'nipple' in waveform - What is it? - diyAudio
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Old 26th November 2015, 09:21 PM   #1
rongon is offline rongon  United States
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Default 'nipple' in waveform - What is it?

I've been working on a push-pull amp in LTspice. I was checking out a design, looking at clipping behavior. Using a 5965 or 12AT7 as a cathodyne phase splitter, I'm seeing a weird waveform coming from the inverting side (plate) of the cathodyne stage (green trace). I've attached a screenshot.

I changed the cathodyne tube to a 12AU7 with 10mA plate current, and that cures the weird 'nipple' in the inverting side waveform. Putting a 6SN7 or 5687 there also cures the problem.

What is it about the higher mu 12AT7 or 5965 that could be causing this behavior when the output stage goes into clipping?
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Old 26th November 2015, 09:51 PM   #2
rongon is offline rongon  United States
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I think it has something to do with the standing current in the cathodyne phase splitter, because if I change things around so that the 6SN7 cathodyne has 4.5mA and 22k plate and cathode load resistors, it too shows that 'nipple' in the waveform from its plate (inverting) output.

Is this a higher frequency caused by slew limiting?

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Old 26th November 2015, 09:58 PM   #3
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EDIT: The following does not make sense.

I don't know a whole lot about tube design, but I can tell you that your phase splitter is sending different signal levels to each of the power tubes. I know you can't get rid of R1 for biasing, but it is the culprit. You need to connect C1 directly to the cathode of U2 and raise the resistance of R21 to match the bottom. You need to add 1.2k so you might just add one in series. (23.2k ohms would be hard to find.)

Basically what you see is one power tube clipping before the other, if I'm not mistaken.

Last edited by barefootwhistler; 26th November 2015 at 10:12 PM.
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Old 26th November 2015, 10:22 PM   #4
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On my guitar amp I got rid of C2, R2 and R22.
This gives balanced 22k on upper and lower legs of phase splitter.
Just make previous stage anode voltage B+/4.
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Old 26th November 2015, 10:29 PM   #5
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So it is still one of the output tubes clipping before the other, right?
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Old 26th November 2015, 10:32 PM   #6
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The top of the waveform is clipping normally.
The lower part is probably too but I haven't seen clipping like that before.
Its maybe some oddity with ltspice.
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Old 26th November 2015, 10:41 PM   #7
rongon is offline rongon  United States
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There was a wave of threads arguing over the characteristics of the split load phase inverter (aka 'cathodyne' or 'concertina'), not too long ago. In the end, it was pretty well proven that the cathodyne phase splitter's output impedance is matched side to side as long as it's working into a high enough impedance load. So that's not the origin of the problem unless the output stage starts drawing grid current.

If I use a 6SN7 with 8mA or more going through it as the cathodyne stage, the nipple in the waveform doesn't appear on its inverting output.

Too see if the unbypassed cathode bias resistor could be adding to the load on the cathode (non-inverting) side of the phase splitter, I got rid of it by DC-coupling the first and second stages using a voltage divider (470k series resistor with a 0.22uF cap in parallel, and a 560k to ground did the trick). No change, the nipple's still there on the plate feed from the cathodyne. I think that exonerates the cathode resistor from imposing imbalance.

I also tried adding a 1.2k resistor in series with R21 (plate load resistor on the cathodyne). That didn't do anything either.

Maybe grid current drawn by one of the output tubes is causing the behavior? OK, so I'll reduce the value of R6 and R7, down from 330k to 100k. Nope, that makes it worse.

Next, I went the other way. I put 1M resistors in for R6 and R7, which actually helps a little. Unfortunately, that wouldn't work in the real world, as 1M is double the permissible grid-to-ground resistance for a cathode biased 6L6 (0.5M ohm is the max for cathode bias).

All through this, the 12AX7 output waveform is looking good.

All I can figure is that it's something to do with whether the cathodyne stage is able to sink enough current into the grid of the output tubes during overload. A cathodyne with 4mA standing current has the problem, somewhat independent of tube type (although the higher mu triodes are more prone to the problem). 6SN7 with 8mA doesn't show the problem at all.

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Old 26th November 2015, 10:43 PM   #8
rongon is offline rongon  United States
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Quote:
Originally Posted by nigelwright7557 View Post
On my guitar amp I got rid of C2, R2 and R22.
This gives balanced 22k on upper and lower legs of phase splitter.
Just make previous stage anode voltage B+/4.

In other words, you DC-coupled the plate of U1 to the grid of U2, correct? I tried that in simulation, and the problem still happened.


Quote:
Originally Posted by nigelwright7557 View Post
The lower part is probably too but I haven't seen clipping like that before.
Bear in mind that the waveforms are from the plate and cathode of the phase splitter, not from the output. I've seen this kind of thing before on a scope when a differential driver stage clips before the push-pull output stage it's driving. This is covered in the RDH4 somewhere, but I can't seem to find it...

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Last edited by rongon; 26th November 2015 at 10:45 PM.
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Old 26th November 2015, 10:46 PM   #9
tomchr is offline tomchr  Canada
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I'd look at the various grid currents. They may provide clues to what's going on.

Tom
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Old 26th November 2015, 10:53 PM   #10
MelB is offline MelB  Canada
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Increase R10 to 300-350 ohms and see what happens.
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