How to Determine Signal Phase with a 'Scope?

[jjasilli]

Why is phase the same in the following situation?

My vintage PA amp has no normal secondary taps in Ohms.  Instead it has a balanced OT secondary with a grounded CT and surrounding voltage taps:  75 25 CT 25 75.  My speaker output is from the 2X 25V taps for ~6Ohms. 


I decided to verify the polarity of the 2X 25V taps using a 'scope.  To my surprise the 'scope shows the same polarity from ground to  either 25V tap.  How can this be?

[sluckey]

I decided to verify the polarity of the 2X 25V taps using a 'scope.  To my surprise the 'scope shows the same polarity from ground to either tap.  How can this be?

It all depends on how you triggered the scope. Either trigger the scope externally from the sig gen or trigger from only one of the inputs. Does that help?

[jjasilli]

Either trigger the scope externally from the sig gen
I don't know what this means.

or trigger from only one of the inputs.
The scope's triggering source choices are: INT, B, Line, Ext. I can get a stable curve switched to B.  But the polarity doesn't change.  Polarity also doesn't change if I scope a 6.3VAC power source.

[drgonzonm]

Give us some more clues. 

Do you have name or model number for the PA.   Who knows maybe we can find a manual. 

Numbers on the transformer, maybe be we can determine the transformer by manufacturer.

Is PA, SE or PP, I suspect its PP.  and then if PP is it a tube PI or transformer? 

What are the power tubes. 

Determine the windings ratio using the 6.3 v on the secondaries, and determine new voltage on primaries.  (Step up transformer).

I would suspect that if you ground the center tap, the two 75's would be 180 degrees out of phase, same for the 25's. 

If all fails draw us a picture. of how you wired everything. 

The last thing that comes to mind, is the two halves of the secondary windings are interlaced. 

[PRR]

Does your 'scope have an external trigger input and option? If it needs a cable, fake one.

They are obviously anti-phase or you would be complaining of zero output. But you should learn how to check that.

[DummyLoad]

Why is phase the same in the following situation?

My vintage PA amp has no normal secondary taps in Ohms.  Instead it has a balanced OT secondary with a grounded CT and surrounding voltage taps:  75 25 CT 25 75.  My speaker output is from the 2X 25V taps for ~6Ohms. 


I decided to verify the polarity of the 2X 25V taps using a 'scope.  To my surprise the 'scope shows the same polarity from ground to  either 25V tap.  How can this be?

just out of curiosity, what make and model scope?


--pete

[jjasilli]

Thanks for your responses.  The scope is a Protek P-3502C, 20MHz, dual trace.  It seems typical.  Attached is an image & basic specs. (The manual is too large to upload; and somehow, I can't find it online.  If needed , I can post relevant pages)


Sig Gen:  vintage Mercury 1600 vacuum tube audio range generator


Amp;  Stromberg Carlson APH 1100: 100W mono bloc; tube PI > 2X KT88's in PP > OT with balance output secondary 75 25 CT (grounded) 25 75.

[Tony Bones]

It's a dual trace scope. Were you looking at both 25V outputs at the same time? Are you sure that neither channel is set to invert?

[shooter]

Sig Gen

It looks like the bnc all the way right on your scope is trigger.  A work-around might be set ch1 to monitor sig-gen, channel 2 over-lay on 1 when you measure each side of your output, you should find one inverted

[sluckey]

Just put a "Tee" connector on your sig gen. One cable to the amp input, the other cable to the scope external trigger input. Set the scope trigger source to external. Now you will be able to reliably see the phase differences between the test signal at different points in the amp.

[jjasilli]

That seems doable, but instead of a TEE:  my vintage sig generator has only banana jack outputs + / -(ground).  Can I stack 2 male/female banana plugs into the sig gen, and run a lead from one banana plug terminated with an RCA plug > Amp Input; and the other lead terminated with a BNC connector > EXT Trigger Input on scope?

[sluckey]

Absolutely. That will work. It's a lot easier if you have a bucketful of adapters. BNC to dual banana is very common. I even have a BNC to 1/4" phone adapter.

[drgonzonm]

Found the manual for the amp.  I am not sure how to wire in the speakers.   25v to CT for 6 ohms, or 25v to ground for 6 ohms? 

[jjasilli]

@ sluckey,.  Thanks for the info.  I've got everything hooked-up, but am temporally sidetracked by other obligations. 


