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Amp square wave testing question, or Ignorance is Bliss

That square wave looks exactly like the output from my cell phone.

I bought a cheap square wave generator ($13 a diy kit?) But still haven't "DIY" it.
 
Read the specs. I bought one "component tester + signal gen + freq meter", but didn't realize it can generate only square waves. And perhaps at a fixed output voltage (I need to check that )
 
My problem is I have hundreds of other things I also want and so I prioritize, even a cheap signal gen would have limited use at this stage of my audio journey. The sound card though limited if understood still tells me enough to feel I’m getting reasonably good audio output. Clipping, good rise time, acceptable tilt, exact wave replication, nothing added. I guess my goal was to just improve on my stock Magnavox 8802 using established mods and methods, my test measurements are used to verify I’m getting the desired results, or adjust. The stock transformers wouldn’t allow this wide of a response at similar output levels, scope verified. When listening to the amp I get the wonderful 8802 sound, now extended well past my hearing range and it can drive my JBL L19s easily to the listening levels I prefer. I'm usually at 1/4 to 1/3 volume range using just a passive attenuator, which is giving me 30 to 20k clean. I guess back to my original question, no the Mac mini sound card isn’t capable of testing to a 10khz square wave, or even a good 1k. So I will have to wait and see if my transformers can reproduce a 10khz square wave for now.
 
I use a cell phone as signal generator for many troubleshooting procedures. Very handy, i can even produce balanced signals with 2 channels shifted 180°.

To check freq response you'll find useful the ARTA software suite. It will send sweeps, white or pink noises to your amp and read the output giving you a graph on the fly.
 
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I went and looked at some of my old notes that I had made on the Rowe jukebox amp that I took these transformers from, and in a discussion with Kward, I mentioned I was getting a pretty good 10khz square wave while optimizing the NFB circuit. It was during that conversation I also mentioned the Eico 377 crapping out. So I think I shall remain optimistic.
 
If this is a one time thing working on the amp, you don't need a new generator, but if you plan to take this as a hobby, you need one.

You need someone with tube amp experience to verify the low frequency distortion is normal and is from the transformer and you are home free. Also, if it sounds good as before, don't worry about it.
 
The symmetrical ripple on the squarewave top/bottom and the sharp reduction in sinewave amplitude between 18KHz and 20KHz are both characteristic of commodity audio DACs. You need a proper audio signal generator, and sooner than later because it offers a huge boost in your learning curve.

The waveform distortion at 30Hz looks a lot like OPT core saturation to me. Those little bursts of parasitic oscillation shouldn't be ignored. Consider yourself lucky to have found this evidence of marginal stability, and do your best to get it fully extinguished.

It's generally best to stick with 100Hz, 1KHz, and 10KHz squarewave measurements for posting and discussion. 1KHz results are often omitted because they're essentially perfect in any reasonably well designed hi-fi amplifier.
 
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What are the typical steps used to deal with output transformer core saturation at low frequencies? I have it set up with common cathode bias but with the left and right channels split apart. The cathode resister value is set to give 17.8 voltage and 28.5 mA per tube, cathodes resisters are also bypassed with 220uF caps. It has solid state power supply, with good clean CLC filtering. Drive tubes have 40uF cathode bypass caps. The .047 and .47 caps are the traditional.01 and .1, I've reduced the NFB to 10db using 3k feedback resister biapassed with a .001 cap. Maybe the feedback cap could be dialed in? I do recall it helped clean it up some in the first place.
 
It can be difficult to distinguish core saturation from output stage limiting due to insufficient OPT inductance at low frequencies. These two problems often occur together with low-cost OPTs, and may be worse in AC-coupled amps due to grid blocking. You can deal with low inductance to a certain extent by giving the output stage a very low internal impedance (e.g. triode finals) and high current delivery capabilities. If core saturation comes on gradually, then negative feedback staves it better than nothing. Core saturation can be helped somewhat in an existing design by adding an input high-pass filter to cut back on LF output power requirements, but the user might just crank in more bass boost and defeat this remedy. In my own amp designs, I reduce core saturation problems by operating the OPT well below its ratings, and I always use DC coupling. Just to stir up trouble, I will suggest that AC coupling is no longer acceptable in modern tube amp designs. ;)
 
If it is the output transformer, not really much you can do but to change the transformer.

