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Old 21st May 2015, 01:21 PM   #1
thehoj is offline thehoj  Canada
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Default Measuring ripple in filament supply (coleman reg)

I'm currently building an 845 amp, and have my coleman regulator's built.. I was wanting to measure ripple on the output of the regultaors..

So I connected the tip of a probe to one side of the filament, and another tip of a probe to the other side of the filament of the 845 being fed by the regulator.

I always seem to have trouble with my oscilloscope, but the measurements seemed kind of weird.. I have both probes set to AC coupling.

Here's a video of what I was reading https://youtu.be/9w-PjBCgHow
I had to have the time/div set pretty low to actually see a waveform.. Anyways, I'm wondering if I'm measuring this correctly, and if what I'm seeing is actually useful / indicative of anything.

The v/div is set to 50mV/div.. So it seems fairly significant.

Can someone take a look and see what they think about this.. ? Or perhaps maybe help me figure out how to take a more useful measurement if I'm not doing this correctly.
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Old 21st May 2015, 06:10 PM   #2
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Can you perform A+B with that scope? you are looking at the voltage at the filament with respect to earth ground, which may be completely irrelevant depending on your design and the grounding. With A+B, you would invert one channel, and the result is A-B, which is what is pertinent.

Optimally, you would want to utilize a differential probe, or just use a multimeter set to AC volts to get a pass/fail result.
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Old 21st May 2015, 06:35 PM   #3
thehoj is offline thehoj  Canada
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Originally Posted by zigzagflux View Post
Can you perform A+B with that scope? you are looking at the voltage at the filament with respect to earth ground, which may be completely irrelevant depending on your design and the grounding. With A+B, you would invert one channel, and the result is A-B, which is what is pertinent.

Optimally, you would want to utilize a differential probe, or just use a multimeter set to AC volts to get a pass/fail result.
I can't do an A+B I don't think... I was kind of wondering if what I had done was even a valid measurement of anything useful.

I have a TrueRMS meter which I've read should give me sort of an idea of my ripple.. If I measure the raw DC that feeds the regulator I get a reading of 0.053V AC, which is RMS, so converted to peak-to-peak equals 150mV or so.. Which is what I had expected to feed the regulator.

And when I measure the AC on the output of the regulator, I get a reading of 0.000V... So I had assumed the ripple was quite low.

But in talking to someone about this they said that the actual waveform is important, so I had kind of hoped to get a visual representation of what is happening on the output of the regulator.



On a related note.. I also wanted to measure the B+ ripple in the amp in question.. The power supply is a CLCLC configuration (10uF | 10H | 52uF | 10H | 52uF .... A lot of filtering!) And when I attempted to measure that ripple, I connected my leads to the + and - of the final capacitor in the B+ rail, set the meter for AC, and it wouldn't stabalize.. Like it jumped around between 0.050V and 0.5V (much higher than I had expected or hoped -- let alone the unstable readout).... So I kind of wondered how valid that sort of measurement was for determining ripple.. When I sim'd the PS out in PSUD2 it calculated that I should see in the neighborhood of 1mV of ripple.. So again, using the oscilloscope to measure this was something I thought might be useful (using a voltage divider of some sort).. But I am still interested in learning how informative or accurate the AC reading on a trueRMS meter is in this application...And why it wouldn't seem to stabalize on the B+ output in my case.

Last edited by thehoj; 21st May 2015 at 06:40 PM.
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Old 21st May 2015, 07:07 PM   #4
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I don't think the scope measurement you did was anything useful. I'm not a tube guy and have no idea what a coleman reg is, but a quick google lets me assume it's a current source. So you will want to measure current ripple, not voltage ripple, which may be different if the load is nonlinear. Just put your handheld meter in series to the load, set it to measure ac current and you are done. If your meter does not have an AC current range, measure AC volts across a know resistor that is also in the load current path. Most current sources use a shunt resistor to generate a voltage for the negative feedback that you could possibly use. Use ohms law to convert your measurement to current afterwards.

Ben
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Old 21st May 2015, 07:18 PM   #5
thehoj is offline thehoj  Canada
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Originally Posted by FlexBen View Post
I don't think the scope measurement you did was anything useful. I'm not a tube guy and have no idea what a coleman reg is, but a quick google lets me assume it's a current source. So you will want to measure current ripple, not voltage ripple, which may be different if the load is nonlinear. Just put your handheld meter in series to the load, set it to measure ac current and you are done. If your meter does not have an AC current range, measure AC volts across a know resistor that is also in the load current path. Most current sources use a shunt resistor to generate a voltage for the negative feedback that you could possibly use. Use ohms law to convert your measurement to current afterwards.

Ben
That makes sense and I'll give that a try, but isn't any voltage ripple present on the filament still of interest?
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Old 21st May 2015, 07:31 PM   #6
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Yes, the voltage that you measured (0.000V rms) is expected - there will be less than 1mV of ripple on the filament with a Coleman regulator, even at 3.5A.

If you really want to measure the output ripple, disconnect the filament and regulator from the amp, power it without the HT supply (so that safety ground is not connected to input or output of the regulator, except where the scope grounds it).

You will mostly see RF pickup at whatever the minimum V/div is on your scope (1 to 5mV/div). Modern scopes with a digital filter will let you see audio band noise, but there will not be much to see if the Regulator is assembled correctly.
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Old 21st May 2015, 07:31 PM   #7
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Quote:
Originally Posted by thehoj View Post
That makes sense and I'll give that a try, but isn't any voltage ripple present on the filament still of interest?
Yes, the filament is resistive, and converts current-noise into voltage noise.
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Old 21st May 2015, 07:40 PM   #8
euro21 is offline euro21  Hungary
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PSRR of Rod's V4 regulator (theoretically) is about 60dB - in 10Hz-10kHz range-.
I measured few years ago about 100uV RMS disturbance on my #26 preamp (1A) heater bias -10R- resistor. It was mostly flicker noise, than hum, due to the well filtered CLCRC raw supply.
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Old 21st May 2015, 07:45 PM   #9
thehoj is offline thehoj  Canada
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Originally Posted by Rod Coleman View Post
there will not be much to see if the Regulator is assembled correctly.
I guess this is what's prompting me to make this measurement.
Everything seems okay, in that it operates as expected, but measurements are always nice to see a confirmation that I've built it correctly.
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Old 21st May 2015, 07:47 PM   #10
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Hi Bela, with filament bias, the current-noise is amplified by the bias resistor, so 100uV will be about right in this case.

The type of resistor can influence the measurements at flicker-noise frequencies - a well-made wirewound is best. Thick film should be avoided!
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