Meanwhile your comments led me to further web research which has all been very instructive.  I never paid much attention to triggering before.  I now see that the balanced nature of this OT is a red herring. The problem exists with any tranny outputting AC whether CT'd or not; or any AC waveform.  The scope will "help" you with its internal trigger by selecting a portion of wave info, and triggering the view-able trace to start on the rise (+) or on the decline (-); thereby disguising phase.  To get a trace of the phase of the waveform the trace must be triggered externally in time with the signal under test. 


@drgonzonm:  Yes, this stuff is very arcane.  There's more than one manual!  Eventually it becomes clear that this is a CT'd OT.  Here's a prior thread:  http://el34world.com/Forum/index.php?topic=18577.msg189795#msg189795


The bottom line is that you connect to the 2X 25V taps in this amp.  Why?  W= V² / R.  100 = 625/R; R = 625/100 = 6 Ohms.  Close enough for 4 or 8 Ohm speakers.  Using only half the winding halves the voltage, which heavily affects the Ohms "output" if you do the math. 

[drgonzonm]

Got it,

so the 2x 25v taps are 12.5 ct 12.5, and the 2x 75 are 37.5 ct 37.5?  much like the 120 ct 120 transformer that feeds my house. 

[drgonzonm]

Why is phase the same in the following situation?

My vintage PA amp has no normal secondary taps in Ohms.  Instead it has a balanced OT secondary with a grounded CT and surrounding voltage taps:  75 25 CT 25 75.  My speaker output is from the 2X 25V taps for ~6Ohms. 


I decided to verify the polarity of the 2X 25V taps using a 'scope.  To my surprise the 'scope shows the same polarity from ground to  either 25V tap.  How can this be?

1.  I just want to verify the ground (blk wire) you are speaking of on this transformer is not the ct tap (blk/wht), but the secondary ground as shown on the PA's very abbreviated service manual.  I ask that seemly less than intelligent question, because the manual I found shows symbolically separate taps and colors for the ground and the ct, and therefore, I would not make an assumption that the CT is Grounded.   If it is the ground, then having the same polarity to me makes sense. if its the ct to the 25v's then I would expect different polarities. 

2.  If I pull up Hammond output transformers data sheets, just about every transformer sheet shows output voltages for each tap,  assuming the the primaries are fed the test voltage.   

[jjasilli]

Per footnote 10 on the Schematic, an "external ground strap" (a jumper) connects the CT (BLK/WHT) to GRD, i.e. ground (BLK).  This is a balanced speaker output.  If the external ground strap is removed, the output is floating.  For more info see the attachment: "Loudspeaker Installation Methods and Connection Techniques", BOGEN COMMUNICATIONS, INC., p.p. 9 - 10.


so the 2x 25v taps are 12.5 ct 12.5, and the 2x 75 are 37.5 ct 37.5?  much like the 120 ct 120 transformer that feeds my house.
Yes.

[drgonzonm]

jjsilli,

the schematic I have only has 9 notes, on it, not that all important 10th note. 

So as you say it depends on which schematic you have. 

thanks for the bogen info

[jjasilli]

Your welcome re the Bogen literature.  Attached: 

*  Yet another Stromberg Carlson manual that makes the balanced nature of the speaker outputs more plain.
*  A schematic with the 10th footnote (I never realized there were different versions!)

[drgonzonm]

I am not a moderator, nor do I want to be, but this thread, probably should have been moved to the tools forum.  Additionally, the thread probably needed to be better titled. for example  "Determining phase on a Balanced OT Secondary using a scope." 
JJasilli, from his posts is an experienced builder and troubleshooter.  Like any experienced troubleshooter, Mr. Jjasilli, wanted to expand his knowledge base.  PRR, Sluckey and others, were able to provide the info on how to set up a scope to perform the task Mr. Jjasilli want to do.   

I apologize for any misleading questions or comments. 


The gist of the thread was setting up  a scope, and triggering to verify expected performance. 

[jjasilli]

Per my Reply #13:  "the balanced nature of this OT is a red herring. The problem exists with any tranny outputting AC whether CT'd or not; or any AC waveform."


After sorting through all this, I now see, and agree, that the problem should have been stated:  "How to determine phase with a 'scope?"  However, if I then knew how to phrase the phase question, I probably would already have known the answer; whereby this thread would never have come into existence.

