69 SB build - SDM-grounding: some questions+wire dress

[young flower]

Hi guys

I have been collecting parts and information for my 69 Superbass build for about 5 months. I finally got my last parts-shipment 2 weeks ago and nearly got all the info I need as a newbie builder, so the build (and corresponding thread) will start soon.

I decided to implement SDM grounding on my build:
http://home.comcast.net/~jbjdav26/1959/ ... ubbers.JPG" onclick="window.open(this.href);return false;

Since I´m totally unexperienced in electronics, I´ll use the 100W Metro build instructions as my main building resource.
I compared the 100W instructions: http://metroamp.com/wiki/index.php/100_ ... structions" onclick="window.open(this.href);return false;
to the SDM-layout above step by step and tried to find all the differences. Most differences were easy to understand, but some things I didn´t understand. I´ll quote the corresponding sections from the instructions here and explain what is different in the SDM layout and what I don´t understand:

difference #1: page 11
"B. Neatly bundle the wires from the PT at the far end of the chassis and tie with two wire ties as shown. There are two plain blue wires for the amp’s bias circuit - one of them needs to route to the same ground lug [note: ground lug on F6] that the heater center tap was soldered to . (It makes no difference which of the blue wires you use.)"

--> If you take a look at the SDM-layout you´ll find that there is no bias wire (ground) attached to the ground lug on F6. First I thought that some PT connections were simply omitted but if you look at the board, you´ll see that the other bias wire (bias 100V, white) is attached to the 27k resistor in the bias section as it should be.



Question: Where does the bias ground go in the SDM-layout?

difference #2: page 22
"C. Route the red w/yellow stripe wire [note: PT CT] as shown to the F4 capacitor. Strip enough insulation from the end (about 3/4”) to join the two positive terminals of the F4 cap together as shown. Solder these connections, but leave some open space on the terminal that the wire goes to first for now; we have more wires to add to this terminal soon."

--> In the SDM-layout the PT CT is not connected to the F4 cap.



Question: Where does the PT CT go in the SDM-Layout?

difference #3: board values: page 56

Some values on the Superlead board from the SDM-layout differ from the 100W kit instructions board layout:



Questions: I´ll probably build a SB amp - but generally speaking: Can I use the values given in the 100W kit instructions or do I need to change some values for the SDM layout?

Hope somebody can give me advice on this and I´m really looking forward to any answers I get. Thanks in advance.

YF

[vh junkie]

For #1: Looks like the green wire is the bias ground
For #2: Looks like the yellow wire is PT CT.

Hopefully, some one else can take a look and verify.

[neikeel]

Long post, multiple questions - tricky to go back and forth!

Are you sure of your wire colours. The HT secondaries are usually blue. The HT centre tap is usually the yellow wire as shown on SDMs pictures.
Normal colours for the bias are green and white. The white is the board wire and green the ground wire.

The ground wire is green on the SDM layout and is the dark green wire that goes to the ground lug on the mains filter cap mounting lug.

Which PT are you using and which wiring colour scheme were you sent with it?

The most significant grounding factor I have found is to take the heater ground (centre tap) and move it to the input ground lug not by the PT.

The V1 330uF cap is probably best as a 25uF or 50uF. If you go SB I recommend 25uF with a 1k5 resistor.

The best tone stack cap for a SB is 250pF (or 270pF) with a 56k slope resistor. The mixer (with the 470k resistors) can be 560pF to 470pF depending on the year/schematic.

The 100k on the diagram is probably best as a 47k and the purple wire connected to the 8ohm tap on the output selector.

[young flower]

First of all: Thank you very much - both of you. .

The HT centre tap is usually the yellow wire as shown on SDMs pictures.

So - to make sure I got it right: When using the SDM layout, the HT centre tap is soldered to the negative lug on the F5 capacitor, as marked with the red circle in the following picture?:

The most significant grounding factor I have found is to take the heater ground (centre tap) and move it to the input ground lug not by the PT.

So - that means moving the heater CT from the F6 ground lug to the F1 ground lug? - Seems quite a far stretch to me, I´d probably have to extend the heater CT - which is not recommended as you said.

