Microphonics from C0G Caps across Heaters

[Magz]

Hello,

I have a sudden issue with microphonics in my tube preamp, and I'm wondering if C0G ceramics are the cause. I added a 0.1uF TDK multilayer radial-leaded C0G across the heater pins at the socket of all the signal tubes in the pre, and it did seem to result in a bit blacker background, so I was happy with them, but I've noticed that if I tap on any part of the preamp chassis, I get a fairly loud thump coming through my speakers. It doesn't seem to be related to any particular tube - they all thump if tapped, as does the chassis - and it was definitely not present before the addition of the caps.

I have read that C0G ceramics are much less prone to microphonics than the other ceramic types, so I'm surprised by this. I plan to take them out or perhaps replace them with film caps, but I have two questions:

1) Has anyone else experienced this problem with C0G caps across heater pins?

2) Might I be better off with two caps per tube (from each side of the heater to ground) than with one cap across the heater pins?


Any help/experience sharing is greatly appreciated.


Here is a link to the exact caps used:

Digi-Key - 445-4756-ND (Manufacturer - FK20C0G1H104J)

[db!]

You should try removing the caps to see if it makes a difference...

Also, have you subjected the tubes to any shock or impact(drop, hard bump, etc) lately?
From what I understand, tubes can go microphonic if the internal support structures are disturbed/damaged...

[Magz]

I will certainly try removing them, I was just looking to see if this had happened to others with these caps. The reason I don't think it's a tube issue is that ALL of the tubes make the thump when they are tapped, and tapping the chassis anywhere also produces the sound. I didn't notice it until a couple of weeks after I installed the caps, but that's probably because I didn't happen to tap on the chassis during that time.

[taj]

C0G/NP0 ceramic caps are a world better than regular (say, X7R) ceramic caps, and are much less prone to microphonics, but you never know.

Also, because you're using them on the heater, I wouldn't expect microphonics to be an issue there, but you never know (at least I never know.).

..Todd

[SY]

Since this happened right after you did a bunch of soldering at tube sockets, my suspicions would be on that.

[jcx]

I would guess a sensitive (and high potential) wire may be in a different location, possilby closer to the chassis gnd than before

or (seems unlikely) the cantilever mass of the added caps at the sockets are mechanically coupling the thumps into the tubes better

electrically the caps seem hard to explain as a source of microphonics

[tomchr]

Some ceramic capacitor dielectrics are piezoelectric. But I doubt that's the case for C0G or NP0 types of dielectric. In any event, the force you're applying to the caps while thumping the chassis is minuscule and if any voltage was induced on the cap through microphonics it would have to couple through the heater to the active elements in the tube. I just don't see that as a plausible explanation.

I'd look for cold solder joints and the like.

~Tom

[Magz]

Thanks for the replies. So it seems that the caps are a longshot as the problem source. I'll need to open it up to remove/change the caps anyway, so I'll do a thorough sweep through and see what I can find. Maybe I'll swap in some new tubes before that just to see if I have a badly microphonic one as well.

Any advice on whether it is preferable to put one cap across the heater pins or two caps, one from each pin to ground?

[SY]

Originally Posted by Magz

Any advice on whether it is preferable to put one cap across the heater pins or two caps, one from each pin to ground?

The latter, for sure.

[Magz]

Thanks, Sy. When I go in there to poke around I'll set it up with two caps per tube.

Thanks to all who answered! It'll be a week or so until I can go in there and check it out since I need to order more caps, but I'll post what I find then.