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Old 29th May 2003, 05:35 PM   #11
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Quote:
I'd seen that AA thread, didn't realize it was you
Wasn't me :-) Just thought I'd point it out.

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Old 29th May 2003, 05:37 PM   #12
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Ah! OK. I was going to say "But now I see the similarity in the varnish", guess it's a good thing I didn't
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Old 29th May 2003, 05:40 PM   #13
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Cigar boxes look quite nice..have something classy about them..

Another alternative would be wine boxes...you live in wine country no?

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Bas
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Old 29th May 2003, 05:42 PM   #14
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Didn't even think about wine boxes, that's a great idea. Though I'm not sure how much the snooty wineries up here would want for their boxes

I found an archived thread with Dhaen's posts about umbilicals. There's some good advice in there, and connectors I didn't know existed. Thanks for letting me know.
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Old 30th May 2003, 01:14 AM   #15
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Hi Saurav;

I had a lot of the same concerns you've expressed when I got serious about scratchbuilding valve amps.

Here goes:

1. Shielding: not an issue. FWIW I always use shielded microphone wire for my signal leads from the RCA jacks to the source selector (or vol control if no selector) and that works fine.

2. Heat (and fire): most woods kindle (IIRC about 400F or 200C) at a temp higher than a correctly functioning trafo should operate at. However, fire is a risk, particularly if you are using old "non fire resistant" carbon comp resistors in a PS - thus I'd limit myself to wirewounds, metal oxide, and other power resistors that are fire retardant for that app. As far as heatsinking is concerned, it really depends on the amount of power that the resistor dissipates. My little wood chassis'd "Gnat" MK II has a PS resistor that dissipates an honest 10.4 watts and it gets HOT - and it thus got a heatsink AND a vent placed in the chassis to help out.

3. Bracing: you can always add a bit more if you need it. I'd do a test fit with just the heavy components before I finalised the layout. FWIW, I had to add bracing following initial construction and testing of load carrying on my 2A3's oak chassis. The best way to get a feel for this is to build it and find out; sorry but there are some things that are best learned by direct experience....

4. Umbilical: I don't have a specific recommendation here, but I know that I've seen decent milspec high voltage connectors at surplus houses from time to time.

Frankly, of the issues you raise, IMHO heating is the biggest issue, but not quite the way I read it in your post. The problem with wood chassis and heat is due to the "mobile" character of natural hardwoods (i.e. they change their shape and size with environmental changes). The fact is that if wood is not dried correctly and completely it will have a tendency to split, "cup" (a kind of warpage), or shrink (it shrinks perpendicular to the grain). Thus, you should make CERTAIN that any wood you're using in this app is thoroughly dried and from a reputable dealer in hardwoods. You may also want to consider adding a thin sheet of plywood under the hardwood decking.

The other heating issue is with the valves themselves - the fact is that the metal mounting plate for the valves acts as a heat sink. Without this you will encounter higher temps on the valves and sockets. You might consider mounting the valves on a metal plate that could be either atop or recessed under the wooden decking, and allow a generous clearance hole for the glass envelopes of the valves. BTW, if the wood is too thick, it may be difficult for you to access the underside of the sockets - when laying out an amp, you should consider the order of assembly with an eye to how much room there is to work, so that you are not getting in your own way.

If heat buildup inside the chassis remains a problem, you can always add a 'computer type' cooling fan for $3-$20 USD and run it off the 6.3V windings of your PS trafo. Some fans run quieter than others, so that's also an issue.

Good luck with your project!!
All the best,
Morse
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Old 30th May 2003, 01:59 AM   #16
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Default speaking of wine boxes

check it out

http://melhuish.org/audio/46amp.htm
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Old 30th May 2003, 04:42 AM   #17
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Morse,

How did you heatsink the PS resistor? And a PC fan is a good idea, some of those run pretty quiet when you drop the voltage down to 6V.
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Old 30th May 2003, 09:08 AM   #18
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Hi Saurav;

You have 2 choices on the heat sinking - one is to use a wirewound that's in an aluminium housing that's got mounting holes for heat sinking (this is the better way IMHO) and the other is to use a ceramic potted wirewound and couple it radiatively to the heat sink (i.e. use a "wrap around" sheet of aluminium that stands off the resistor by 3 or 4 mm which is tied to a much larger heat sink). The obvious danger here is that the aluminium MUST NOT be allowed any way of making contact with (or worse, between) the leads of the resistor!! Careful measurement and mounting of the pieces is absolutely required.

In the case of the Gnat MK II I used the bottom plate of the chassis as the heat sink and mounted the resistor 4mm above it (this way any flexing of the bottom plate in handling would not be able to flex the leads to the resistor). There's also a sheet of the same aluminium that I bolted in that goes up alongside the resistor. It's got to be dumping it's heat pretty efficiently, since the bottom plate directly underneath and alongside the resistor heats up pretty quickly! As an additional safety measure, I used a 25w rated NTE made resistor (I compared the NTE to the Philips ECG and the ECG was a bit smaller - thus had less surface area and a smaller thermal mass, even though both resistors were rated at 25w) - better than a 100% overdesign for the power dissipation involved.

Frankly I would have preferred to use a 50w aluminium potted wirewound (Michael Percy has 'em the last time I checked) but I used what I had on hand.....

Fans are a bit funny - I've got some that are really noisy and some that are really quiet - but you're right, it's not that hard to make one work in this application. I'm surprised that more people don't use 'em. The last fans I bought were $3 each and dead quiet at 6.3V!

All the best,
Morse
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Old 30th May 2003, 06:35 PM   #19
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Thanks for that information. I didn't know that you could get high power resistors that can be attached to heatsinks. It's a good thing I haven't started shopping for parts yet.
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Old 30th May 2003, 11:51 PM   #20
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Hi,

I hate to point this out...

Does fire hazard mean anything to you at all??

Personnally, I'd rather use a metal topplate to attach anything that could possibly inflame...
That would surely help me sleep better at night.

Never forget that wood, when moist, conducts surprisingly well...not that I'd want to experience the proof.

Hate to spoil the fun,
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