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Gary Pimm self bias CCS 500kHz 'ringing'
Gary Pimm self bias CCS 500kHz 'ringing'
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Old 7th February 2018, 09:47 AM   #11
Rod Coleman is offline Rod Coleman  United Kingdom
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Gary Pimm self bias CCS 500kHz 'ringing'
Yes, you can try the 1K with the probe, but if you have a x10 probe, there should be no need. The output should not be sensitive to a few pF of capacitance.

The experimentally determined value of the gate resistor may work reliably for a reasonable range of parts, but beware that the IRF820 is almost generic - available from ON, ST, Vishay/siliconix,as well as the IR original. It could turn out that the processes have changed since the article was written, or that some vendor's parts have different capacitances, gm etc. and that this compensation method is not guaranteed to work.

If it really oscillates at 200kHz, even with cleaned-up probing, the effect is not self-oscillation of the FET, but an unintended oscillator composed of several different components. This kind of problem can almost always be fixed in a reliable way, without compromising the gate stoppers.

OTOH, Ale's 'gyrator' boards should be able to host an equivalent to this anode-load circuit, and has been tried by plenty of constructors. It's probably easier to get a set of these, that try to debug a circuit like this.
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Old 7th February 2018, 11:35 AM   #12
SemperFi is offline SemperFi  Wake Island
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Looks more like overshoot that rings... And if fine on the tube's plate, and sounds good...?
The ringing (overshoot) is the same independent of frequency b/c it is the fast edge and not the frequency that causes the ringing. Does it ever get a fast edge signal under normal use?
Anyways when probing high frequencies, fast edges have very high frequencies, you must have a very small loop on the probing.

I have a very similar circuit used as a stepped/pulsed load to test power supplies, and it oscillated when I came over some current level. Tried all sorts of gate resistances etc, layout is tight. Ended up with a snubber over the load MOSFET (IRFB11N50) and all is good. I used 1n and 22ohms, didnt spend time optimising, I just got it working and happy.
Perhaps a snubber will help damp out your ringing from the overshoot?

Oh and am I correct with where your square waves are measured?
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Old 7th February 2018, 05:40 PM   #13
Hanze Khronye is offline Hanze Khronye  Australia
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Thanks. Fast edge with normal use, well.. no, but there is quite substancial voltage gain increase at ~550kHz with a sinewave of any amplitude.

The two scope shots are taken at either end of R1 in the schematic wrt ground, and no.. not the shortest loop and on x1 and not x10 and without any series resistance etc (yet). I did think about just taking the output from the tube plate and be done with it, but combination of mild OCD and that I am wanting to use the 'mu output' for lower output impedance is enough to keep me interested in sorting this out .If indeed it turns out to be a probing issue, or requiring some adjustment to the gate stopper based on production inconsistencies as suggested above, I'll be happy.

Just need to find the time now.

Last edited by Hanze Khronye; 7th February 2018 at 05:42 PM.
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Old 8th February 2018, 05:32 AM   #14
Hanze Khronye is offline Hanze Khronye  Australia
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Fairchild IRF820B, based on above no need to change gate stopper values. Beach Boys 'Wouldn't it be Nice' from Pet Sounds, if it were just a simple measurement quirk?.

Thanks to all.
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Old 9th February 2018, 08:03 AM   #15
Hanze Khronye is offline Hanze Khronye  Australia
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Would you believe it..

My nice square wave was taken with the load disconnected (a flurry of twenty different actions all on a whim). And the load is... an AVC, and the secondary of that AVC is .. essentially unloaded, 1M.

I start looking around and I find this excerpt which looks very familiar to me (http://jensen-transformers.com/wp-co...rs-Chapter.pdf)

My thinking was based on what I believe to be a misunderstanding that I was wanting lowest drive impedance, highest load impedance and the smallest sized coupling capacitor to fit the LF requirements.

I think to keep the CCS load, take the output from the plate of the tube to increase slightly the drive impedance to ~2kohm, and load the AVC by reducing amplifier input impedance from 1M to 100k.

Stevens & Billington Limited Audio transformers TX-102

Thanks to all for advice and suggestions, hopefully you can see the funny side of it. The waveform in the excerpt is identical.. and my apologies to Mr Pimm for any suggestion that his CCS was the issue here.
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File Type: jpg 2.jpg (80.6 KB, 66 views)

Last edited by Hanze Khronye; 9th February 2018 at 08:08 AM.
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Old 9th February 2018, 10:08 PM   #16
SemperFi is offline SemperFi  Wake Island
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Lol. It happens and we all learn stuff or at least get something to ponder. Good for the grey masses.
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Old 10th February 2018, 01:13 AM   #17
Hanze Khronye is offline Hanze Khronye  Australia
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Just to follow up;

Changing the load across the autoformer from 1M to 100k made no change to the square wave response.

Adding an RC across the autoformer (with the 100k load still in place) squared things up and rolled of the HF response accordingly. The only values which really worked were 10nF+33R, 4700pF+8R2, or straight out 56nF without series resistance. I removed them.

Adding series resistance from the mu output allowed to damp the ringing (Image 5) and without visible distortion to over 200kHz where it started to roll-off, and of course no huge amplitude peaks between 500 and 600kHz.

Taking the output from the D3a plate provided square wave response as shown in (Image 6), no visible sine wave distortion but started rolling off gradually at 15kHz.

Value of series resistance as determined by rheostat was 190ohm. Not sure if it matters whether this is before or after the output coupling capacitor.

I did stumble across this; Electra-Print.com Tech Blog but for the cost of two resistors and response without ringing out to 200kHz, I'm ok with the resistors.
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