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Old 19th October 2003, 06:58 PM   #1
Lico is offline Lico  United States
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Default a few questions for my first time

Hi, I'm new to diy amps and everything, and I'm about to start the wiring of my first amp in a few days. I just have a few questions before I start.

1. My power cable has 3 wires, hot, neutral, chassis (safety) ground. When I was looking at grounding schemes, it seems that chassis ground is the only ground used, ie. all grounds connect to the chassis. How do I wire this? Hot and Neutral connect to the transformer in and chassis ground connects to chassis where all other grounds connect?

2. In the power supply, I need 250v and 180v. In the schematic, the resister isn't labeled that drops the voltage from 250 to 180. How large of a resister do need?

3. I need a volume knob. Where can I get a nice one? The knobs at radio shack were ugly as hell.

Click the image to open in full size.


Thanks
Andy
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Old 19th October 2003, 08:46 PM   #2
dhaen is offline dhaen  Europe
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Hi Andy, and welcome to diyAudio

As far as earthing is concerned, you are spot-on. With regard to Live and Neutral, it is safest to run the Live through a fuse, then a switch, then to the transformer primary. The Neutral can connect directly to the primary, if only a single pole switch is available.

The value of the resistor that drops the 250v to 180v will depend on the current taken. Do you know? Or, do you have the amplifier schematic?

Try a search of the forum for Volume Knobs. There have been a couple of threads about them.

Cheers,
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Old 20th October 2003, 07:42 PM   #3
Lico is offline Lico  United States
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here's the amp schematic.


Click the image to open in full size.

thanks
Andy
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Old 21st October 2003, 02:06 AM   #4
AndyN is offline AndyN  United States
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Your proposed grounding scheme will work fine.

It means that the neutral lead (white) and the safety ground lead(green) will be at the same potential, which is perfectly OK. That's how it's supposed to work.

As a point of good construction practice (schematics dont' always tell the whole story) What you'd be shooting for is to have the "audio ground" connect to the safety ground at only one place. Generally, that's very near the input signal connector.

The center tap of the high voltage secondary is the place to think of the "audio ground" returning to the transformer (it's sometimes called B minus for historical reasons, and B minus is usually at ground potential, but not necessarily). Ground is a slippery term, so hold it loosely, rather than in a mental grip.

As a more advanced noise reduction consideration, people sometimes put back to back "Ying Yang" diodes in between the audio ground buss and the safety ground point. Diodes drop about a volt across themselves, so the two "grounds" can differ by about that amount in either direction. Noise less than that volt can't get through, but any more serious imbalace can. Note that this sort of thing is NOT ok with the Underwriter's Labs, or your friendly home insurance firm. Mostly I mention it out of academic interest, not as a sugestion for a first timer.

I agree with dhaen's comment about the fuse and the switch - the fuse should always be the first thing attached, and it's also ok to put two fuses in both legs of the primary. The idea is that if it blows, everything goes cold, no matter how it's plugged in.

Volume knobs - you might stop by a thrift store, things fall off of old stereo gear all the time... (Hey, that's evil) Think about repurposing items - get a drawer pull from Home Depot and drill it out to 1/4" for the standard shaft size. Or glue the head of a GI Joe doll on a stick. My next project might use trumpet mouthpieces for knobs. The best part about DIY is that things don't have to be shiny black or brushed aluminum.

Have fun!
Andy N
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Old 21st October 2003, 04:59 AM   #5
SHiFTY is offline SHiFTY  New Zealand
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Cool schematic, that looks nice and simple and should be good for a watt or two. I am building a similar thing at the moment, using 6SL7. You might want to add a capacitor (say, 100uF) across the 255 ohm (strange value BTW?!!) cathode resistor of the 6V6... Otherwise might be a bit anemic in the bass.

With regards to the power supply resistor, just use a bit of guesstimation (how much current is flowing, V=IR etc) and try... I am guessing 10K or less... Don't worry too much about getting the voltages exact, 6V6 can be run at up to 400V as long as the total plate dissipation is not exceeded. Adjust the cathode resistor to get the required value...

Also keep in mind that your earths should be separated, and joined only at one place- ie signal earths (input & volume) cathode resistors, power supply earths.

You should also ground one side of the filament.
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Old 21st October 2003, 08:42 AM   #6
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Quote:
Originally posted by ShiFtY
Cool schematic, that looks nice and simple and should be good for a watt or two.
450mWrms with the cathode bypassed.
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Old 21st October 2003, 12:10 PM   #7
dhaen is offline dhaen  Europe
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Quote:
Originally posted by AndyN
...snip....

it's also ok to put two fuses in both legs of the primary. The idea is that if it blows, everything goes cold, no matter how it's plugged in....snip..

Andy N
Hold on
In Europe it is not OK to fuse the neutral side. The reason is that if the neural fuse blows first, it leaves the whole primary side "live".The only exception to this is medical instruments where other special precautions are taken.

Perhaps someone who is UL literate can advise whether the practice is allowed in the US on domestic equipment.
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Old 21st October 2003, 05:05 PM   #8
AndyN is offline AndyN  United States
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Quote:
Originally posted by dhaen

Perhaps someone who is UL literate can advise whether the practice is allowed in the US on domestic equipment.

I hasten to say that I am NOT in any way UL literate. The 2 legs fused business was something I picked up along the line from an old HAM years ago.

On further reflection, I think there's a distinction to be made between the modern US 3-prong cords, and the older US 2-prong unpolarized cords. In the day of unpolarized cords, it was a 50-50 chance that the unit would be plugged in with the nominal hot side attached to the unfused leg, and if the grounding were to fail, with the chassis were attached to the hot side, bad things could happen.

I think I ought to retract my statement about fusing the two legs, and leave it at "Use a polarized plug, and fuse the hot side. When in doubt, consult a professional."

Here's one reference:
http://home.comcast.net/~radiowarren/safety.html

I searched UL but they want $200 for pdf files of thier standards

Apologies for my Ameri-centrist assumptions...
Andy
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Old 21st October 2003, 08:26 PM   #9
dhaen is offline dhaen  Europe
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Quote:
Originally posted by AndyN



snip..Apologies for my Ameri-centrist assumptions...
Andy
Not at all The original question was from a US member. That's why I tempered my statement with the UL disclaimer. After all 230v is a lot more dangerous than the usual 110v lounge voltage, and standards do differ across the pond.
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Old 22nd October 2003, 02:30 PM   #10
Ralph is offline Ralph  Netherlands
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Hi John,

In Europe with 230V AC, it doesn't really matter which wire to fuse, am I right? There're is no such thing as hot and neutral. (I don't count the 3rd ground wire of cause).

Ralph
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