The V1 330uF cap is probably best as a 25uF or 50uF. If you go SB I recommend 25uF with a 1k5 resistor.

The best tone stack cap for a SB is 250pF (or 270pF) with a 56k slope resistor. The mixer (with the 470k resistors) can be 560pF to 470pF depending on the year/schematic.

The 100k on the diagram is probably best as a 47k and the purple wire connected to the 8ohm tap on the output selector.

So - that means I can choose all the components like in the Metro 100W instructions except the V1 330uF cap?
Actually, I already got all the parts - so I already have a 330uF and a 250uF cap for that position. I guess it´s oke to use the 330uF cap isn´t it?

PS: I know the wire colours on my PT (Heyboer 1203-80-ML) are different. I got a colour chart with it and will take care to apply that code properly i.e. replace the colours in the instructions with the correct colour on my tranny.

Thanks again
YF

[SDM]

So - to make sure I got it right: When using the SDM layout, the HT centre tap is soldered to the negative lug on the F5 capacitor, as marked with the red circle in the following picture?

Yes.

I guess it´s oke to use the 330uF cap isn´t it?

Fine to use 330uf, just overkill in boosting lows, may find a lower uf value suits your tastes better (or not). Neil was just suggesting alternate V1 values to try in that regards for a SB.

PS: I know the wire colours on my PT (Heyboer 1203-80-ML) are different. I got a colour chart with it and will take care to apply that code properly i.e. replace the colours in the instructions with the correct colour on my tranny.

Just to make things clear as I can concerning on my layout above, the "top left" (as pictured/drawn) grommet has 4 wires coming from it, 2 blacks, a solid green, and a green wire with yellow stripe. Blacks are both your heater wires, solid green is bias winding to ground wire, green/yellow stripe is heater CT. "Bottom left" grommet has 6 wires coming from it, 2 blues, 2 reds, a yellow, and a white. The 2 blues are the LV (lower voltage) secondary taps, red HV (higher voltage)taps, yellow is CT for both, and white is bias winding AC to board/bias supply input. Lone rightmost grommet has 4 wires coming from it, these are all the AC primary wires, orange=common, red=120VAC tap, blue=220VAC tap, violet=240VAC tap, use the appropriate tap for your region of course.

[young flower]

Thank you very much for your detailed explanation Steve - everything is clear to me now. Hope I´ll do a good job using your grounding layout.

Last thing that remains unexplained would be Neil´s suggestion:

The most significant grounding factor I have found is to take the heater ground (centre tap) and move it to the input ground lug not by the PT.

and my question about that:

So - that means moving the heater CT from the F6 ground lug to the F1 ground lug? - Seems quite a far stretch to me, I´d probably have to extend the heater CT - which is not recommended as you said.

Greetings
YF

[JimiJames]

IIRC hi-voltage grounds are separated from low.

...Last thing that remains unexplained would be Neil´s suggestion:

The most significant grounding factor I have found is to take the heater ground (centre tap) and move it to the input ground lug not by the PT.

and my question about that:

So - that means moving the heater CT from the F6 ground lug to the F1 ground lug? - Seems quite a far stretch to me, I´d probably have to extend the heater CT - which is not recommended as you said.

Greetings
YF

[young flower]

Oke - here is my final layout for the wiring and wire dress on my PT (the PT has a kind of special wire configuration which is why I made this layout in the first place). All PT connections plus SDM-specific connections around the PT area are shown in this layout. It incorporates SDM grounding (with the heater CT going to the input grounds instead of the F6 ground lug - as suggested by Neikeel) and a slightly altered power switch wiring where the pwr switch is switching live and neutral at the same time (it´s from a TAD Superlead layout)- completely cutting off any live current from the wall plug when turned off.
I´m glad about everybody having a look at it and giving me feedback if it´s basically oke or if there are any wires that need to be routed differently, kept shorter, twisted together with others or kept apart from other wires for hum, ghost note rejection etc.:

https://dl.dropboxusercontent.com/u/184 ... al.png.png" onclick="window.open(this.href);return false;

Thanks
YF

EDIT: layout design updated Thursay May 30, 2013 05:00 pm UTC+1