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Old 3rd August 2003, 05:50 AM   #1
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Default Do these scope traces mean something is oscillating?

I don't have a function generator; this is just the amp's noise with the inputs shorted. The scope is uncalibrated, so I'm not sure how much to trust the signal magnitude it shows. Also, this looks really bright here, but I had the intensity down to a point where I could barely see it. My digital camera had a hard time figuring out what was going on, and these are the best pictures I have.

My question is about the spikes in these images. What do they mean? And if it's oscillation, any ideas on what it could be, and what I could do to fix it? All the images are the same signal, with different timebases - 5, 2 or 1 ms/div. And I just noticed that the spikes seem to appear with fairly consistent frequency, once every 4ms or so. Which means something kicking in with a frequency of 250Hz? 240Hz? I have a tube rectifier on the B+ supply, and all heaters are AC. What event(s) occur(s) with a frequency of 240Hz in an SE tube amp?

Also, every alternate peak of the main noise signal is of a different amplitude. That's just a result of 60Hz noise combining with 120Hz noise, right? I can see the waveform changing as I adjust the humpots, and this is about the lowest I've been able to get it.

Click the image to open in full size.

Click the image to open in full size.

Other images:

http://mywebpages.comcast.net/saurav/2a3/scope1.jpg
http://mywebpages.comcast.net/saurav/2a3/scope3.jpg
http://mywebpages.comcast.net/saurav/2a3/scope5.jpg
http://mywebpages.comcast.net/saurav/2a3/scope6.jpg
http://mywebpages.comcast.net/saurav/2a3/scope7.jpg

Thanks a lot in advance,
Saurav
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Old 3rd August 2003, 07:28 AM   #2
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The short peaks look like standard diode noise to me, check the ground path between the transformer CT, 1st filter cap and the rest of the circuit.
Are your heaters floating? That's a possible source of this kind of noise.

That it's going up and down every other cycle means there's 60Hz with 2nd harmonic distortion, i.e. 120Hz, or probably more accurately, 120Hz with 60Hz modulation. That probably means the rectifier is unbalanced - one side of the PT's CT has a lower DCR than the other, so one charges the filter cap faster. The filtering will attenuate 120Hz at least twice as much as 60Hz, damping the 120Hz making the 60Hz appear stronger.

Please get your scope calibrated so you can take measurements with it.. and adjust the focus too

Tim
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Old 3rd August 2003, 09:01 AM   #3
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Ahhh, focus! I hadn't even noticed that little screwdriver slot. That's the control I was missing. Thanks Getting this scope calibrated will probably cost more than what I paid for the scope. I'll certainly do it some day, but not right now.

Quote:
The short peaks look like standard diode noise to me, check the ground path between the transformer CT, 1st filter cap and the rest of the circuit.
When you say 'standard diode noise', would a tube rectifier have this noise too? I don't have any silicon diodes in this amp. I'll check the ground path as you suggested.

Quote:
Are your heaters floating? That's a possible source of this kind of noise.
Well... the 2A3 heaters are grounded through the cathode bias resistor, and the 6SL7 heaters should be grounded through the voltage divider used to get the heater closer to the upper triode's cathode potential. I can try putting a cap on that voltage divider to see if it takes more AC to ground. I think I've seen that done before. And all my heaters are AC.

Quote:
or probably more accurately, 120Hz with 60Hz modulation.
That's what I thought it was. I noticed that adjusting the hum pot seemed to affect mainly 60Hz noise (both on the scope and audibly), the 120Hz hum stayed mostly unaffected. So a less-than-perfectly balanced hum pot would cause this too, I think.

Thanks a lot,
Saurav
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Old 3rd August 2003, 09:19 AM   #4
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For a cheap and crappy cal, you could take a known AC voltage, say filaments (is your voltmeter accurate?) and set horizontal to display one 60Hz cycle in 16.6msec, and for vertical remember that the voltmeter reads RMS, so multiply by 2.828 for peak-to-peak voltage.

That's the procedure recommended for my 1971-era Heathkit scope, it has a 1Vp-p output on the front panel. Now that's accurate calibration!

Tim
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Old 3rd August 2003, 09:24 AM   #5
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Hmm, hadn't thought about that. I had thought about using a signal/function generator, but then I wouldn't know if that was calibrated.

My voltmeter is a Radio Shack digital cheapie, but it should be good enough on 60Hz sine waves, I would think. That's something to do for tomorrow.

