Why are the two sine waves out of sync

[skal1]

ok lads don not know much about wave forms but this does not look right .


PIC of input/out of vacuum tube stage top wave form is o/p ,bottom wave form i/p..


What would cause this effect and can it be corrected.

cheers

skal

 

[nielsk]

Have you sent the same signal to both scope inputs to check the scope?

 

[Nora]

I see no difference except in amplitude and dc offset (or difference in vertical position on your scope)
I don't see any phase difference or delay?

 

[JohnRoberts]

Yes, if you turn off the scope the difference will stop.    8)
======
Seriously that just looks like a small amplitude difference, or gain/scale difference between the two inputs, and a slight DC level difference between the two.

The two signals could actually be slightly different levels or the two scope inputs could be scaled slightly differently.

What exactly is your concern?

JR

 

[skal1]

The pic is showing the difference between the input and out levels, both inputs on the scope are set up identical

@JR  i think you are right on both counts it seems i need to re work the stepped pot.


ok lads thanks so i have learnt a new word { DC level } . What does this new word me and can i correct it ?

no concerns , i just did not know how to evaluate the findings

cheers

skal

 

[Nora]

wait, are you sure the vertical position on both channels is identical?
When you short both inputs to ground, you should only see a single flat line. (most scopes have an "input-grounded" next to the AC/DC setting, just for this)

If that's the case, then yes, you have a dc-offset problem

What output is driving the scope?
Ac- or Dc-coupled?
Since you said it's a tube circuit I'm guessing ac-coupled because the dc-coupling would show a much higher offset.
Try loading it with a reasonably sized resistor like 10K
A scope has a very high input impedance, designed to "measure" instead of loading or interfering with the dut.
In an ac-coupled output circuit, the capacitor might need a dc path in order to remove small offset voltages.

 

[skal1]

wait, are you sure the vertical position on both channels is identical?
When you short both inputs to ground, you should only see a single flat line. (most scopes have an "input-grounded" next to the AC/DC setting, just for this)

If that's the case, then yes, you have a dc-offset problem

Yes Arno the "input-ground" lines appear as one when {Gnd} is switch is on

What output is driving the scope?

TUbe output stage

Ac- or Dc-coupled?

Transformer coupled

Try loading it with a reasonably sized resistor like 10K

Wot should i be loading ,the output tx or the input from sound card.




Cheers

skal

 

[skal1]

Are yeah howshould i be connect the scope to the pushpull tx probe to 1 side and ground to the other ? , just checking

skal

 

[Nora]

Probe should connect one side of secondary, ground connects to the other side.
Transformers are always meaured between both sides of a winding. There is no "ground" since they are floating. You do connect the scopes ground because it has an unbalanced input...

A transformer winding cannot generate a dc offset.
Does your probe have a transformer? Otherwise I'm guessing the dc offset should come from the scopes input.
Have you tried swapping probes and inputs?
I've seen unreliable weird offsets on a scope when you have improper connections. Eg, disconnected ground.

Use a dmm to verify that you have no dc on the transformer due to incorrect wiring...

 

[skal1]

@Arno

OK just check with dmm reading say 0.01 mv on 200m scale



skal1

 

[etheory]

Could you please post the portion of the schematic that you have the probes attached to and mark the precise points that you are measuring with the probes, and then let us know which point refers to which signal (say something like point A refers to the smaller amplitude trace, and point B to the larger amplitude trace).

There are just far too many possibilities to even discuss without a concrete reference.

For instance if you measure the input after the coupling capacitor, or before it (depending on whether your circuit is AC or DC coupled) then clearly you get different results, or if you measure one side or the other of the output transformer.

I'm sure there is a simple explanation.

 

[CJ]

unless the tube stage is a perfect 1:1 amplifucation factor, you will never see the same waveform at the input and output,

guessing one probe is set to 1:1 and the other is a 1:10 and the tube has an amp factor or slightly less or more than 10?

this one is tough without being there,

 

[rafafredd]

Did you put any load other than the scope leads on the putput transformer? You could also load it with two matched resistors on the secondary, grounding the middle point, like a virtual center tap.

Also, are the tube preamp and the scope grounded on the sma epower system? If not, you could try connectint grounds together, or using the same wall power inlet.