HF oscillation bursts at low ( < 40Hz) frequencies - diyAudio
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Old 15th June 2012, 01:13 PM   #1
bigwill is offline bigwill  United Kingdom
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Default HF oscillation bursts at low ( < 40Hz) frequencies

Hi everyone,

It's an oscillation thread

I am experiencing oscillation with a P-P amplifier VERY similar to the symptoms described by Wavebourne in one of his old threads: Weird oscillation

The bursts (seemingly around 200Khz, one period being around 1 division on 5S/div timebase) only seem to appear on the negative SLOPE of a sine wave, and appear when you feed the amplifier 40Hz or lower, and near full power. It seems to correlate to the transformer being near or at saturation.

Annoyingly, it only seems to do this on one channel. The amp uses negative grid bias, and tweaking it seems to move the position in which the burst happens. It seems to be the one valve of the pair causing it - if you bias the suspect valve nearly off and leave the other to do its best to reproduce a low frequency sine through an ungapped TX - these bursts don't occur. I say valve - it seems to be any output valve in that particular socket.

The amp uses negative feedback, has an ultralinear EL34 output stage with 1K screen stoppers. Interestingly, despite the amp having a dominant pole on the input stage at around 10k, the oscillations DO appear on the anode of the first stage, at the same relative amplitude as the output. This strikes me as slightly odd. If the oscillation was entirely output stage bound (snivets?) - wouldn't they be naturally attenuated by the first stage?

The amp seems otherwise stable and already has a zobel network across the speaker terminals, and the frequency response doesn't have any strange anomalies. Oddly, the channel that has the issue has a slightly smoother HF response than the stable channel.

Any pointers would be MASSIVELY appreciated

Last edited by bigwill; 15th June 2012 at 01:20 PM.
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Old 15th June 2012, 02:05 PM   #2
bigwill is offline bigwill  United Kingdom
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OK, well there was a happy outcome to this - when scoping the grids of the output valves, the oscillation ceased. Turned out the little bit of capacitance from the probes damped some parasitic coupling somewhere, and my g1 lead was a bit too long
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Old 15th June 2012, 02:29 PM   #3
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Try to add a 10K 1/2 watt grid stopper, the body of the resistor as close to the socket as possible.
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Old 15th June 2012, 06:03 PM   #4
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whenever possible use a 10:1 probe with your scope!
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Old 15th June 2012, 06:57 PM   #5
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Capacitive coupling back to an earlier stage? One of the output valves is the right phase to give positive feedback. If the OPT nears saturation then it might begin to decouple the two output anodes from each other. Or maybe one output valve is approaching cutoff (trying to drive the OPT) so leaving the other free to oscillate.
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Old 15th June 2012, 07:17 PM   #6
bigwill is offline bigwill  United Kingdom
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Quote:
Originally Posted by Osvaldo de Banfield View Post
Try to add a 10K 1/2 watt grid stopper, the body of the resistor as close to the socket as possible.
I do this as standard practise, this is the only time due to a number of reasons when I don't have one right at the socket, and look what happened
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Old 15th June 2012, 07:22 PM   #7
bigwill is offline bigwill  United Kingdom
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Quote:
Originally Posted by Juergen Knoop View Post
whenever possible use a 10:1 probe with your scope!
Don't worry - I always use a 10:1 probe on valve circuits. The exception being a loaded and known to be stable OPT secondary. I am willing to be wrong if this is generally a bad thing to do.

Quote:
Originally Posted by DF96 View Post
Capacitive coupling back to an earlier stage? One of the output valves is the right phase to give positive feedback. If the OPT nears saturation then it might begin to decouple the two output anodes from each other. Or maybe one output valve is approaching cutoff (trying to drive the OPT) so leaving the other free to oscillate.
Before I scoped output grids, I put the scope onto the cathode resistors, and the oscillation seemed to correlate with the highest peaks of anode (+screen if you want to be pedantic ) current on one of the phases - so it is probably something similar to what you suggest. When I say peak, the actual current waveform was very distorted at this frequency / power so it was more of a 'nipple' of high current (obviously due to the OPT). The global feedback was doing a shockingly good job of keeping the output waveform clean, however!
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Old 15th June 2012, 07:25 PM   #8
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"puede fallar" (may fail) told an old character, characterized by doing fooling in TV programs. One time failed, and he died.
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Old 15th June 2012, 08:09 PM   #9
DF96 is offline DF96  England
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At low frequencies, where the OPT is running out of inductance, the global feedback will push the output valves nearer cutoff in an attempt to maintain the correct output voltage. Of course, the other valve will be pushed harder into conductance. Taken together, this means that the output stage is quite unbalanced for a while. Output valve gain, and hence Miller effect, will also change.

I once had to put an electrostatic screen in to stop a LTP phase splitter from oscillating due to the rather large coupling caps to the output grids being too near the input stage.
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Old 15th June 2012, 10:31 PM   #10
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It is OT guilty (mine was ringing right above 35 kHz 10K P-P 100W transformers). I cured it by increasing local feedback around output tubes; as the result global one that goes from secondary decreased.

You probably have better tubes and as the result slightly higher open loop amplification factor in that channel that is ringing.
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Last edited by Wavebourn; 15th June 2012 at 10:35 PM.
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