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Old 23rd April 2008, 01:09 AM   #1
vax9000 is offline vax9000  United States
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Default which type of rubber cushion under transformers?

I finished my El34 amplifier about a month ago. However, it has several problems. It is micro-phonic, there is hum magnetic field conducted by the mild steel chassis, and One or more power transformers/chokes hum. As the first step to attach those problems, I am going to add a rubber cushion for each of the transformers, to stop magnetic field from spreading, and to reduce vibration.

I went to ebay, search for rubber sheet, and I end up with several options. They are, "gum rubber", "solid gasket material rubber", "MACLA GUMMI", "neoprene", "Durometer", "Silicone Rubber", "Synthetic Crepe" etc.

I am really not familiar with this rubber thing. Would you please recommend a type? I guess it should be able to sustain the transformer, does not smell, does not decay under heat, have long life, and serve its cushion purpose well. Thank you!
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Old 23rd April 2008, 02:04 AM   #2
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I would suggest forgetting about feeBay and go to www.mcmaster.com

Two ways to start: from the main page, scroll down and select rubber from the raw materials catagory and go from there.
Or, type in "page 3428" in the search box and start reading.

Durometer is the international hardness scale that applies to rubber and plastics. Page 3428 will explain this. Begin reading about the various rubbers and narrow down to what you need. McMaster has no minimum and almost always has everthing in stock and ships the next day.

For output transformers that don't get very hot, regular SBR or Buna-N rubber would work. The power transformer which will run hotter will need a better grade temperature wise.

However, your microphonic problems are caused my the tubes themselves. And hum could be caused by several other things including transformer orientation or positioning with respect to each other and/or other componets. Power supply filtering and stray pickup. Improper grounding techniques, general layout, lead dress, etc, etc. All of which must be considered.

Victor
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Old 23rd April 2008, 02:04 AM   #3
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Don't think you'll have much luck stopping a magnetic field with rubber, but it might help with vibration and it's certainly cheap enough to try. If the transformer is anchored with screws, washers should do the trick. If there's a bigger surface you will need to cut out a sheet, as you said. Either way just go to mcmaster.com and supply "rubber" + "sheet" or "washer" in the search window. Good luck.
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Old 23rd April 2008, 04:25 AM   #4
vax9000 is offline vax9000  United States
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Thank you. I went to mcmaster and ordered silicone rubber stripes. Before I placed the order, I came up with another idea: furniture feet pads. However, for the best result, I ordered rubber stripes anyway.
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Old 24th April 2008, 05:07 PM   #5
Trout is offline Trout  United States
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Has anyone used lords mounts to mount their transformers? Just curious.

I thought about trying it to see if it reduces vibration from a PT.

I have used them in other applications like mounting model aircraft engines with lots of success. I figure if 4 of them held the motor on a 12lb model pulling 10G's they should work ok just sitting there in the chassis.

They come in lots of sizes, male and female and also a wide range of durometers.

Click the image to open in full size.
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Old 24th April 2008, 05:26 PM   #6
kvk is offline kvk  United States
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Quote:
Originally posted by Trout

They come in lots of sizes, male and female and also a wide range of durometers.

Click the image to open in full size.
who sells these retail (e-tail) in small sizes?
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Old 24th April 2008, 06:23 PM   #7
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McMaster sells them. They're called bumpers in the catalog. Seek and ye shall find.

Victor
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Old 24th April 2008, 06:33 PM   #8
KSTR is offline KSTR  Germany
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You might want to check out wire rope isolators (google for those). Advantage, besides that they work perfectly, is that they are virtually 100% fail-safe... which is a concern here.

- Klaus
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Old 24th April 2008, 08:16 PM   #9
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I use McMaster Carr p/n 94708A212.

Simple mechanical isolation of the transformer from the chassis. Below chassis, I use a fiber shoulder washer, so all hardware (and therefore the xfmr core) is electrically isolated from the chassis.

The one of the mounting hardware is bonded to chassis for single point ground. I like stainless steel for hardware; non-magnetic (not that it matters) and won't corrode long term. Cheap from McMaster.
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Old 25th April 2008, 01:19 PM   #10
oshifis is offline oshifis  Hungary
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I use common steel chassis, the transformers are screwed down to the chassis and my amplifier is silent dead. No microphony either. I think tight fix or rubber padding are two ways to fight mechanical hum. For me the first method seems easier, and it works, at least in my case.

http://tube.fw.hu/pa1.jpg
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