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Old 1st June 2012, 09:26 PM   #1
Feagil is offline Feagil  South Africa
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Default Oddblock Wooden Case

Hi everyone

I have a quick question for anyone that has built a valve amp from scratch. I want to build the project featured in this link.

OddWatt Audio 5751 SRPP / KT88 Push-Pull Monoblock Tube Amplifier Kits

I think most of you have probably seen this amp as a project in one form or another. I live in South Africa and shipping is prohibitive on items like output transformers and the chassis. I got the transformers custom wound here and thought I might build the case in wood instead of attempting to work sheet aluminium. I see many of these types of amps use the case as a ground plane so I was wondering what changes / precautions I should take if I make a wood case?

Do the transformer casings need to be grounded as they would be if bolted to a metal chassis?

Are there some reasons to avoid wood like EMI interference / hum?

Thanks for any help.
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Old 1st June 2012, 10:01 PM   #2
tomchr is offline tomchr  United States
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If you don't like metal work, I suggest building a wood frame and attaching a sheet metal top plate to it. I think with an all-wood enclosure, you'll have issues with material thickness that interferes with tube sockets and the like. Drilling a couple of holes in a sheet of aluminum is really quite easy and you can use most of your wood tools for this anyway.

The insides should be covered in metal. I'd use thin metal screen like the stuff used for decorations and such. You can generally find copper or brass screen material in arts & crafts stores. I've also found some galvanized steel screen used for vents and such at my local building hardware store. That would work as well. The screen should be grounded.

All metal parts that can be couched by the user should be grounded as well.

~Tom
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Old 1st June 2012, 11:43 PM   #3
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Back in the olden days of tube radio most of them came in a wooden case. Better screening from electrical interferences is achieved with a metal case and this is an important consideration with pre-amps and tuning circuits where low level signals are involved but not as much of a problem with power amplifier amplifiers. Still - anything you can do to minimize interference is always desirable.

Here is a link to something similar to what you are considering and there are many more examples available on the 'Net - do your research and look around.
DIY 6T9 Push-Pull Tube Amplifier Project
DIY 6T9 Tube (Valve) Single-Ended Amplifier Kit
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Old 2nd June 2012, 09:01 AM   #4
M Gregg is offline M Gregg  United Kingdom
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Hi just for interest..

Post 3848 and 3851 in this thread:

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The main thing is make sure the top and case can carry the weight of the Transformers...You can put circuit board under the top to give screening and grounding..if you use it as a ground plane then use a tool to engrave slits in the copper coating between any components so you don't have any conduction arround holes for valve bases...make sure if you take a supply cable through a hole in it bring the return supply cable through the same hole (eddy currents)..Yes Tx chassis must be earthed..


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Old 2nd June 2012, 09:36 AM   #5
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My first ever amp was all wooden chassis. Dead quiet.

Recommended. RF rarely is a problem. I would not bother with shielding at all.

Amps made with a wooden frame with an aluminium or copper top plate are very common and have no problems either with shielding. Even if you have an all metal enclosure. That is still no guarantee that it is shielded properly.

Go for it. With regards to safety you should have all exposed metal parts such as transformers connected to your ground.
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Last edited by Bas Horneman; 2nd June 2012 at 09:39 AM.
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Old 2nd June 2012, 10:10 AM   #6
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This one resides in South Africa. Built for my mate Jacques Beukes.
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Last edited by Bas Horneman; 2nd June 2012 at 10:13 AM.
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Old 2nd June 2012, 10:19 AM   #7
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by Bas Horneman View Post
This one resides in South Africa. Built for my mate Jacques Beukes.
Ok,

Where is the underside shot..its always the most interesting..well I think so anyway..

I like wood its better than a sterile chassis....Birds eye maple would be nice....I think I even prefer it to a metal top plate....and I hate shellac but I keep touching it and cant stop looking at it.....LOL Shellac dosen't like heat either but who cares...I would use it again its just such a pain ....I can agree that this is one of the quietest I have made dead silent..

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What is the sound of one hand clapping?

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Old 2nd June 2012, 10:59 AM   #8
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I love shellac.

This is the underside. A bit of a rush job. As I had to finish it before my holiday to the most beautiful country in the world South Africa.
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Old 2nd June 2012, 11:12 AM   #9
M Gregg is offline M Gregg  United Kingdom
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I see,

you have screened the caps..I do that as well..it works great, even with aluminium foil and drain wire..wrap with tape and varnish. or copper foil and solder.

This getting me thinking again...

The OP tubes have to be isolated from the wooden top plate..I recessed them in and mounted on silicone washers..also the Transformers to reduce any vibration. I guess you could just distance them using fixed sockets..(The type that the mounting ring is fixed to the socket/base).

I used long thread "red copper" speaker jacks..compared to my silver plated ones they are much better.. Also the input phonos can be mounted on a drilled copper plated PCB board and Gnd to board and isolate the Jacks from the board so they have a seperate star Gnd. with PCB shield...it makes it easier to mount in a square cut out..spray the board black works well..drill some glass for the front panel or use tiles.. mount the volume etc on PCB again behind the tile or glass and ground the copper on the board..(creates a shield) don't etch it just use it as a mounting material as though it was copper sheet..screw it (inside) to the wooden chassis then you can remove it to replace the volume if you need to without removing the glass front plate. No screws showing.

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M. Gregg
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What is the sound of one hand clapping?

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Old 2nd June 2012, 11:46 AM   #10
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Quote:
even with aluminium foil and drain wire
Good tip. That copper tape is expensive!
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