Smash Microphonics

[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.

 

[rogerfederer]

i have already bought 2 extra pairs of 4P1Ls and 2 extra OD3s.  they can't all be bad.

i have taken it out and will get around to checking all the voltages again.  maybe there is a wiring issue?

 

[Grainger49]

There are just too many Smashes out there for it to be the design.  It has to be something about your setup. 

What are the components, source to speakers, in your system.  Is the Smash sitting on something flexible? 

I doubt that a wiring mistake has caused this problem.  I agree, going through 2 sets of 4P1Ls says it isn't the supplied tubes.  Keep with us, your problem can be solved!

 

[Paul Birkeland]

Can you post a photo of how things are laid out in the room? I'd also second the idea of adding mass to the chassis and squishy-ness  under it.

Additionally, having a ton of gain after the Smash with sensitive speakers is another potential issue (alluded to by Grainger's questions) .

-PB

 

[rogerfederer]

it is now out of the room.  given up on it for now in that rig.  i'm trying to figure out if it can go somewhere else.  this issue really is limiting. 

i had 24db attenuators between the pre and the amp so that should have reduced the sensitive speakers issue.

 

[galyons]

Additionally, having a ton of gain after the Smash with sensitive speakers is another potential issue (alluded to by Grainger's questions) .
-PB

I go from Smash/UP to Paramount 300B, with 6CG7 drivers, to 102dB/1W/1M 3w-way horns.  My Paramounts are  lower gain then original version or V1.1. (My guess would be an input of about .4v for full power.)

I concur with PB...mass and isolation.  I  live on a steep hillside in a 2 story wood framed/sided house, (actually 3 stories downslope). Doors closing, footfalls and general activity all play havoc on the music system.  I had to do extensive, wall mounted isolation and  mass loading of my turntable shelf! I have a rack with sand filled supports, spiked shelves and soft feet under the Smash.  No microphonic issues from environmental feedback.  I will design better mass loading than what I am currently using, plus add isolation for the tube sockets and switches. But that is to address the microphonics when switching volume and source.

Cheers,
Geary

 

[Doc B]

Another area that has not been discussed is room treatment. Conceivably a room could have a resonance mode that is right around the resonance of the filaments. That could definitely create a feedback situation. Re-positioning the preamp and or the speakers to reduce the presence of that mode might be worth trying too. A room measurement rig might be a helpful tool to see if this is a potential source of the problem.

 

[steve j]

I have 4p1l tubes from different years and suppliers and all sound slightly different but microphonics are much the same although one tube which when first powered up will ring on its own for about 5 minutes until warm.
I have wrapped the tubes with teflon plumbers tape which has made a difference and as others have used rubber washers between tube sockets and top plate. I'm not able to hear any microphonics while listening to music only when coming into contact with the preamp changing volume just my 2cent's worth.
Steve.

 

[Samsquatch]

When I get some time I'm going to do some more top plate and socket dampening. Frankly I don't believe I really NEED to but this is the fun part of these amps. They're big kid toys.

 

[rogerfederer]

Not to be discouraged, I have now moved the smash to a new location, with auspicious results.  It is in a heavy target rack in a different room.  I added some feet with soft felt tips and mortite to where the top plate joins the wood frame (replaced the blu-tac).

Now in day 2 of listening and NO microphonics at all!

So maybe the other location (room, rack) was especially subject to vibrations?  New feet helped?

 

[Doc B]

So maybe the other location (room, rack) was especially subject to vibrations?

That was my guess. Good job figuring out a solution!

 

[Brent]

Just wanted to report that I've had good results with the vinyl coated flask rings Doc identified above. I got a pair of them from Amazon for about 40 USD shipped. The Smash sits on 0.5 inch ID, 1.5 inch OD, 0.5 inch, thick 30 duro washers, also from Amazon, sandwiched between similar sized steel washers of normal thickness.  I also have a few random heavy things sitting on the top plate, but I'm not sure that this is doing anything. Also also, the top plate is screwed to the base, and everything is on a rigid, wall-mounted shelf.

With this isolation and damping setup, ringing is a non issue with music playing and is much reduced in level and duration when handling controls or otherwise touching the Smash itself. Before, I would occasionally hear ringing after big orchestra hits & other loud-silent sequences.

The Smash connects to Paramounts with 300Bs driving 90dB Fostex FE103-SOL speakers with a Fix in between, so results might vary with more sensitive downstream components.

 

[dpwoos]

Our Smash is microphonic to the point of being unusable. Honestly, disappointed that Bottlehead would offer a kit like this. How much of the preamp is useful in some other design that uses different tubes?

 

[Paul Birkeland]

Our Smash is microphonic to the point of being unusable. Honestly, disappointed that Bottlehead would offer a kit like this. How much of the preamp is useful in some other design that uses different tubes?

What amplifier and speakers are you using?  Too much gain after the Smash can make the microphonics unbearable, but this issue can be addressed as well. 

 

[dpwoos]

The amp is a Bottlehead Seductor, and the speakers are ADS L420. Our house is on a street with a lot of traffic, and the (simply awful) microphonics seems to be related to that. BTW, my son and I have built a bunch of amps and preamps - both kits and from scratch - and never experienced anything like this!

https://www.hifiengine.com/manual_library/ads/l420.shtml

 

[Jameson]

I also have a Smash. I get some light ringing with heavy footfalls but nothing noticeable when music is playing. I have it sitting on a marble tile and some rubber padding. I think it’s a pretty incredible preamp.