[PRR]

> the problem should have been stated

I believe, as OP, you can go-back and re-name the thread.

If you can't(?), or if you think it should be in the Tools section, use the "Report" link under your post, leave a Comment what you want done, and one of us mop-pushers will do the deed.

[jjasilli]

I just tried renaming this Thread. EDIT:  It didn't work.


I have no objection to renaming this thread:  "How to Determine Signal Phase with a 'Scope?" or to re-placing it in the Tools section.

[drgonzonm]

Per my Reply #13:  "the balanced nature of this OT is a red herring. The problem exists with any tranny outputting AC whether CT'd or not; or any AC waveform."


After sorting through all this, I now see, and agree, that the problem should have been stated:  "How to determine phase with a 'scope?"  However, if I then knew how to phrase the phase question, I probably would already have known the answer; whereby this thread would never have come into existence. 

I have a scope, an old HP gathering dust, along with some tube based toys.  I along with others did not know the need for an external trigger. 
IMO, Your question, is applicable to the forum, as you are probably not the only one to run into this problem. It was a good topic. 
regarding the "red herring", because, the tranny was set up as a ct tranny, did the phase become important, and hence, your all important question, "why can't I display the two phases on the balanced transformer output", and the discussion led to the need for an external trigger.

Slightly off topic, but relevant to the amp,  Stromberg Carlson, as far I as can tell, allows you to set up this amplifier as non-ct output.  If you look at the 115v output, a different winding is wired in by jumpering the ground lug, to the nearest 70v lug,  I believe in order to keep the transformer from smoking, the factory installed ground to ct external jumper would need to be removed. 

Again thanks for the interesting thread. 

[DummyLoad]

higher end scopes will usually have a trigger source selector that allows selection one of the channels as the trigger source for both channels. that negates the need to use the external trigger source.


--pete

[jjasilli]

@drgonzonm:  Be careful if your scope is old enough:  Ground means Ground & floating measurements are not ok -- attaching one of those scope's ground lead elsewhere will cause a short circuit and likely destroy something somewhere.  (Like the typical VTVM.) I hope that this thread has been generally helpful in exploring the trigger function as necessary to phase determination.


@Dummyload:  Yes, my scope's Trigger Source Switch has:
* INT (Internal):  the source of my initial confusion, because phase is disguised
* B                  : probably meaning CH2, but it would be nice if the manufacturer kept the nomenclature consistent.  This must be what you refer to; and I'll do that too.
* Line              : I'm clueless
*Ext (external) : What sluckey said.

[sluckey]

Get the manual for your scope. It's freely available.

Your scope vertical inputs are called "A" and "B", not channel 1 and channel 2. So, nomenclature is consistent.

Your trigger sources...

INT means trigger on whichever input is being used. Could be A or B. If displaying both channels the trigger will alternate between A and B (this is usually undesirable, especially when comparing phase or measuring time between two inputs). INT trigger is no good for viewing phase relationships.

B means always trigger from channel B, even if displaying channel A. You can see phase changes when looking at channel A but you will not see phase changes when looking at channel B.

Line means trigger the scope from the AC power line (usually 60Hz).

Ext means trigger the scope from a signal applied to the EXT TRIG input. This requires an extra cable but it will allow you to set the trigger level independent of the signal level applied to the vertical inputs. No fumbling to readjust trigger level as you change the Volts/DIV switches or move the probes to different points in the test circuit. And you will ALWAYS be able to see phase changes. If you view a signal that starts off swinging positive on a tube grid you will see that signal start off going negative on the plate, just like the text book says.

All scopes are slightly different so get the manual and play with it. A more expensive scope might have these trigger source options. Source labels make a bit more sense to me...

[Ed_Chambley]

This video along with others that loaded helped me learn external trigger.  This uses the same scope Sluckey has I think. 




https://www.youtube.com/watch?v=OFGm-Pel4Hg

[Ed_Chambley]

And this one is very long and covers a lot.  There is annoying background noises, but anyone interested in understanding more uses for their scope it is very worthwhile.  I must admit it is very possible to build amps without one, but to me it has become essential.  I think back on the amount of time I spent troubleshooting with a DMM as a huge waste of time now.  I should have been using the time learning more about scope usages. 


