The howling and the cure

[VacuumVoodoo]

So what do you do when a combo amp howls
just sitting there with nothing plugged in the input?
O-rings on the V1? - didn't work...
Selecting a silent tube...nope, didn't work.
Isolate the tube socket mechanically from the chassis.
Mount it on a small metal plate with vibration damping grommets?
Yes. The cure. Holidays are for creativity
[img:600:600]http://tubewonder.com/IPDOCS/small_prob ... val_02.jpg[/img]
I might do a run of 100 pcs or more, just need to get a quote for laser or water cut and anodized plates. Making them by hand one-by-one? I don't think so. The price of grommet + special mounting screw set is high enough already and it uses 2 of those.

[Reeltarded]

that is a piece of machine art right there! nice!

[ampdoc1]

Kudos on your fix. You might find a market for those on this and other builder forums.

On a few amps I've built, I bored out the standard mounting holes on the tube socket, and put small soft-rubber grommets into the wider holes, allowing the grommets to have a bit of play. Then I loosely mounted the screws and nuts, using a spot of Lock-Tight on the screw ends. Probably not as much isolation as your your fix, but cheap and the parts are easy to find at most hardware stores.

adoc1

[pdf64]

My 1957 Gibson GA20 uses this same method. Perhaps the early 12AX7 were more microphonic and so such measures were necessary even for low gain designs?
Pete

[Richie]

My 1957 Gibson GA20 uses this same method. Perhaps the early 12AX7 were more microphonic and so such measures were necessary even for low gain designs?
Pete

A few of those models used that method,i believe some even were used back on the octal preamps. They used rivets,and through the center went a flexable woven ground lead,to ground the socket base. Some amps would solder the ground to the tube shield.

I think that is a great idea that would help a lot of amps,or tubes. EF86 or pentode preamps too.

[VacuumVoodoo]

It's not any particular tube type to be blamed for an amps willingness to howl but total gain of the signal chain + mechanical coupling between loudspeaker and amp chassis. This is a much more frequent problem in combos then in heads. At least one can always place a vibration damping mat between the head and speaker cab to get rid of it..
Another sensitive piece of equipment is any condenser microphone with internal tube preamp.
A number of various fixes have been used in the days of old, from very simple to way "over engineered".
One thing that hasn't been addressed before is breaking the vibration path through cabling to the socket.

[Structo]

Yeah I have seen devices like that on EF86 tubes.

Someone here found a dampening or isolation socket online that worked well.

Couldn't you just take some rubber grommets and put them above and below the socket mount holes then use screws with nyloc nuts?

Or is there not enough room on the socket plate for the rubbers?

[VacuumVoodoo]

Couldn't you just take some rubber grommets and put them above and below the socket mount holes then use screws with nyloc nuts?
Or is there not enough room on the socket plate for the rubbers?

This and the fact that "general purpose" grommets are often too stiff and/or get compressed by the screw/nut. The grommet+screw set is designed so it prevent the grommet from getting compressed. That's the key to the whole trick of balancing between too simple and over engineered. It was developed specifically for vibration proofing sensitive electronic circuit boards.

[Structo]

Thanks for the clarification Aleks.

[Milkmansound]

I've had that happen before and lead dress fixed the problem

[VacuumVoodoo]

I've had that happen before and lead dress fixed the problem

You had capacitive parasitic coupling between gain stages due to flying leads of some sort.
It caused the circuit to oscillate, this can be really hard to fix sometimes so you did good.
I'm dealing here with strictly mechanical/acoustic feedback and microphonics.

[NickC]

Beautiful!

[roberto]

Well done Aleksander!
Very smart!

[rp]

To quote Crocodile Dundee, "That's not a shock mount, THAT's a shock mount." I posted this before, it's something I found on the web from some IIRC German hifi or pro audio gear.

Just kidding though. + 1 on beautiful work. I'd likely buy at least one from you.

If I was gonna try one of these DIY at home I'd probaly do the whole pre section just because it's easier to hack up big pieces with just basic hand tools. Something like pict 2, which is also from the web somewhere. Cheers to whoever the orig was.

[VacuumVoodoo]

Cool. THE Shock mount in first pic - notice how lower springs are not participating in the team work. Of course, I can't see if that thing was perhaps meant for operation in vertical position.
Hagstrom did something similar in some models of their amps. They suspended whole main board with springs, basically just like the sled in a reverb tank is suspended.
Second pic is more like it, something I'm looking at too.
The thing most of these miss is the fact that stiff cables connecting the socket to the circuit is enough to lead vibration into the tube and thus reduce effectiveness of this gizmo or even make it useless.
Making the connections with spiral formed low current thin Litz wire is the thing to do. It's highly flexible and when wrapped in double cotton layer stays silent.