But try take pictures at 30Hz at lower and higher amplitude. If it is core saturation, you might get better waveform at low amplitude. If the transformer is not really for 30Hz, the distortion might look the same.

Stressing "might". I don't know enough about transformer. But I am pretty sure BinaryMike can tell more with the pictures.
 
The 30Hz looks normal as I increase the input voltage, then the sine wave kinda jumps and I start to grow this little snivet distortion you see above and it gets bigger while the sine wave loses its symmetry. The jump almost looks like phase shift.
 
Schmidlapper said:
The 30Hz looks normal as I increase the input voltage, then the sine wave kinda jumps and I start to grow this little smi it distortion you see above and it gets bigger while the sine wave loses its symmetry. The jump almost looks like phase shift.
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We crossed posted, you might have answer my question.

Jump is NO GOOD. I test stability of amplifier looking at how sensitive it is to changes. Graduate changes tends to indicate stable, sudden changes that are jumpy usually indicates trouble. It's like poking a raw nerve!!!!

I let the oscillation go in your picture ONLY because there usually no easy way to fix. There is small ( emphasis very small) chance the filter cap is so old it just loss it. BUT usually it's closed loop feedback problem that is engineering. Oscillation is not easy to fix and involve compromising of performance vs stability trade off. That's the reason I told you if the sound is always good and is still good, look the other way.

I just went through 3 to 4 weeks working on my amp, testing in all different situations, low high signals, sweep frequencies, power up and down. I also hang capacitors at the output to repeat the test to make sure it is unconditionally stable. It's engineering design work. You might not have to knowledge.....particularly don't have the equipment to do that.

Bottom line, my advice is if the amp sounds good to you, move on. You said you don't have time, you don't have the equipment.
 
Both transformers are testing this same way. These Rowe Ami jukebox amp output transformers are rated up around 30 watts and are generally well thought off. I imagine I need to adjust the feedback and bypass cap some to tweak it in better.
open loop voltages @1KHz was in .3Vrms and out 7.76Vrms.
Closed Loop w/ 3K0R as Rf, @1KHz is in 1 Vrms and out 5.7 Vrms
 
Oscillation might not be only on global negative feedback. If you want to get to the bottom, you are going to have to commit some time which you said you don't have. You are going to need a better generator that you already said you don't want to buy. You cannot use a single programmable frequency. You need to be able to turn the knob fast to trigger oscillation. You need to have a good squarewave generator.

If your number is correct, your open loop gain at 1KHz is 7.76/0.3=25.8=26. which is very low., You closed loop gain is 5.7. You feedback gain is 26/5.7=4.6. I don't think your oscillation is from the GNFB, it very well be from components. That's where it gets tricky. Don't think just because you know a little about GNFB theory and you can just tame it.
 
Schmidlapper said:
Original design for 8802 Rf was 2200 ohms with stock trannies using 6 ohms, I’m using these at 8 ohm
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I don't think this matter.

You need to decide what are you willing to do first. If you are serious about fixing it, post the schematic as the first step. It's starting to talk in circle. You want to fix it or not?
 
My earlier comment regarding “priorities and other wants” was referring to money, not time. Time I got, money I have to pace myself or my wife will. I’ve been working on this amp off and on for over three years. I think my open loop gain is ok, I may have just documented it incorrectly. The output I’m getting out of this amp is inline with what is to be expected of a 8802. Tomorrow I will remeasure, and try a few things. I am not an expert on audio tube amp diagnostics, but I’m capable and have found people on here to be helpful.
 
Post the schematic. It must be a simple amp, you can just trace the circuit out. You serious in working on it, trace out the schematic. Then we go from there.

With feedback gain this low, I really don't think it can sustain oscillation, you have something going on. Try to get a picture where you can see the frequency of the oscillation.
 
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