Thanks once again,
Saurav
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Old 3rd August 2003, 11:16 AM   #6
dhaen is offline dhaen  Europe
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Wink Just adding my bit....

In this case it does look like focus is the main reason for the thick trace. However, other reasons for trace thickening (apart from oscillation) can be a local radio transmitter or local equipment radiating.

Cheers,
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Old 3rd August 2003, 02:31 PM   #7
SY is offline SY  United States
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If you've got a calibrated AC voltmeter, you can use that to check the voltage from a filament transformer and use that as a reference, like Tim recommended. Or DC couple, and check the voltage from a new carbon-zinc C or D cell- that's 1.56 volt (an old Dynaco trick).
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Old 3rd August 2003, 05:51 PM   #8
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Alright, I did a rough check on the calibration, and it seems to be reasonably close. I don't have a way to accurately generate signals in the millivolt range, but filament supply voltages seem to be correct both in amplitude and in frequency.

I did some more checking, and the B+ supply of the 2A3 does not have these spikes. They appear on the plate of the 2A3, and on the OPT secondaries. So I decided to check the grid of the 2A3, and this is what I got (that's the best I could do with the focus control. It gets a little thicker when I change from viewing ground to viewing the signal, and the camera's auto-exposure thickens it some more):

Click the image to open in full size.

That's at 10mV/div. The trace is the same at the cathode of the upper 6SL7, and the plate of the lower 6SL7 (the driver is an SRPP, I removed the cathode bypass cap a few days ago to see if I liked the sound better this way). The AC ripple on the 6SL7 B+ is pretty small, to the point where my scope has a hard time triggering off it, and here's the best I could do (5mV/div):

Click the image to open in full size.

Is this normal? That looks like a really strange waveform to me... almost triangular with other stuff in there. And doesn't triangular mean a lot of odd order harmonics?

Any ideas would be most appreciated.

Thanks,
Saurav
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Old 3rd August 2003, 09:07 PM   #9
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What time/div on horizontal?

My guess would be that last waveform is charging spikes (the filter cap only charges at the peak of the cycle), and getting in there by ground loop. This could happen with something like this:

Buss wire >---HV CT------3rd filter cap-----1st filter cap---

With the 1st filter cap's return path across the 3rd cap (which is the cap on the 6SL7's supply... this assumes a CLCRC supply, I don't know what you have ), the resistance of the buss wire, however small, produces a voltage due to the high current spikes, and these are coupled by the filter cap to the HV rail.

If you switch the position of the caps, the charging spikes stay between the CT and 1st cap, while the 3rd is isolated from that circuit.

BTW, do you have a 1x or 10x probe?

Edit:
My best guess: the noise is obviously coming from the first stage. Try putting some caps from heater winding to ground (about .1uF) and see what that does.

Tim
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Old 3rd August 2003, 10:14 PM   #10
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Hello, Suarev!

I've browsed this forum from time to time, but this is my first post.

I'd just like to offer a word of caution. From my experience, I've come across transients like these in many different types of equipment. I've spent more hours than I'd like to admit chasing these snivets down, only to find that they're not really coming from the circuit in question.

I suggest a sanity check. Start with the same conditions that produced the 'scope pictures in your first post. The ground clip from the 'scope probe should be attached to your chassis ground. Check first to see that the pulses are still there. Next, leaving the ground clip grounded, unhook the probe tip, and hold it near the measurement point, but not on it. Still see the pulses? Next, hold the tip of the 'scope probe to ground at the same place that the probe's ground clip is grounded. Now do you still see the pulses?

If it seems like the pulses are everywhere. then try removing the probe's ground clip from the chassis, and clip the ground and probe tip together. Did that make the pulses go away?

There are many possibilities that should be considered before tearing into the guts of you amplifier. Bear in mind that you are using the 'scope in unbalanced (single-ended) mode, which requires connecting 'scope ground to your chassis ground. Together with the third wire AC power connection, ground loops are created. The 'scope amp is also at a very high sensitivity, so it will amplify signals in that loop(s), irrespective of their source. It is possible that the glitches you see could be coming from the 'scope itself, of some other piece of equipment that's on nearby (TV, VCR, microwave oven, etc.). Try turning things off, one at a time, to see what effect it has.

I didn't mean to run on so long, but when I saw this thread, it was like a conditioned reflex. Or perhaps better to say deja vu?

Hope this was helpful.

M. W.
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