from a schematic to a drawing.....

[danielzink]

Fairly cryptic title 'eh ?

Here's the scoop.....I can figure out a schematic in the most basic sense of the word. As in - if I'm told to replace the .047's on the trem circuit......after a few minutes of staring at the schematic I can find 'em and even sort 'em out on the board itself.

But let's say I've got a schematic that I want to build into a homemade amp.

How do I get from this:


to this ?


(and these are just examples for illustrative purposes...)

I can follow a drawing.....but hey - I'm just an amateur "fiddler" here....I can't translate the schemo into an easy to follow drawing....

[Enzo]

A schematic shows the electrical relationship of hte parts, while a wiring diagram shows the physical relationship of the parts. The schematic shows how it works, and the wiring diagram shows where to put everyting.

I'd have to think about "how" to do it. At this point, I can look at the schhematic and see the thing. I have no conscious process that I follow, it just "is."

But if you have the circuit, you know what parts there are, and it appears you plan to use an eyelet board. (A turret board would look the same here.) Hopefully you have decided on a chassis format and it is large enough for all your stuff. DOn't make the chassis too small, why cramp your own style? Leave that to Mesa.

Gather your parts. You gonna mount the chassis on edge with controls on top and tubes hanging down? Or will the controls be out front with the chassis flat and tubes sticking up (or down, depending if the chasis is on the top or bottom of the cab.) That determines where the tube sockets go - flat surface or on one edge panel. The controls will almost always be along the front edge of the chassis. FIgure the dimensions of the chassis. Draw a rectangle of that proportion of a convenient size. Life size if you want even. You know the power transformer would mount in the corner, and the OT must go somewhere. If they mount on the top, other stuff can live on the inside of the chassis in the same spot. Line the controls up along the front in the order you want. And line the tube sockets up towards the rear.

Now all that is left is the wiring and small parts.

Within your chassis rectangle, make a smaller rectangle to represent the eyelet board. Try to keep things to scale. I often draw on graph paper. You can start by making rows of eyelets down the two long sides of the eyelet board. If you have no board design yet, then draw them in as we go. Usually in pairs, one across from another so the resistor or whatever wil stretch across the board.

You can practice. Pick one stage - the first stage perhaps, input jacks to the volume control. It will involve one triode, so pick one side of the 12AX7. Pins 1,2,3 sound OK to me. We will draw three wires from those pins to the eyelet board.

The parts mostly stretch across the board, though some might stop at an eyelet in the middle. But for now, they all span the thing. So we will need the cathode parts. Here is a 1.5k. If there was a bypass cap, it wuold sit along side and use the same holes. We can put it in the first set of holes at the end of the board. The end of the resistor closest to the tube sockets will get the wire to pin 3. SO draw a line from pin 3 of that socket to the end of the resistor. The other end of the resistor will go to ground.

Now the grid of the tube. The input jacks have three resistors. It is possible to stick them on the eyelet board, but it seeems to me they mount just fine on the jacks. So each jack tip has a 68k coming from it. So draw a resistor at each jack connected to its tip terminal. The first jack has a second resistor from its tip to ground, so draw the 1meg resistor from the tip terminal to the sleeve terminal there. Now the two free ends of the 68ks? Conncet them together. From there draw a wire back to pin 2 of the tube. We could add shielding, but some other time.

Now the remaining tube element is the plate. The schemo says the plate has two things connected there - the 100k resistor and the 0.02 cap. They mount on the board. Put the cap across at the second set of holes - along side the first resistor. From the end of the cap nearest the tube, draw a wire from the cap to pin 1. In the same hole as the cap, draw one end of the 100k resistor. Draw the resistor across and its other end goes into the hole next to the cap's still free end. SO it angles one hole. SO there is a wire from pin 3 to the resistor and cap on the board. At that point in the schemo there is pin 3, resistor, cap - nothing else connects there. On our drawing we have pin 3 wired to the other two things. They are the same thing now. We have the free end of the cap. Draw a wire from that hole to the top leg of the volume control. The free end of the 100k resistor will connect to B+.

We go through the circuit at each point looking at what connects to what, then placing parts on the layout and making the same connections.

Since the second stage is fairly similar, we have another 100k (and at this point, geez woulddn't we like to have part numbers like R2, R3, C5, etc?) running from pin 6 of the tube to the same B+. So draw another 100k on the board. SInce both 100ks connect to B+, draw one end of this new 100k into the same hole as the B+ end of the first one. From there eventually we will draw a wire over to wherever the B+ comes from. The remaining end of the new 100k will draw to a new hole on the tube socket side of the board. From there we draw a wire to pin 6.

Pin 6 also has a 0.02 cap runing from it, so we draw a cap with one end at the hole where pin 6 wire and 100k meet. The other end can be across the board in a new hole.

Move along the board one and draw another 1.5k across a new pair of eyelets. It is the cathode resistor. One end gets a wire drawn to [pin 8 of the tube, and the far end will go to ground.

DEpending on your ground arrangement, these grounds we are accumulating can either be drawn as soldering to the chassis at each point, or they can be draw with a jumper wire to each point, and from there to some master ground. Or other options.

What's left? Pin 7, the grid. From pin 7 just draw a wire to the center leg of hte volume control.

We have got 2 of 3 legs of that control wired now. The third leg gets a wire drawn to ground.

Note that from pin 8 of the tube, ther is also a 22k resistor for the feedback, so draw a 22k spanning the board with one end sharing the hole with the pin 8 wire and 1.5k resistor. The other end gets its own hole, and from that hole a wire will be drawn to the output jack.

We go through the circuit, starting with a blank eyelet board, but knowing where two tube sockets are adn knowingf where the row of controls are. We then go stage by stage through teh amp and make al the connections on the schemo.

I don't claim the result of this exercize is the best layout, it just shows a way to do it. I made the two plate resistors side by side so they could share a common B+ eyelet, but then teh cathode resistors were eparated. I could just as easily drawn the two cathode resistors next to each other sharing a ground hole and drawn the plate resistors separated and ran a jumper wire to B+_ from each.

There are layout caveats, like keeping leads to the power tubes short as possible and so on, but one thing at a time.