Rubber or silicone gaskets for tube socket bases to reduce vibration?

[dchang0]

I've got to mechanically decouple an EF86 socket from the chassis to reduce microphonics/vibration. Many discussions mention using o-rings around the screws and some mention using a bigger o-ring around the socket base. Others mention using grommets instead of o-rings, installed in the same places.

It made me wonder: are there silicone or rubber gaskets that fit the purpose?

It could be made for some other industry and purpose (automotive, plumbing, etc.), so long as it fits the hole for the tube socket and the two screws on the side.

Here's a photo of an unrelated gasket that shows what such a gasket would look like:



Anybody have any idea where I might get a fairly thin gasket that would fit a 9-pin tube socket? I am thinking the fit doesn't have to be perfect--the center hole could fit snugly against the ceramic socket base or go up as wide as the chassis hole (22mm or 7/8in).

Thanks in advance!

[Leo_Gnardo]

A solution I saw in a couple of old amps (Danelectro? late 50's?) had the tube socket mounted to a small square of steel, say 2x2", and that steel plate floated over a hole in the chassis with 3 bolts, each with a couple of rubber grommets as shock absorbers where bolt passed thru sheet metal. Advantage, no specially shaped rubber parts needed. If you had some spare silicone grommets from modern reverb tanks, they'd come in handy for this kind of setup. Or you could source grommets/bumpers from a hardware store or home center.

[dchang0]

Thanks for the tips, Leo_Gnardo:

Ted Weber makes something similar to what you described with the Danelectro: SM9 Shock Mount

Suffice it to say that it's a lot further than I want to go... Especially since someone who reported using the Ted Weber approach said they had to heat shrink all the wires running to the socket to prevent the socket pins from shorting on the chassis.

Another absolute-last-resort method is the one shown in this guy's video: https://www.youtube.com/watch?v=z7yHtoxkdb8
Take a look at the chassis around 1:00 mark: it looks like he cut a huge hole and pop-riveted a thick sheet of silicone rubber to the chassis, then attached the tube socket to the rubber. He can wiggle the tube around like crazy.

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I saw somebody's close-up photo on a hi-fi forum who had square-cross-section silicone o-rings--I think they are these:

McMaster-Carr

SKU 1182N006 is probably the part that was in their photo. Four of these, one above the socket ear and one below the socket ear on each screw would serve to suspend it above the chassis. This idea requires the one-piece plastic sockets where the metal ears are permanently attached to the plastic socket, unlike the ceramic sockets where a separate aluminum ring with ears holds it down.

Maybe if the o-rings are big enough in diameter, they can hold the ceramic socket up off of the chassis, avoiding having to change the socket out for a plastic one with permanently-attached ears.

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I did find that there are gaskets with 20mm inner diameter holes like the one in the photo in my first post that are intake manifold gaskets for pit bikes like so:

20mm Intake Manifold Gasket Seal Set Pit Bike CRF50 SDG SSR 107 110 I GS27 | eBay

But the rubber is very thick--probably far too thick for the job. Might be easier for me to source a sheet of silicone rubber and cut the gasket myself with an X-Acto knife.

[dchang0]

BTW, I just tried a trick I saw in some other forum saying to use heat shrink tubing over the tube itself. That works well to quiet the tube but doesn't do anything about vibration transferring from the chassis to the socket.

And I saw on this EF86 tube comparison page that one should not use the Fender-style metal shield on Tung Sol EF86s, as it increases the microphonics. I have no idea why, but it turned out to be true in my case. http://blog.thetubestore.com/get-best-ef86/

I saw elsewhere that one can dampen the tube socket itself by putting Permatex silicone gasket maker on the outside of the tube socket (not the same as putting it under the socket between the socket and chassis). So I combined these three tips and put a giant sleeve of heat shrink over the entire thing--covering the tube and the 1/2" of shielded base.

It seems to work. Prior to all the dampening efforts (which so far is just using heat shrink tubing), I could only tun the knob up on the EF86 channel to 5.5 out of 10. After adding one layer of heat shrink tubing to the tube's exposed glass, I could go to 7/10. Adding 2 layers didn't do much at all, just 7.5/10. Adding the 3rd layer that covers the whole socket including the tube made it so I can go to 8/10.