Also, videos covering probes are very helpful.



https://www.youtube.com/watch?v=niBGkqlh2XY&t=296s

[Tony Bones]

Pretty much every scope I've ever used has the same set of basic features. Triggering on either/both channels is a new one for me and seems dumb. I would probably end up using ch B most of the time and consider A the 'other' channel just for that reason, and pretty much never use the 'int' mode. Some scopes have trigger modes unique to television. You can just ignore them. They make no sense with digital TV anyway. 

So, once you get your head around basic triggering functions like slope and level, etc, you should be able to use almost any scope without much trouble. It's really not rocket science.

[jjasilli]

Problem solved, in any one of 3 ways.

I must confess I never used both channels of the scope simultaneously before. 



Finally did the test and here's my findings, using both channels of the scope, both probe's ground clip > OT CT (which is grounded to amp chassis); one probe tip > ea 25V OT terminal:


Position of Trigger Source SW:
* INT : Ch A+; Ch B (-)     [Tony Bones was right-using both channels at the same time is one solution.]


* B      : Ch A(-); Ch B (+)  [Dummyload was right, though with reversed polarity]


* Line  : couldn't get stable traces and gave up on this setting.


*Ext (with signal input from signal generator)   : Ch A+; Ch B (-)     [sluckey was right]


Thanks to all!   

[pompeiisneaks]

This is great info guys thanks! I need to watch these videos later from home!  Trawling a few forums at work is okay, watching hours of video, not so much :)

~Phil

[sluckey]

* Line  : couldn't get stable traces and gave up on this setting.

That's because the ac power from the wall is nonsynchronous with the signal from your generator. If you set your sig gen frequency to 59.9Hz or 60.1Hz you would see the signal slowly slipping across the screen. Slowly tune the frequency farther from 60Hz and the display will begin to slip across the screen faster and faster until you can no longer tell it's a sine wave.

Line sync or trigger is only useful for viewing signals that are derived from the AC line, such as viewing PT winding voltages. Yes, you can see phase reversals on the secondary windings. Just look at any winding that has a center tap. You will see that one side will be 180° out of phase with the other side. Also good for looking at 120Hz ripple on the B+ rail.

So, line trigger does have some good uses. Play with it a few minutes. Take a look at your PT windings and B+ ripple. You're about to move into the visual world of electronics! 

[Tony Bones]

To add a tiny bit to Sluckey's excellent post, sometimes I use a scope instead of DMM to probe voltages just to get a rough idea of levels and also to see how clean my supposedly DC really is. Since it's basically DC there's nothing to trigger off of. "Line" trigger is handy in those situations to keep the trace moving across the screen.

[jjasilli]

@Ed_Chambley: so far I got through 1/2 of one video.  Very helpful.  Thanks!


@PRR & sluckey: measuring B+ ripple sounds good, but my scope"s manual says to stay below 300Vpk (600Vp-p) at the point of measurement.  In an old thread PRR building a device to safely probe B+, but the attachment is gone:  http://el34world.com/Forum/index.php?topic=3974.0  Can someone resurrect this?

[pompeiisneaks]

@Ed_Chambley: so far I got through 1/2 of one video.  Very helpful.  Thanks!


@PRR & sluckey: measuring B+ ripple sounds good, but my scope"s manual says to stay below 300Vpk (600Vp-p) at the point of measurement.  In an old thread PRR building a device to safely probe B+, but the attachment is gone:  http://el34world.com/Forum/index.php?topic=3974.0  Can someone resurrect this?

One thing you can do is get a higher voltage probe.  Basically the input can't take over that amount, but the probes can go higher.  a good 100X probe will cover anything you'd use, but I've used my dual 1x 10X probes on 10X because as I understand it, they can handle 10X the input's, as they lower it by a magnitude of 10.  (someone correct me if wrong)>  So if you've got a 600V input and put the probe on 10X mode it can handle, in theory, 6000V because the probe has a resistor in it that reduces the voltage and current.  Now you also need to look at the voltage rating of the probe too, I think my probe that's 10X says 1000V max but no amp I've been on yet exceeds 1000V so I'm good.  (Please someone correct me if wrong, I don't want to nuke my scope or kill myself!)

