which type of rubber cushion under transformers?

vax9000 · 23rd April 2008, 12:09 AM

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!

HollowState · 23rd April 2008, 01:04 AM

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

CarlyBoy · 23rd April 2008, 01:04 AM

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.

vax9000 · 23rd April 2008, 03:25 AM

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.

Trout · 24th April 2008, 04:07 PM

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.

kvk · 24th April 2008, 04:26 PM

Quote:

Originally posted by Trout

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

who sells these retail (e-tail) in small sizes?

HollowState · 24th April 2008, 05:23 PM

McMaster sells them. They're called bumpers in the catalog. Seek and ye shall find.

Victor

KSTR · 24th April 2008, 05:33 PM

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

zigzagflux · 24th April 2008, 07:16 PM

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.

oshifis · 25th April 2008, 12:19 PM

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

23rd April 2008, 12:09 AM	#1
vax9000 diyAudio Member Join Date: May 2006 Location: Cleveland	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!

23rd April 2008, 01:04 AM	#2
HollowState diyAudio Member Join Date: Aug 2006 Location: Taxland, New Jersey	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 __________________ "The supercomputer is technologically impossible. It would take all of the water that flows over Niagara Falls to cool the heat generated by the number of vacuum tubes required." ~ Professor of Electrical Engineering, New York University

24th April 2008, 05:23 PM	#7
HollowState diyAudio Member Join Date: Aug 2006 Location: Taxland, New Jersey	McMaster sells them. They're called bumpers in the catalog. Seek and ye shall find. Victor __________________ "The supercomputer is technologically impossible. It would take all of the water that flows over Niagara Falls to cool the heat generated by the number of vacuum tubes required." ~ Professor of Electrical Engineering, New York University

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23rd April 2008, 01:04 AM	#3
CarlyBoy diyAudio Member Join Date: Aug 2006	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.

23rd April 2008, 03:25 AM	#4
vax9000 diyAudio Member Join Date: May 2006 Location: Cleveland	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.

24th April 2008, 04:07 PM	#5
Trout diyAudio Member Join Date: Nov 2004 Location: Midwest Madman	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.

24th April 2008, 05:33 PM	#8
KSTR diyAudio Member Join Date: Jul 2007 Location: Central Berlin, Germany	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

24th April 2008, 07:16 PM	#9
zigzagflux diyAudio Member Join Date: Oct 2006	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.

25th April 2008, 12:19 PM	#10
oshifis diyAudio Member Join Date: Mar 2004 Location: Budapest, Hungary	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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