That's acceptable to me for so little effort. Next, I will buy a tube hand-selected or guaranteed to have low microphonics, and that should make for a gig-worthy amp.

That said, if someone does know of a tube socket gasket, please still post it here.

[Enzo]

I don't understand, you say sleeving the tube quiets it, but not the socket. I thought to point of this exercise was to stop tube noise. The socket itself ought not to make noise, but the tube in it vibrating does. if you can quiet the tube, then what is left?

But to your question... Don't ever-think it. The material doesn't much matter. You can mount the socket in a chassis hole slightly oversized so the socket doesn;t touch the edges, and put rubber grommets or O-rings between chassis and socket mounting tab with screws through them. The hardware store will have O-rings and grommets. You cannot crank down the screws tight, so use a nylok nut on the screws.

You can also use a larger O-ring that sits snug around the body of the socket, and pinch it between socket tabs and chassis. But most sockets I see have a tab sticking out on opposite sides, but no flat flange all the way around. So your blue gasket would have nothing over it on the sides.

If you want to cut a gasket, an auto parts store will have gasket material. I think cork would be preferable to some other materials for this.

Home Depot, Lowes, a good hardware store, will have grommets and rings, just go look through them.

I have a large selection of O-rings with square cross sections, but here we call them idler tires for record players, tape decks and other motor driven things. I bet any TV repair shop has them around from their VCR repair days. if you are near Lansing, MI, you can look through mine. Google "idler tires" for lots of pictures.

[dchang0]

Oh, I guess I should've been more clear about it. In this amp, there is some sort of mechanical transfer of high frequency vibrations from the chassis to the tube such that if I tap the tops of the chickenhead knobs with my fingertips, I can clearly hear a "ting" in the EF86 channel's output. At 8 or higher on the EF86 channel, the chassis itself will start a very high frequency feedback loop, just like a squealing microphone.

This is in addition to the microphonics caused by the tube's innards vibrating and the tube's glass vibrating, the latter of which seems to be solved by the first two layers of heat shrink on just the exposed portion of glass.

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Thanks for the offer, tips, and terminology, especially the suggestion to try cork. That will be easy to find and cut to shape!

If the gasket idea does not work, what I will do is use a 22mm inner diameter o-ring or idler tire (intended to go between the ceramic socket and chassis) and four small 4-40 o-rings and nylock nut for the screws. That should do the job nicely without requiring any desoldering of the socket. All I would have to do is remove the screws, lift off the aluminum shield base, snap the 22mm o-ring over the top of the ceramic socket and snug it under the socket, then replace the shield and screws with the little o-rings suspending them off of the chassis.

[dchang0]

Here is a photo of the current solution using only heat shrink. This gets me from 5/10 volume to 8/10 volume without the microphonics feeding back. I hope and expect the decoupling of the tube socket from the chassis to get to full use of 10/10 of the EF86 channel.

I guess one other benefit of decoupling the socket is that it will help in the situation when I have to change the EF86 tube out at a gig--I won't have heat shrink or a heat gun there to apply new heat shrink over the socket. At that point something like Herbie's $30 tube dampeners will probably be a more practical solution.

[mozz]

I wouldn't use heat shrink tubing, too tight, heat retention issues. Premier 60's working on now, has factory grommets, even on the 5y3. I made a 5c3 and with grid leak bias and most all 6sc7's were howling. Glass tung sols were ok, i ended up putting grommets on the pre amp tubes, works fine now. Harbor freight, buy the assortment, you'll be set for a long while.
What were the old MIL SPEC tube shileds, EIRC ? They had lots of fingers inside, i have some here somewhere, will try them spur of the moment when i find them.

[dchang0]

Thanks, mozz--Harbor Freight it is!

I do have one old MILSPEC tube shield somewhere, but I would be surprised it if helped cut down on microphonics. I'll try it when I find it (won't be soon--it's probably lost).

