Avoiding Microphonic ef86 in v1

[Ed_Chambley]

I have seen many products and different ways to chassis mount preamp sockets.  From spring loaded tube sockets to floating a metal plate above the chassis.  I was wondering what some of you have found to work the best?  I do not think the silicone rings would work as the problem of vibration doesn't seem like it could be reduced by putting rubberbands over the glass of a tube.  Even though I have some nice NOS mullards which generally run fairly quiet, the current design I am working on will be producing much more bass and wattage.

It will be a head/cabinet configuration.

Thinking of floating the preamp section above or below the chassis (probably below as I prefer the look of the tube inside a punch hole in the chassis), but I have never needed to do this.  Is it common practice to use rubber spacers or springs.  I am thinking rubber spacers with locknuts to allow for a snug fit, but I am open to proven suggestions.

[John]

I am thinking rubber spacers with locknuts

Even with my limited experience, I've seen that on quite a few old relics on the small pentode socket.

[kagliostro]

When I go to electronics fairs often I can get some old Geloso rubber mounted sockets at 2.00€ each

if you can't get something similar, you can punch or drill a larger hole and use a rubber foil as to achieve an elastic mount for a standard socket, Tubenit has experimented this with success

the contemporary mount of rubber rings can improve the result

K

[tubeswell]

How I do it

[tubenit]

I use this. Very easy and inexpensive. It works for me.

With respect, Tubenit

[Ed_Chambley]

Thanks, these look like the simple approach.  I like simple.

[jojokeo]

Hi Ed,

I just run the tubes at a higher voltage w/ success (ie lower plate resistor & scale screen resistor accordingly is easiest to fiddle with). If the tubes are going to scream because of the design condition - rubber, spring, silicone, or condom mounting "may" not help w/ the extra prevention all that much? Nothing is 100% effective like pulling the goalie. Okay humor aside the seperate head & speaker cabinet approach is the best method for isolation. As in backflow prevention devices - the simple "air gap method" is the most effective, safe, and sure way for many things.

[sluckey]

These tube shields have internal fingers that provide good contact with the glass and help dissipate heat. They also help dampen tube vibrations. If simply grasping a microphonic tube with your fingers makes it less noisy, this shield will also help. Lot's of things help but I don't know if there is any single cure all.

[jojokeo]

Lot's of things help but I don't know if there is any single cure all.

IMHO I just explained how to do it 100% of the time. YMMV if you don't interpret it correctly?

[sluckey]

I wasn't responding to your post. Just throwing out another band aid suggestion. Those shields are too expensive for most people.

[jojokeo]

Hi Steve! No worries mate, I was just giving the two best methods that actually do work - operational and mechanical (absolute isolation).

[Ed_Chambley]

Hi Ed,

I just run the tubes at a higher voltage w/ success (ie lower plate resistor & scale screen resistor accordingly is easiest to fiddle with). If the tubes are going to scream because of the design condition - rubber, spring, silicone, or condom mounting "may" not help w/ the extra prevention all that much? Nothing is 100% effective like pulling the goalie. Okay humor aside the seperate head & speaker cabinet approach is the best method for isolation. As in backflow prevention devices - the simple "air gap method" is the most effective, safe, and sure way for many things.

Hey Joe, where you goin' with that gun.  What type of voltages you speaking of here.  Do you have any notes you can share on voltages.  Most run plate voltages low on ef86, least the schematics I have looked over.

[kagliostro]

This are the standard values in some amp with EF86 Tube



and to prevent microphonic authors like Merlin Blencowe council is to lower gain lowering plate values and so on

---

About the shields, those that Sluckey proposed here in Italy can be found at €1.00 each at Ham Fest Flea Market

however also $3.00, if only one shield is required, I think that can be affordable

http://www.surplussales.com/tubes-sock-acc/TubeShields-1.html

K

[jojokeo]

I'd have to check any notes from various schematics and see if I can tell if there is a magic range but I couldn't actually say offhand. Higher gain is inversely proportional with plate voltages which goes against your instincts - at least mine. That's why if you increase the plate resistor on a 12ax7 from the standard 100k to 220k or 330k - you're increasing the tube's gain but lessening the plate voltage (there's more going on like operating point/biasing but you get the general idea). With the EF86/5879/or other pentode you lower the plate (& screen) resistor to get a higher voltage which helps lower the tube's gain. There's plenty of gain on tap anyway and this helps reduce/eliminate microphonics especially when the tube's "pushed". You don't need to go after an absolute voltage, you do this to fine tune whatever amp you're dealing with and what helps to correct the problem. People running low voltages as you speak of is why they can have more of this issue, not less. Look at Geezer's past designs w/ the 5879 (Hoso, 56T, & others) and you'll see the plate resistors lowered also. I don't know if he's done this for this reason but it helps this tube in the same way and still sounds & plays very well.

[sluckey]

however also $3.00, if only one shield is required, I think that can be affordable

http://www.surplussales.com/tubes-sock-acc/TubeShields-1.html

Unfortunately they are 'sold out' of the size necessary to fit an EF86 or 12AX7 size tube.

[John]

Look at Geezer's past designs w/ the 5879 (Hoso, 56T, & others) and you'll see the plate resistors lowered also. I don't know if he's done this for this reason but it helps this tube in the same way and still sounds & plays very well.

IIRC he used 56k/plate and around 470k/screen, but started with a lower B+ too. I think he said the lower voltage was important from a tonal standpoint. (? never trust my memory)

[kagliostro]

May be here you can find it

http://www.ebay.it/itm/IERC-TUBE-SHIELDS-SHIELD-1212AX7-5157-12AU7-7025-12ADF-CV4004-CV492-6681-QTY-1-/221227618128?pt=Vintage_Electronics_R2&hash=item3382318f50

http://www.ebay.it/itm/IERC-TUBE-SHIELDS-SHIELD-SHEILD-5670-WE396A-TR6-6015-QTY-1-/221227612432?pt=Vintage_Electronics_R2&hash=item3382317910

But I admit, the shipping cost is more than the shield value

K

[sluckey]

But I admit, the shipping cost is more than the shield value

I agree. I'll never pay $8 for a single tube shield. The only reason I have some is because I had the opportunity to salvage a bucket full from our old radar system before it was hauled off last year.