Is it normal for VFD tubes to emit acoustic noise?

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5 months 3 weeks ago #12944 by blinkman
Bit of a newbie question here.

I have a "Jenny" clock, an older design by Mr. Nixie that drives 6x IV-12 tubes. The tubes produce a small amount of noise, just barely audible when the room is quiet. I'm wondering if this is just typical acceptable behavior, or if it might be something that could be improved.

Thanks in advance for any insights!

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5 months 3 weeks ago - 5 months 3 weeks ago #12945 by Torsten Lang
Are you sure that the tubes themselves are the source of the noise? I own a Thyratron clock by Mr. Nixie wich does not use a "real" switcher based on some dedicated hardware but uses a simpe driver stage driven by the uC. It makes horrible chirping noises under load and even seems to drive the coil into saturation under maximum load so that the high voltage starts to fluctuate when maximum brightness is combined with many active segments.

With switching frequencies within the audible range you may hear noise from the coil and/or the MLCCs.

With my first own project I decided to use a dedicated switcher that is working with very high frequencies (if I remember right it is around 600kHz..700kHz), so the power supply is silent.
Last edit: 5 months 3 weeks ago by Torsten Lang.

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5 months 3 weeks ago #12946 by Ian
It's a bit unusual for such small tubes to resonate - on large tubes it's definitely "a thing", but I'd tend to agree with Torsten that it's more likely the boost circuit.

If you can locate the source we can reason a little more about how to solve it:

If it is the electronics, you only have a few workaround options available (e.g. you can epoxy or hot glue the offending component to damp the noise). A real solution is not possible: Really the firmware would have to be revised to run the boost circuit in a slightly different way, and I don't think you'll get any support from Mr Nixie any more - the clocks all seem to be end of life or gone, there are only a few left overs in the store now.

If it is one or more tubes, you could check them to see if they vibrate due to some internal structural issue and possibly substitute if needed.

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5 months 3 weeks ago - 5 months 3 weeks ago #12947 by Ty_Eeberfest
Clock in question is an original Jenny (2010), not a Jenny Reloaded, right?Jenny Reloaded generates its HV using PWM from the microcontroller, but original Jenny uses 2ea MC34063A (one in boost mode for the HV and one in buck mode for the filaments). They run at >30KHz and the reservoir caps are respectably sized, so switching "shouldn't" be audible. But maybe the inductors or something are resonating at a subharmonic of the fundamental frequency.

Schematic attached,

Look into it later when the dust is clearing off the crater.
Attachments:
Last edit: 5 months 3 weeks ago by Ty_Eeberfest.

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5 months 2 weeks ago #12948 by blinkman
Thanks, Torsten, Ian and Ty for your replies.

Ty, this is indeed the original Jenny.

I had assumed the noise is coming from the tubes themselves, because there's no noise when the tubes are removed. And this may also be a clue: With only one, or a few tubes installed, the noise is louder. When the sixth tube is installed, the noise decreases significantly.

But on closer examination––especially with only the rightmost tube installed––I can hear the noise coming from the opposite end of the board, in particular (you guessed it) the inductor coils. I can even dampen the noise a bit with a thumb on the coils.

When I originally assembled the clock I did apply some clear nail lacquer to the coils. I could try a more thorough coating of epoxy... Or, I wonder if they could be replaced with a similar part (22 uH inductor) that's "better" (i.e. less susceptible to noise).

I will investigate this. If you think of anything else, please share. Thanks again!

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5 months 2 weeks ago #12949 by Ty_Eeberfest
Not much to add. If this is the same Jenny you posted about in 2010 you might consider replacing capacitors C9 and C16 on general principles, especially if the inductor noise is a new development (you never really said if it's always been noisy or not). Replacing the inductors seems reasonable as well.

Avoid eBay, Ali, etc. for the new parts. Stick with branded parts from Farnell or DigiKey or Mouser or the like.

Look into it later when the dust is clearing off the crater.

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