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Hi,
I had a 26 tube preamp built by a good friend of mine or he became a good friend from our dealings with the build(did a phenomenal job bye the way). I had built a few of them in my past on my own but I wanted to see what this filament bias design was about and me not being all that experienced with it I had it built for me using the filament bias. I just love the 26 tube preamp. I have not heard the 4P1L that Andy Evans uses but maybe someday I will get to hear it or own one. I know there are others as well but for now I am familiar with I think it is as good as it gets. BUUUUUUUUUTTTTTTTTTTTT!!!!!!!!!!!!!!! As the tubes got more and more used the tubes seem to have become more microphonic. Not all the time but you can hear if there are loud passages or anything that might create excitement in the room. I like it but I was wondering if it can be controlled to some extent. I am using the globe version 26 tube. I have the ST type but the builder recommended I use the globe.
Thanks!
Follow Ups:
I made my 26 Preamp tube mounts using 3mm thick aluminum plate, two pieces sandwiched together around a layer of Soundcoat sheet, that mounted to wooden supports using Deflex bushings I cut out of Deflex sheet, that mounted to the aluminum chassis, and finally a teflon tube socket to help aborb any vibrations that get through that gauntlet of dissimilar materials.Works like a charm! Zero microphonics.
Edits: 08/26/14
I know it is customary to gush over photos of other people's projects, but I cannot help saying that you've done beautiful work there. Can you show a photo of the fully assembled unit, including the front panel? What is the 9V battery doing? Thanks.
Here's the completed preamp.
The 9V battery applies grid bias to the 26. It is fed through the LL1676 input transformer secondary so that there is no need for a DC blocking cap in the grid circuit. It makes for a much cleaner sounding preamp; I tried it with a cap first (Duelund CAST, no less) and it sounds better without it. Here's a schematic:
Just for completeness, the 26 has a cascoded CCS on the plate, with the mu output of the CCS feeding Bent Audio (Slagle nickel core) autoformer volume controls at the output in a lossy parafeed configuration; the autoformers function as both volume control and output transformer in this arrangement.
Power supply is LCLC for plate and filament. B+ is regulated using CCS-fed VR tubes (0A3 +0D3 to get 225V). Filament is regulated using Rod Coleman regulators.
It maybe that your tubes are getting too used and need to be replaced. I found that microphonics can be an indication of a tube reaching the end of its usefulness at least in a more sensitive or a preamp circuit. regards, Dak
A possible solution ... see pic in link below. I believe he mounts his tubes using 2 pcb boards with a foam sandwich in between and has rubber stand offs as well.
Cheers
FM
I've gotten good results with Sorbothane as part of the solution. Half-sphere feet easily available as well as sheets of the stuff, all in many sizes.
WW
"A man need merely light the filaments of his receiving set and the world's greatest artists will perform for him." Alfred N. Goldsmith, RCA, 1922
Although not an option for your application, I built a 22dB phono tube pre that handily sounded better than a high-end SUT. Final version will have the 6922 immersed in oil.
or lacking the ability to have one of those I would try to mimic it.
Place the component on a board with as low a pressure as you get away with bicycle tube (thin wall - not a mountain bike tire tube) underneath.
Since it is the insides making the noise I cannot imagine the envelope making that much of a difference.
I am in the midst of collecting parts for a 4P1L and will be dealing with its sensitivity to microphonics soon enough.
Best of luck.
IIRC, the "bicycle inner tube" was done years ago from the good folks on Tweaks.
Search the archives if interested.
My advice is to suspend the 4P1L on a small subchassis using soft rubber grommets as suspension . Run the fils in series at around 3.7V .
ps packets of crisps aka 'potato chips' are excellent , quick ways of providing isolation and can be eaten if you don't like the sonic result ;)
Al
I like what the fellow from Germany does(forgot his name for the moment). He suspends all his tube sockets but being that they look to be suspended with springs would that be a bit rigid to control some of the microphonics?
Why go to all that effort ? All I do is add a bit of damping sheet to the top plate , suspend the valve socket on grommets and back off the filament volts , this tends to reduce microphony . The older types , especially globes or types with filament hangers tend to be a lot more microphonic . I recommend avoiding globe 26 for this reason
Al
The filament voltage was set just below the 1.5 with the filament bias. Even though globes are beautiful, truly NOS ST tubes will fix the issue. Hard to tell with the globes how much use they really had. People used to leave their radios on constantly. STs still sound very good.
I had no trouble getting my ST type 26 quiet , the globes I used tested as new but were prone to hum and were very microphonic . I could clap my hands next to the valves and there would be a quiet 'poiiiinnnnggg' through the speakers that lasted about a minute . Sold the globes to a man in HK who loved them...
Al
I think they sound fuller and a bit warmer than the ST. I have no problems with microphonics, but the mounting scheme I use is a bit extreme (see above for pics).
I'm seeing super-flexible wires soldered directly to the pins, and the tube is submerged in a bottle of mineral oil...
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