My overly complex approach to tube damping

[Deke609]

I've finally gotten around to the final rebuild of my Beepre in a huge aluminum chassis (that I call "Bigby", as in "big bee"). One of the challenges was figuring out how to deal with the extra height of the chassis - with feet and tubes installed, it wouldn't fit my rack. So I figured I'd make big cutouts for the tubes and install the sockets about 1"+ below the top plate.  And then it occurred to me that I could mount the socket subplate so that it is sandwiched between thick silicone washers: bolt head, steel washer, silicone washer, subplate, silicone washer, steel washer, standoff.  To minimize (hopefully eliminate) vibration transferred from the bolt threads to the subplate, I wrapped the top 3/4 of the threads 2-3 times in electrical tape -- just in case the side of the bolt makes contact with the inside of the subplate mounting holes.  I'm hoping the combo of ptfe sockets and the additional damping will give the 300Bs extra isolation from any chassis vibrations.

Here's some pics. I punched the silicone washers out a sheet of 3/16 silicone.

But it occurred to as I was putting it all together that a simpler and better approach would have been to really oversize the mounting holes on the subplate and install thick silicone/rubber grommets. Ah well ... next time.

cheers, Derek

 

[Paul Birkeland]

I used the ribbed anti vibration grommets from McMaster on my personal BeePre.

 

[Deke609]

I'm assuming for the power transformers?  Or did you install really big grommets in the socket holes?

 

[Paul Birkeland]

I have a suspended sub plate that the 300Bs hang out in.

 

[Deke609]

That's pretty cool. And single power trafo (Kaiju?) for a more compact layout.  What's that toggle switch for? Pre-heating the 300B's?

I looked at those ribbed grommets at M-C, forgetting that they don't ship to Canada. Too bad - MC has pretty good prices. Digikey and Mouser will ship me something similar for 3X the price. Grainger operates in Canada, but doesn't appear to carry anti-vibration grommets.  I briefly thought about getting really beefy regular silicone grommets and cutting the ribs myself -- but I don't think I'm OCD enough for that. ... but maybe.

cheers, Derek

 

[Paul Birkeland]

Why would you preheat 300Bs?

 

[Deke609]

I meant preheat the filaments before B+ and signal.  I seem to recall some people claiming that it is better for the life of the tube.  But I could be misremebering things.

If the toggle switch is not for that, what's it for?

 

[Paul Birkeland]

It's a switchable pad.  300Bs conduct fully after a second or two, so preheating them isn't all that helpful.

 

[Deke609]

It's a switchable pad.

That's cool. Will there be an announcement about this, or is this just a personal modification? If the latter, can you say more about it?

300Bs conduct fully after a second or two, so preheating them isn't all that helpful.

That's good to know. Thanks.

On further thought, I don't think my silicone washers are going to do much. I'm no physicist or materials engineer, but my gut sense is that the silicone is too rigid to do much damping for such a light mass load. I think I'd need a material with a consistency closer to partially set jello -- that, or suspend the tube socket subplate in a thin silicone web (think thin rubber bands).  But that would pose challenges for inserting/swapping tubes.  And it would probably work best if you could suspend the tubes upside down so that they don't fall to one side. If it could be made to work, that would be one really weird looking amp.  I'll likely never know.

cheers, Derek

 

[Paul Birkeland]

That's cool. Will there be an announcement about this, or is this just a personal modification? If the latter, can you say more about it?

This is just a personal project. 

On further thought, I don't think my silicone washers are going to do much.

I'm not so sure the anti-vibration grommets I'm using are doing much either.

 

[grufti]

Thomas Mayer of vinylsavor [sic] suspends many of his tubes on subplates. He has lots of images up on his blog.

 

[Doc B]

Mass is your friend with this kind of stuff. That's why the lead rings on the tubes work better than most other attempts at damping. You could make your sub panel out of 1/4" thick lead.

 

[Deke609]

No joke: when I first started planning the Beepre rebuild about a year ago I gave serious consideration to adding lead bars to the inside of the chassis.  I think what stopped me was the cost of shipping!  I was going to rubberize them and build brackets into which they could be slotted.

I've been pretty happy with Herbie's Isocups with acrylic balls. If you poke the amp it wobbles once like jello, then comes fairly quickly to a gentle stop - so I assume the opposite is also true: wobble the rack and the amp will stay relatively fixed in space.  But they only help with mechanically transmitted vibration -- don't do much if anything for sonically transmitted forces. Not a big deal for me b/c I use headphones. But I may try those lead rings all the same.

cheers, Derek