Building Questions

[rlyach]

I have been going through the manual pretty carefully while I wait for my amp to ship. I noticed that the power switch looks to be wired with 24 gauge wire. the power coming in through the IEC is probably at least 18 gauge and the internal wiring is all 20 gauge. Is this a problem?

Also I was planning on twisting up some wire ahead of time using a drill, and while I was calculating how much to pre-twist I noticed that the kit is 4" short of red wire. Since I was short I was going to buy some green and yellow 20 AWG solid 300V for the filaments only. I could get some 600V cloth 20AWG but I don't think that is necessary. I assume 300V rating is plenty for the filaments.

 

[Paul Birkeland]

The kits will generally ship with a little extra wire, just in case.  (When I pull wire,  I will pull an extra 3" or so to be sure nobody gets short changed)

The amplifier uses a 1.5A slow blow fuse in the IEC/fuse module, which gives us a good idea of what the maximum instantaneous current might be (which is brief and during startup).

On the page below, there is a table that rates wire based on current and application:
http://www.powerstream.com/Wire_Size.htm
As you can see, for chassis wiring, 24 AWG wire is OK to provide 3.5A of current, which is more than enough for an idling Stereomour.  When you look at the Power Transmission column, you'll see that 16 AWG is approximately rated for the same current, and this is the size of the power cord we ship with the kit.


If you wanted to twist up some green wire to be traditional with the filaments/heaters, the 300V rating will be sufficient and 22 AWG or thicker should work well.

(I'd always suggest building the kit with the parts supplied first, then making any changes thereafter)

 

[rlyach]

Thanks, I will probably use Green/Yellow for the filaments just because it will look nice and add a little more color to the underside.

 

[Grainger49]

Randy,

The heater wiring is a good place to get tight loops using a drill.  I like the look, that is what I did with my last build, an Eros.

 

[rlyach]

Grainger,

how many twists per inch is good for the heaters and outputs. I think the Bottlehead power cord uses 3.

Randy

 

[Grainger49]

For the heaters I had the two feed wires spread apart, turned on the drill and held on to the "Join" point pretty tightly.  So maybe 20 turns per inch?  I couldn't count.

This is a flash back to Reliance Electric DC drives.  The most delicate signal in the drive was the tach feedback.  They had a similar twist ratio.  Even the speed reference wasn't protected as well.  If you lose reference the drive stops, if you lose feedback it runs away.

 

[rlyach]

Wow... that is a tight twist. I built a power cord and used a vise to hold the wires on one end and the drill on the other. I will run a tight twist on the heater wires then. I assume that the output transformer wires shouldn't be wound that tight?

 

[Grainger49]

Randy,

I don't have a picture of the tight twisted wire.  I'll make one. 

Here is a link to my shielded Bottlehead power cord.  I got the twists to "set" by leaving them twisted for a while before relieving the tension.

http://www.bottlehead.com/smf/index.php/topic,2890.0.html

The output transformer is a higher voltage, probably lower current and I don't tight twist them.  Maybe I should???

 

[rlyach]

I just over twisted my power cord so that when I let it loose it relaxed to 3 twists per inch. I wonder if I should twist the heater leads to the 12AT7 one direction and the 2A3s the other, since those two lie on top of each other on one side of the board. What do you think?

 

[Paul Birkeland]

Three twists per inch is a nice amount.

 

[rlyach]

One more build question... I noticed that terminal 13 is only grounded through the chassis plate. The drain wires of the input connect here but there is no direct ground wire to ground them. Should that be added or is the chassis connection enough?

 

[Paul Joppa]

"ground" is an arbitrary reference point; in this case T13 is the reference point.

 

[Paul Birkeland]

One more build question... I noticed that terminal 13 is only grounded through the chassis plate. The drain wires of the input connect here but there is no direct ground wire to ground them. Should that be added or is the chassis connection enough?

Randy, having built nine Steremour amplifiers myself (some with powder coated top plates), I can assure you that we have not run into any "issues" that need addressing (or mistakes in the schematic).

There are, however, occasions when addressing problems that don't really exist create problems that do.