[Mick Bailey]

Matchless isolates the tube socket by drilling the chassis holes oversize to take red silicone grommets. The centre hole when in place is an easy fit for the screw. The retaining screws have a plain washer under the head, and then a silicone washer. So the socket flange is sandwiched between the silicone washer and top-side of the grommet. Inside the chassis there's a plain washer then a nut with captive star washer.

They use a capacitor chassis-mount clamp lined with heat resistant fabric strip as a tube retainer for octal rectifiers/power tubes. When these fail I replace them with silicone sheet cut from the base of a silicone cake-baking pan.

[dchang0]

Thanks, Mick, for the detailed description of the way Matchless does it.

Looks the 1x 22mm I.D. o-ring/grommet and 2x or 4x 1/8" I.D. o-rings is the best way to go. I'm abandoning the gasket approach (even though cork sheets are very easy to get at the local auto parts store).

I've ordered silicone square-profile o-rings from McMaster Carr, SKU 1182N006 and 1182N02. I figured the square-profile would be stay put better than the round-profile, and I chose silicone due to its high temperature rating (takes much longer to harden) and relative softness (70A durometer).

I avoided the grommets because I don't want to enlarge any chassis holes. This should be good enough suspension to cut out the high-frequency feedback noise.

It'll be: 4-40 screw, metal washer, silicone square-profile o-ring, mounting tab, silicone square-profile o-ring, chassis, 4-40 nylock nut. One large square-profile o-ring will only be under the ceramic socket. It is the same 1/16" thickness as the smaller o-rings, so the final effect will be like a gasket.

I came across silicone rubber sheet in the McMaster Carr catalog:

McMaster-Carr

I like how you can choose the softness and durability/temperature rating. Prices are on par with the cork sheets at the auto store.

[dchang0]

One 7/8in I.D., 1in O.D. square-profile silicone o-ring under the ceramic tube socket. Did not require de-soldering the socket to install. Does not lift the socket high enough from the chassis for the pins to possibly short out. Does not fit snugly to the ceramic socket (it's a tiny bit loose).



Bought from McMaster Carr, SKU 1182N02.

I will use two 1/8in I.D., 1/4in O.D. square-profile silicone o-rings on the tube socket aluminum base, on top of the mounting tabs and just under the heads of the 4-40 3/8in screws. The McMaster Carr SKU for the smaller o-rings is 1182N006.

Actually, 1182N006 is a bit too large for the task. SKU 1182N005 (3/32in I.D., 7/32in O.D.) is probably a better bet, as it will wrap around the 4-40 screw snugly and won't press too hard against the side of the aluminum base (which pushes the screws outwards, away from the socket's center and causes metal-on-metal contact between the screws and the mounting tabs).

I will come back and post a picture later of the completely-installed socket later, when my 4-40 nylon-insert lock nuts arrive.

[Randall]

Maybe I'm missing something here, but isn't the microphonic problem with the tube, ie you have a microphonic tube. Why not replace it?

[dchang0]

You are correct, but as I understand it, swapping EF86 tubes to achieve low/no microphonics can get expensive quickly. I forgot where I read it, but someone who had been buying $50 to $90 NOS EF86 tubes hand-selected for low/no microphonics had noticed that they became microphonic after just a few months of use in his combo amp.

Others noted that they had to buy large batches of EF86 tubes and finding only a fraction of them that were not microphonic in their combo amp. I can't afford this approach with the cheapest EF86s going for $15 each. (Buying 10 tubes at $15 = $150, and if only one is usable, that effectively means I paid $150 for one $15 tube. It also means I took 9 tubes off the market that could be usable to people who are not concerned about EF86 microphonics because they drive them at lower gains, etc.)

By shock-mounting the socket, it should protect low/no-microphonic EF86 tubes and squeeze longer lives out of them. It may make microphonic tubes like the one I currently have acceptable for use, and it seems that in this case, I probably will be able to make this microphonic tube usable for some number of months.

(I have some tubes on order right now, but who knows whether they will be microphonic or not?)

[Enzo]

Exactly, some tubes are prone to be microphonic just naturally. So with a little construction consideration, you prevent it ahead of time. At least to some extent.