Newbie square wave question

[Linx1]

Thanks for looking I have a newbie question. Is this sloping I see on the leading and trailing edge a problem? This is a 10k square wave test I have been experimenting with.

 

[nerdorama]

It's pretty clean but the slope on the edges generally indicates a limited high frequency capability. What is the amplifier?

 

[Brice]

Agreed. This trace is not bad.
Here is a simple diagram which might help.

 

[Linx1]

The amplifier is a Heathkit w5 circuit with a Hammond power transformer 279x and opt 1620 transformer(older unit.) I was told to remove the feedback network and steps to get a baseline; I will follow this advice. Seeing I have made some pretty basic mistakes I wanted to make sure the slope I am seeing is not being caused by something I’m doing wrong.

 

[whoaru99]

There are a lot of smart folks who can help, but based on that I'm not sure I'd do much tinkering.

If you properly align the trace to the scope graticule markings you can derive the bandwidth based on the 10/90 rise time.

 

[wdecho]

Not what I would call an awful square wave at 10khz for a tube amplifier. I just built two PP EL34 amplifiers with better looking ones at same frequency but not perfect either. My two builds had really square, square waves a 1khz. Without any ringing I would not do any tinkering with yours if I were you.

 

[wdecho]

Agreed. This trace is not bad.
Here is a simple diagram which might help.

Thanks for graph. Never seen it before. It will be saved by me.

 

[wdecho]

Seeing I have made some pretty basic mistakes

I have built over 30 amplifiers the last 20 years or and there were only a few that I did not have some kind of mistake on fire up. Trouble shooting is part of the fun of building.

I remember Nelson Pass bringing an amplifier to the Burning Amp Festival years ago and would not take the cover off. He said it was neat and nice underneath when he brought it but then he had to get the dang thing to work. Made me feel much better about my building abilities.

 

[Olle]

It is recommended that you use x10 probes with your oscilloscope when checking waveforms, especially pulsetrains and squarewaves. Make sure your probes are properly compensated.

 

[Linx1]

Thanks for the advice and words of encouragement. I have to admit that I have been enjoying the whole process and it’s humbling how much I don’t know. I have spent more time researching and asking questions than actually building.

As for the correct probes I am using the x10 11.8 pf probes that were supplied with the scope. Not sure if they are correct but they appear to be ok.

 

[whoaru99]

Many scopes have an integral square wave test point.

Does yours, and if so have you used it and adjusted the probe compensation to show a square as possible square wave?

 

[wdecho]

Thanks for the advice and words of encouragement. I have to admit that I have been enjoying the whole process and it’s humbling how much I don’t know. I have spent more time researching and asking questions than actually building.

As for the correct probes I am using the x10 11.8 pf probes that were supplied with the scope. Not sure if they are correct but they appear to be ok.

Watch this for tons of info.

 

[kward]

Maybe try to obtain an authentic pair of W5-M output transformers. Peerles 16458 or 16309. Here are the square waves my W5-M's produce. Hammonds typically have HF extension to 30 KHz (see caveats below) while the Peerless models go out to 60 KHz.



Also the issue I have observed in builds with Hammond transformers (1650 models primarily, but the 1620 use the same secondary winding scheme) is that for best stability using global feedback from the output transformer secondary, you should wire it for 4/8 ohm connections and tap the feedback from the 4 ohm tap. Then limit the amount of feedback to 10 dB or so. Maybe 12 dB if you push it. Any amount of feedback higher than that creates stability margin issues in the HF, and you need to compensate for it with aggressive step filters or Zobel networks, essentially negating the feedback at frequencies above approximately 15 KHz.

Heathkit W5-M's in stock form apply right around 20 dB feedback, and you will have a very difficult time applying that amount of feedback with Hammond classic wind output transformers.

On the other hand, Hammond classic wind OPTs seem to have exemplary LF response and I've observed no real issues there.

So stating this bluntly: The Peerless OPT models used in the W5-M are all around better transformers and will elevate your amps to the best they were designed to be.

 

[grindfix]

Thanks for the advice and words of encouragement. I have to admit that I have been enjoying the whole process and it’s humbling how much I don’t know. I have spent more time researching and asking questions than actually building.

As for the correct probes I am using the x10 11.8 pf probes that were supplied with the scope. Not sure if they are correct but they appear to be ok.

10x probes are not needed to test the output. They are sometimes useful for testing high-impedance signal levels. Typically, the 10 kHz square-wave output is viewed at an equivalent output of 1W, which is 2v/div into 8 ohm load. Does your scope auto sets for 10x probe? If not, you are viewing 10x the signal level, so a 10 kHz square wave will never look as expected.

 

[gadget73]

just a quick mention about x10 probes, if they aren't properly compensated what you get on the display doesn't really mean much. Most scopes have a calibrator built in to check this. Probes usually have a trimmer cap, often at the BNC connector.

 

[ConradH]

I don't see any value in using 10 kHz square waves. The harmonics are vastly higher than that and you're really feeding the amplifier something it wasn't designed for and likely at higher amplitudes than will tell you useful things. It's remarkable it looks as good as it does. Try square waves in the 300 Hz to 1 kHz region. And/or do a frequency response curve using a good 'ol sine wave at up to 30 kHz.