I need some small rubber blocks (e.g. 2" x2") for acoustical isolation
- specifically to mount valve (vacuum tube) bases on a pre-amplifier to avoid microphonics. I need to build up a raised platform on which to mount two octal valve bases so they are decoupled from the base plate. Any ideas welcome. I'm in Hammersmith London, so anywhere local would be great.
Chunks of solid rubber may well be sold as some other kind of product (door stops?) so lateral thinking is welcome. Andy
When I was a research student, we used to use large rubber bungs for such purposes. I don't know where you get them from outside laboratory suppliers, though, or whether they go up to 2". Home-brewers' supplies maybe?
Continuing to think laterally, there is a form of dense packing material that looks like rubberised matted horse-hair. I think I have a slab somewhere looking for a good home.
You could also try a garage or breaker's yard; I think I've seen big rubber blocks in cars.
Chris (SE London)
Rubber door stops any good?
I saw a similar request recently in another forum where the suggestion was a hockey puck. These supposedly cost about a quid each. Any good?
Bob
This was the link quoted
I think 2"x2" will be way too much rubber to mount something as light as a valve base. It'll be so stiff the sound absorption will be poor. Try something much lighter.
NT
Hmm, strange thing to build in this era, however a solid block of rubber may not help much as the valve itself is so light that the mechanical resonance will be at a high frequency and below this frequency there won't be any decoupling. So for isolating say loudspeaker vibrations which are usually at a few hundred Hz, it may not be ideal. You could try to find some stong compression springs that will take the traditional 4BA bolts screwed in the ends. I think at the time they used to have special anti-vibration valve holders which were like a flat disc of phosphor bronze pressed out into a shape something like a flat gyroscope or chronometer mounting.
john2
mount the pre amp on a sturdy shelf on the wall
In the past I've mounted valveholders using rubber grommets in the chassis holes, obviously with washers behind the nuts to protect the grommets from the nuts.
The standard way of doing this was to use a paving stone supported on an inflated tyre inner tube, but that's for the whole thing, not just a valve.
You've just created the image in my mind of Eusebius sitting listening to his loud music, as 4BA nuts are coming loose and launching the valves into space in the background...
I would have though you would need to create an isolated platform (concrete pad perhaps) that you can mount on rubber mounts (e.g part number RMM7) and fix the electronics to that:
I've decided to try a couple of Chinese ceramic octal sockets (heavy) for the two valve bases, and mounted them on a sheet of solid teflon about 6mm thick. Just thinking how to attach this to the base plate. First thoughts are a couple of big bolts say M6, and decouple these with rubber grommets so they don't touch the teflon.
It then occurred to me that ordinary erasers from stationary shops might be rather good - the rubber is soft.
I may have to do a few experiments - microphonics are quite severe. The base for the two octal sockets may have to be heavy - like 10mm alu. I have another preamp with directly heated triodes (notoriously microphonic things) where the sockets are mounted on a 4mm alu plate and that's much better for microphonics.
Back in my days of playing around with valve amplifiers (before any of the valve resurgencies), a valve which had gone microphonic was regarded as dead.
I think you probably want high density foam rubber as valves are not that massy.
It won't solve microphony though..thats more down to direct coupling of the glass and grid stuff. You need an acoustic shroud over the valves for that.
Or better still, use FETS instead. Valves have some slight advantages in power amplifiers, none whatsoever in pre-amplifiers. Particullarly hi-fi ones.
They are marginally nice in gutar amps where non linearity and microphony adds a little color to a rather dull pickup sound. The non linearity is easy to fake in solid state, the microphony is not.
Its probably down to valve design rather than mounting.
We used to 'ping' the pre-amp valves in Marshall valve amps with a fingertip and any that were vile got used in the power amp instead ..all ECC83 IIRC..some brands were always worse than others.
Those were always a bit microphonic though and the heaters need to be run on DC to avoid hum problems in very low level input stages for gram pickups and tape heads, etc.
Mullard introduced the EF86 specifically to overcome those two problems.
Back in my days of playing around with valve amplifiers (before any of the valve resurgencies), a valve which had gone microphonic was regarded as dead. >>
Different thing, it isn't malfunction as you rightly refer to - these directly heated triodes were used in radios in the field and operated from batteries. The filaments were something like 50mA at 1.4v - nothing at all. So the filaments were tiny and fragile, and it's these kind of filaments that tend to sing in a microphonic way. It's a well-known issue with these particular type of DHTs. Despite this, they sound so good that all obstacles simply have to be overcome.
It won't solve microphony though..thats more down to direct coupling of
the glass and grid stuff. You need an acoustic shroud over the valves for that. >
Yes, this could well be true (some DHT users report good results), or failing that some rubber damper rings
Or better still, use FETS instead. Valves have some slight advantages in power amplifiers, none whatsoever in pre-amplifiers. Particullarly hi-fi ones. >>
Unfortunately this isn't true at the highest levels where you want to squeeze the ultra details and timbre out of the system. DHTs for all their many problems just go that little way further in clarity and tone. You'd have to hear them to know what I'm talking about - they are so arcane (20s and 30s as well) that there's no body of opinion to guide your thoughts, so I wouldn't expect you to to believe all this just on my say-so. .
They are marginally nice in gutar amps where non linearity and microphony adds a little color to a rather dull pickup sound. The non linearity is easy to fake in solid state, the microphony is not. >
this is another field - stage gear for effects - I'm talking about hifi as above. As I say, all this is hard to accept without actually hearing DHTs and there is no production preamp that uses them since they haven't been made since the 30s and 40s. I don't expect you to believe anything on faith alone, so I guess we just have to differ on this one. Andy
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