right.
It never did this before. Am I looking at a scope problem of some sort.?
Sorry, I just have no experience with these things.
Thank You

[Tom Phillips]

... when I connect the scope to any point I did before, the trace is real unstable. When I adjust the frequency knob to a position that I can read the amplitude, the whole trace moves across the screen from left to right.
It never did this before. Am I looking at a scope problem of some sort.? ...

I think that you just had a situation where the scope could not reliably (and repetitively) trigger on the signal. This is because you had both your signal of interest and some of the 60 Hz signal being picked up by the scope input. That creates a messy waveform shape.

To show a stable trace the scope must start each sweep across the screen at the same point in the waveform being observed.If this doesn't happen then you get the result you described. Also the waveform needs to be repetitive which is usually not a problem with audio signals. Sometimes you can get a better display by changing the sweep rate but what you really need is a stable trigger. There are some good scope tutorials on YouTube. Try searching for "scope trigger." Seeing how to adjust the trigger will be much clearer than I can describe in narrative. You need to understand how to set the scope trigger controls even when you have a clean waveform.
Have fun learning,
Tom

[trem]

OK Tom -
I may have a scope tutorial somewhere.....or can find one on "The Net".
Thank You

[olddawg]

OK Tom -
I may have a scope tutorial somewhere.....or can find one on "The Net".
Thank You

See if there is a switch labeled trigger. If there is put it in the "auto" position. Also if it is a dual trace scope, make sure that you have it selected to trigger off of the channel you are monitoring unless there is a reason not to. Otherwise the scope will free run. Some scopes trigger better than others and is one of the first things to malfunction on aging scopes. Old B&Ks in particular. Some scopes also have an expandable time base so you can look at parts of a complex waveform. Make sure that is off. You should learn to be able to measure both frequency and amplitude with your scope, but many scopes have an output to attach a volt meter or a frequency counter. More expensive newer models even display them.

[g1]

So when I put the scope across the speaker jack, I get a much more normal sign wave. Thank You
I hate to ask, but now, when I connect the scope to any point I did before, the trace is real unstable.

To look at anything other than the speaker output, you must move your scope ground back to the amp chassis.