Bottlehead Kits > Smash

Smash Microphonics

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Samsquatch:
Okay I'll bite. Just finished reading the Smashup Install thread where it finished off with Doc and Geary suggesting this has it's own thread.

My understanding is that the 4P1Ls have an inherent micro phonic quality and there's nothing to remedy this entirely. What I have done is take two hydraulic system o-rings, that were slightly stretched, to fit over the glass envelope of the tube. While they don't entirely eliminate the ringing after adjusting volume or changing the input,  I find they settle down faster. That's my two pennies and I'm overly happy with the amp itself.

Mike

Alonzo:
I'll take your two pennies and raise you 2.  I did that and added fiber washers between the tube and socket.  Also added some diving weights to the top plate behind the tubes.  It sits in a small sandbox like my turntable.  While they will ring when first turned on, after warm up they settle down much quicker and it takes more force/disturbance to make them ring.
Alonzo

mcandmar:
Its pretty much a trait of all DHT tubes, the most common solutions i have seen are,

-Isolate the chassis from vibration with soft rubber feet, i think tenderfeet was a brand mentioned before?
-Rubber damper rings on the tube itself which is cheap and effective.
-Adding weight, Doc posted about large lead donuts for the 300b tubes in another thread.
-Isolating the tube socket itself, either with rubber rings between tube sockets and chassis, or rubber mounting the entire socket on rubber insulators.

In my own 4P1L build i used soft rubber chassis mounts, damper rings on the tubes, and rubber mountings for the tube socket which prevented ringing while playing music but the stepped attenuator will still make them ring.   Another interesting option is the old Russian tube sockets that have metal clips to keep pressure on the glass but i didn't use them due to the awkward top mounting, figured the rubber rings would work just as well.

There is also "filament starving" which is a popular concept on DIYAudio by running the filaments under voltage. While testing i found there was actually some merit to it, anything over 1.95v made my tubes more prone to a high pitched ringing so i run them slightly under that at 1.9v.   I have no idea how the filament supply is configured on the Smash so that may not have any relevance at all.

rogerfederer:
I've tried the lead diving weight on top of the power trafo, blue-tac between the top plate and the wood frame, 24 db attenuators on the amp input, 2 additional sets of 4P1Ls and 2 new OD3s.  The tubes are noisy around the start up and then will erupt -- usually for no clear reason (maybe vibrations from the floor above?) -- multiple times during the day.   At this point I'm going to take it out of my rig and look for a new location for it.

This is what Paul had to say about microphonics last april: "The locking socket really grips onto the tube, so the approach to minimizing microphonics with this preamp will be more along the lines of lowering the resonance of the entire preamp as a whole."  As I recall, the manual suggests that you bend the socket so it is easier to get the tubes in and out (which I did).  maybe this suggestion reduces the ability of the sockets to reduce microphony. 

I'd love any other suggestions.  For example, what would a "starve the filaments" mod look like?  Anyone found any lead rings to put over the tubes?  There are lead rings used in labs to keep beakers from tipping over; not sure if they are the right size...

Doc B.:
If the tubes are going into a feedback mode seemingly at random I would guess what you are guessing, something not necessarily music related is setting them off. It sounds like a fairly extreme case of microphony and I wonder if other 4P1Ls might be less susceptible. You don't mention isolation feet. If the vibration is coming up through the shelf, really lossy sorbothane or similar feet might help.

Re: the lead rings they come in different sizes. I have not looked to see if there is one to fit the 4P1L, but there may be something that would work. There are some interesting weights here, notably a 19mm donut and some vinyl coated wire that might work:

http://scicominc.com/flask-weights-a-supports

The vinyl coated wire is mighty spendy. I'm thinking one could achieve the same effect with some lead fishing weight wire and maybe some FEP tape.

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