Any cure for Microphonic power tubes?

[nevetslab]

It always bugs me finding an amp where one or more power tubes in a quad set are microphonic. In the past, I've always had enough used 'pulls' from our large rental inventory to search thru and cobble another working set of power tubes together, same mfgr, near-same plate current @ idle, and save having to order yet another matched pair or quad set of power tubes.

I've used up all my 6L6GC Groove Tube 'pulls', and once again, have a near-new Twin Reverb with a pair that's microphonic. Does anyone know of a cure for that microphonic behavior, or is this just tough titties......need a new matched quad set again, adding the other good pair to the collection of pulls for the future?

[Leo_Gnardo]

or is this just tough titties......need a new matched quad set again, adding the other good pair to the collection of pulls for the future?

That right there ^^^. No fix for rattly or microphonic tubes. Like you, I keep a collection of good used pulls & try to save customers some $$$ when I can. Bad enough when I get a rattler in a new pair or quad, it puts me thru a minor hell when the customer has bought their tubes from some supposedly reliable dealer & I have to tell them their new output (and sometimes preamp) tubes are dodgy. I also get the chance to offer what appears to be self serving advice: "if you bought them from me, I'd have swapped out bad ones before you ever had a chance to hear them." Which may be true, but try to tell the friendly customer who sees those ads for the tube vendors all the time. "I thought Groove Tube - or whoever - was supposed to FIX THIS !?!" Well, get on the phone and demand a new set, and go to the post office & send back your baddies. Rinse, lather, repeat. And hope your new set is better than the ones you just bought, 'cause it might not be. And and ... sit there with a busted amp whilst you wait!

[xtian]

Obviously, you can't get inside the glass envelope and tighten up the mica spacers. But you can mitigate the problem with added mass (those red, automotive silicone O rings) or physical pressure (add spring retainers with silicone rings, the flat, milky kind). If you can silence rattling or howling tubes by pressing them with your fingers, these methods may work.

[galaxiex]

Sorry to barge in, but I'm confused about the difference between "rattling" tubes and "microphonic" tubes.

Is there a difference, or is this just the same issue?

I've had pre-amp and power tubes make rattles at high volume,
where the noise goes away if I lightly touch the glass.

I've used these things to cure the problem...

Hi temp rubber O rings.
Shrink tubing.

Installed on the tube in the area where the internal support wafer touches the glass.

I guess you could also use some o them hi tech "Tube Damper" things too...

[drewl]

Unfortunately it's just one of those things we have to deal with.
And yeah, it's a pain when a customer supplies their own supposedly "good" tubes.

I've got a nice stash of old RCA and Mullards for my own stuff.
Nothing sadder than this beautiful pair of Mullards EL34's I was running in my Plexi and one got too microphonic to use anymore.....

[Reader]

- - - those red, automotive silicone O rings- - -

- - - Hi temp rubber O rings / hi tech "Tube Damper" things - - -

Are the high temp orings available from suppliers such as McMaster Carr or are they all sold through specialty suppliers that have marked them up as fancy audiophile parts.

[g1]

Sorry to barge in, but I'm confused about the difference between "rattling" tubes and "microphonic" tubes.

Is there a difference, or is this just the same issue?

I don't think there is any real difference aside from the manifestation of the problem. I've never heard a power tube squeal, only low frequency type 'rattling'.
Preamp tubes tend to squeal when they are excessively microphonic, rather than the low frequency type rumble you might get from a power tube.

[Leo_Gnardo]

I've never heard a power tube squeal, only low frequency type 'rattling'.

Just a couple weeks ago I had an amp in that gave out a low moaning "moooooooo" soon as it warmed up. No rattles, just the cow imitation. Bad output tube. Like bells, tubes can ring at a frequency appropriate to their size. Yes some rattle too, but not that one. I'm sure in time you'll run across Ferdinand-in-a-bottle, and you can have a big smile as you think "yeh, there it is all right, I finally found one!"

Pre tubes can make all sorts of outer space noises, in fact I've saved about a dozen or so prime offenders just for fun. Not so for output tubes. Just rattles & moans like Marley's ghost. Boring... mooo, clank!

I've found external fixes like O rings & shrink tube may help a tube that's marginal, but the heat/cool cycling they undergo drives that marginal tube into unacceptable range soon enough. Maybe that can work in hi fi gear, but not so much in combo amps or heads that are stacked atop speaker cabs.

[g1]

Yes, I've heard what you mean Leo. My choice of the work 'rattling' was poor in that context. I meant that even when a power tube oscillates, it's at the low end of the frequency range rather than a higher pitched squeal. Moo indeed!

[galaxiex]

Are the high temp orings available from suppliers such as McMaster Carr or are they all sold through specialty suppliers that have marked them up as fancy audiophile parts.

This guy has tube damper o-rings for pre-amp tubes ONLY!

$2.99 for 6 rings.

https://amprepairparts.com/tubeaccess.htm

scroll down a bit...

[Reader]

This guy has tube damper o-rings for pre-amp tubes ONLY!- - -

Thanks. It's interesting that they look like standard oring material. As you pointed out the ad copy says "These dampers are NOT for 9-pin power tubes, such as the EL84 or 6BQ5." so I assume the product offered will not survive the high temperature. I suspect they should not be used on the 12AX7 reverb driver tube in classic Fenders because those get really hot.

[xtian]

Tons of listings on eBay and Amazon, e.g.,

https://www.amazon.com/USA-Sealing-I.../dp/B07BHPFS3Y

[galaxiex]

Good to know the temp range of different types of rubber...

http://mykin.com/rubber-temperature-range

[Enzo]

In my world, a rattling tube is one that physically rattles, the innards are somehow loose enough they rattle against the glass sidewalls. EL84s in CLassic 30 PV amps were famous for this. You can hold such a tube up in front of you and rap on it with your knuckle, and hear a bit of rattle.

A microphonic tube is one that makes a signal out of mechanical shock. Like tapping on a mic, you tap on a microphonic tube in a live circuit, and you will hear it out the speaker. MAny, if not most, input triodes in guitar amps tend to be microphonic.

A microphonic tube may not rattle, probably doesn't, it is just sensitive to vibration.

A rattling tube may not be microphonic at all.


And of course, any tube could be both or neither.

[trobbins]

Yeh I'd just suggest keeping microphonic/rattling output tubes for amps that (a) aren't subject to external vibrations (eg. from speakers or floors), and (b) either have a tube clamping mechanism that has some compliance (eg. the spring loaded clamps sometimes used for inverted operation) or have modified valve bases that provide mechanical compliance to suppress vibration transfer. Some diyer may fit in that category if they have time and incentive.

Output tube temperature is certainly an issue to contend with, as you don't want any 'attachment' to obstruct air flow, and only touch any glass where it is coolest (ie. certainly not 'between the mica support disks). Luckily high temp silicone rubber o-rings or matts (eg. oven matts) or a tube of RTV, are cheap and easy to come across, and can survive touching a cooler region of glass.