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Old 30th March 2010, 04:23 PM   #1
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Default shielded wire

Where and when should I use shielded wire in a tube amp project? I'm building a single ended 5-watt guitar amp (http://www.schematicheaven.com/newam...at_minicat.pdf)

Also, is 18 gauge wire okay for everything? Single or stranded?

Where is the best place to get wire?

Thanks for any and all answers.
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Old 30th March 2010, 04:45 PM   #2
TheGimp is offline TheGimp  United States
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Shielded wire is good for eliminating hum pickup, however there are other tecniques which should be used first.

(1) Three wire power cord properly fused, switched and grounded.

(2) Single point ground for all active circuitry with respect to chassis ground. Possible dual inverse diodes between chassis and circuit ground.

(2) Good layout with power and output transformer cores out of allignment.

(3) Isolate input and output jack grounds from chassis ground.

Following those, I'd use shileded wire from the input jack to the input circuit (I would locate the input resistors near the tube).

Solid vs stranded is a matter of choice.

I'm using cloth covered wire like this:

Radio Daze-Cloth Covered Hookup Wire

I got mine before I found this source, so mine is not 600V rated. Then again, I'm only using it at 250V.
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Old 30th March 2010, 04:51 PM   #3
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Single point ground means create a "ground bus" for the amplifier circuit and then ground that to the chassis only at one point, or am I interpreting that incorrectly?

And isolating the input and output jack grounds, how do I isolate them from the rest of the grounds?

Thanks for the source, I'll order a few colors.
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Old 30th March 2010, 05:12 PM   #4
TheGimp is offline TheGimp  United States
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Yes, single point means that the amplifier (and power supply) only connect to the chassis at one point. Also it is good practice to connect the power supply ground to the rest of the amplifier at one point.

Use isolated jacks that have plastic isolation from the chassis. This allows you to connect the input and output jacks to the common amplifier ground at a choosen point and helps to avoid ground loops.

I also twist the wires that go from the output transformer to the output impedance selector (if one is used), and the wire pair that goes from the selector to the speaker jack.

In the case of the schematic you posted, there is only one output tap (two wires) so no selector is needed and the two wires can be twisted and connected directly to the output jack. Since there is no GNFB in this amp, you could use a grounded speaker jack and let that be the single point ground for the output (although I don't like doing this because I have seen corrosion cause problems in a Traynor PA Amp.).

If there were GNFB, this would create a ground loop.
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Old 30th March 2010, 05:13 PM   #5
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Default 600 v wire

600 V wire is pretty important for tube circuits. For non-shielded wire I'm using 22 ga. with teflon insulation because if I bumble around with the soldering iron I don't have to re-do any work. I found 100' rolls of 22 ga to be about half the price for Belden brand stranded at Newark vs Alpha brand at Mouser. There are not a lot of colors in teflon 600v 22 ga Belden, perhaps when the current stock is gone it will be all Alpha wire every where at 60 cents a foot.
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Old 30th March 2010, 05:14 PM   #6
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I'm still a little confused. Do I connect the input ground to the output ground, and then ground that to the chassis so both the input and output are touching the chassis at the same point?

Sorry for being dense!
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Old 30th March 2010, 05:31 PM   #7
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I just ordered 50' of each color in a pack (I am probably going to need it all down the line anyways). Its 18 gauge solid core with cloth covering rated for 600V, so I should be all set. I will probably take some coax wire and cut it and use it for the input lead.
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Old 30th March 2010, 06:22 PM   #8
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Default tube returns

The input jack ring should be connected to the minus of the power supply at the input tube. The output jack ring should be connected to the minus of the power supply at the output transformer, or the minus power supply at the output tube if the transformer doesn't have a minus lead (push pull). Historic tube circuits used the steel chassis as the common for the minus of the power supply but this is kind of obsolete. For one thing, if the power plug was upside down, and the cardboard insulation was damp between chassis and the steel case, your 1954 Philco TV would shock you (my Mother's did). It is best to chain the minus power supply around in wire these days, then connect to the safety earth of the input cord at one point. For hum shielding the chassis may also be connected to the input cord earth at the same point, but in 2010 the chassis shouldn't carry minus power supply current as it did in the old days. You may also want to put a metal oxide spark arrester that is UL CSA or VDO rated between the power supply minus and the power cord neutral(western hemisphere or Korea).
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Old 30th March 2010, 07:57 PM   #9
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Now I'm all sorts of confused when it comes to grounding things.

So all the little grounds in my schematic (http://www.schematicheaven.com/newam...at_minicat.pdf) in the tube area of the circuit are all connected to a ground bus. Then, at one point, I connect that ground bus to the ground on the secondary side of the power transformer.

Do I connect this huge mass of ground connections to the chassis ground that is connected to the main ground (3rd pin on input power jack)?

For the input, I connect the ground of the input jack to the ground at the base of the cathode biasing resistor/cap? What about the 1M resistor?

For the output, I tie that jack's ground to the ground of the transformer, and never tie it to the rest of the circuit's grounds, so it is completely isolated?

Sorry again, but I need some clarification. Maybe someone has some zoomed in pictures of what this all looks like? I checked the AX84 project and its builders guide did not mention grounds, although the schematic has the input ground grounded to the chassis, and the output ground grounded the power supply's ground, and the power supply's ground never comes into contact with the chassis ground, which is contrary to some advice I've gotten here.
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