~Phil

[jjasilli]

The best practice is to follow the scope's manual which limits me 300V_pk at the point of measurement.  I have amps with 600VDC B+.  Under signal conditions that could double or more! 
Relying on a 10x probe is not a good idea.  I could set it wrong, which believe me, I WILL eventually do.  Also, if the probe's ground clip comes off, or is faulty, I think it then automatically becomes a 1x probe. 

Moreover, the probe's voltage division factor does NOT determine safe voltage.  The voltage rating of the cap in a 600V probe may only be 600V.  6000V will kill the cap and possibly wreak other havoc with the scope and/or the device under test.  IOW a probe should not be used for voltages higher than its rating.

I'd rather build an outboard attenuator, say 10X, and probe that.  Then, 600V becomes 60V at the point of measurement; AC ripple should still be large enough to measure. 

[pompeiisneaks]

The best practice is to follow the scope's manual which limits me 300V_pk at the point of measurement.  I have amps with 600VDC B+.  Under signal conditions that could double or more! 
Relying on a 10x probe is not a good idea.  I could set it wrong, which believe me, I WILL eventually do.  Also, if the probe's ground clip comes off, or is faulty, I think it then automatically becomes a 1x probe. 

Moreover, the probe's voltage division factor does NOT determine safe voltage.  The voltage rating of the cap in a 600V probe may only be 600V.  6000V will kill the cap and possibly wreak other havoc with the scope and/or the device under test.  IOW a probe should not be used for voltages higher than its rating.

I'd rather build an outboard attenuator, say 10X, and probe that.  Then, 600V becomes 60V at the point of measurement; AC ripple should still be large enough to measure.

First, if the ground clip comes off, the probe is no longer in circuit and is therefore not conducting anything.  Not sure I understand how it becomes 1x at that point, it becomes open circuit X.  the rating of the probe is the cap you're talking about so the designer better have used a 1000V cap in the probe or they're full of crap,and would be sued if someone got hurt/killed at 900V on a 1000V probe.  My Manual for my Rigol states clearly:

Max Input Voltage
(1MΩ)
Maximum input voltage of the analog channel

CAT I 300 Vrms, CAT II 100 Vrms, transient overvoltage   

Nowhere does this say "at the point of measurement" it says 'input voltage'

From what I understand a 10X probe (or a switch to 10X) makes it have a 10MΩ input, which should make the input from 600V see 60V maximum on the other end.  the 100X goes to 6V. 

I was explicitly told here that a 100X probe would make any amp safe for a scope.  I just wanted someone to make sure I wasn't misstating something in my comments above. 

I'm curious as to how I may be misunderstanding all of this though? 

~Phil

[shooter]

which limits me 300Vpk at the point of measurement

Not sure why your manual says this, I have used HV probes on scopes measuring up to 52Kvdc on scopes with 300vdc max in.

EDIT:  not recommended for the faint of heart

[jjasilli]

OK, I'm going to start another thread.


EDIT:  With the probe ungrounded you will not get a reading.  But that's a mind trick.  With no ground there's no voltage division, so full voltage under test will attempt to charge it's way through the probe and into the scope.

[shooter]

With the probe ungrounded

you don't normally want an ungrounded probe, that's kinda like a 2 pronger amp

[jjasilli]

Correct, but I'm talking about an abnormal, fault situation.  Let's stop this thread now and go to the other.

[Ed_Chambley]

OK, I'm going to start another thread.


EDIT:  With the probe ungrounded you will not get a reading.  But that's a mind trick.  With no ground there's no voltage division, so full voltage under test will attempt to charge it's way through the probe and into the scope.

Keep them coming. I too have really begun using my scope a lot more. In the past scope threads didn't seem to get attention because you can build an amp without one, but actually seeing and measuring tube selection and not just going by the data sheets helps me understand what is really happening.


After completion of my breadboard, the first thing I want to setup on it is a Push Pull 300b amp for Hifi, but sigmoid amps is offering one for guitar, so when I build my Hifi RAII tube phono and line inputs I can build a guitar one too.
Of course the scope knowledge will help a lot.

[shooter]

there are a few of us dinosaur types  that used this function of a scope to test diodes, caps, etc
it's also just plain fun

https://en.wikipedia.org/wiki/Lissajous_curve

[jjasilli]

My scope has a component test function, with a circular recessed input jack.  I can't figure out what type of plug it accepts.  The same type of input appears on other scopes.  Banana plugs fit too loosely.