Carolina songbird plus

[Bugman3183]

Good morning all.  Happy new year! I'm currently starting a build of the Carolina songbird plus and had questions about how the mosfet is wired and how it connects to the pots.  The layout and schematic in this area seem to differ from one another.   Admittedly, I'm no expert when it comes to schematics but getting better though. Typically, I tend to read on this board to go with the schematic in case of discrepancies but was hoping yo get some verification. Any advice would be greatly appreciated.   Thanks.

[Bugman3183]

Schematic and layout I am working from

[PRR]

> The layout and schematic in this area seem to differ

What looks wrong to you?

I don't think the ".01" belongs there; it may be leftover from an earlier drawing. If that is used, then the MOSFET needs some other way to bias its Gate(*), which is a different complication; just DC biasing from the triode is clean and simple.

Also:

EDIT: eliminate the coupling cap between V1-6 and the mosfet CF grid/gate.

(*) "All tubes must have a Grid resistor!!!" (FETs also!)

[Bugman3183]

On the layout the mosfet is labeled x=grid, y=plate, and z=cathode.   Layout has b+ from node d connected to y, schematic has it connected to x.  Layout has x jumped to where zener and "z" meets,  schematic does not.

[Bugman3183]

Upon furthur inspection, I don't believe the layout and schematic are actually the same amp.  On the layout x and y of the mosfet are connected to the mid pot, and the schematic appears the have "z" connected to a trim pot.  These were the only documents I could download as I am unable to use jschem and thought they were compatible.

[sluckey]

On the layout x and y of the mosfet are connected to the mid pot, and the schematic appears the have "z" connected to a trim pot.

No! The X, Y, Z designations for the MOSFET are all UPPER CASE. And they don't connect to anything other than the turrets on the board. The y and z on the mid pot are lower case. They connect to the mid switch. Don't get the UPPER and lower cases mixed up.   

IMO it's a poor choice for labels. There should have never been any XYZ designators for the MOSFET. Those should be DGS. Less chance for confusion.
 

[shooter]

not sure how many birds there are, fwiw here's the build link for the SE version, looks to have the FET figured out

http://el34world.com/Forum/index.php?topic=11889.0

[PRR]

> x=grid, y=plate, and z=cathode.   Layout has b+ from node d connected to y, schematic has it connected to x.

Looks like a tube-dude made-up his own pin names. It maybe would be OK just "P-G-C" for D-G-S (until you try to find them on the MOSFET datasheet). Going the extra step with XYZ is a bit much.

Does this crayon help?

[Bugman3183]

Thanks for  the help.   I see how I got confused with the mid pot (poor labeling choice indeed).  I am also on the same page now about the coupling cap that needs to be omitted.   What I'm unsure about still is whether the b+ should be connected to the source or the gate (I would think source but would like to be sure) and if the drain should actually be jumpered to the zener.   Please excuse my ignorance on the subject matter, still trying to learn.  Thanks.

[Bugman3183]

Sorry, layout has gate jumpered and drain connected to b+

[sluckey]

What I'm unsure about still is whether the b+ should be connected to the source or the gate (I would think source but would like to be sure) and if the drain should actually be jumpered to the zener.

The DRAIN (Y) connects to B+ node D on the schematic and the layout. Why don't you see that?
 

[Bugman3183]

I don't see that.   On the layout b+ is connected to middle leg "y" and on the schematic it appears yo be connected to top leg of mosfet "z".  Maybe it's just me.

[sluckey]

I don't see that.   On the layout b+ is connected to middle leg "y" and on the schematic it appears yo be connected to top leg of mosfet "z".  Maybe it's just me.

I'm looking at the schematic you posted and there is no "z" anywhere near the MOSFET. The only xyz appear down in the lower left corner of the schematic where tubenit put the MOSFET symbol signal with his label designations. Even there the top leg is clearly labeled plate, y, and drain.

Can you post a pic of what you are seeing? Maybe a screen capture?

[Bugman3183]

You are 100% correct that the is no "z" designation on the schematic.   I'm just tying to equate it ti what I see on the layout.   I'm not familiar with mosfets and have already mentioned I'm not great at reading schematics.   With this being case, the designation really doesn't mean a lot to me.   Could be x,y,z  right, middle, left leg or top middle bottom.   When I look at the schematic the are 3 lines for the mosfet symbol.  There's a dot at the top line (which to me indicates a connection point) leading to node d.  The top leg on the layout is designated as "z".  doesn't seem to be a dot (or connection to the middle line) or "y" on the layout.   It seems to me I'm looking at this all wrong and have more studying to do apparently.   Thanks.

[sluckey]

I think the only thing you need to realize is that circuit symbols are "usually" put on a schematic following a logical convention. In the case of a MOSFET, the drain is shown on top, gate in the middle, and source on the bottom. This follows the same "usual" convention for a bipolar transistor where collector is top, base is middle, and emitter is bottom. I say "usual" because there will always be exceptions. This makes it easier to understand the workings of a circuit.

But solid state devices come in many different shapes and pin configurations (case styles) and these rarely follow the same logic as used when drawing a schematic. Case in point is the IFR820. You will usually see the schematic symbol with the drain shown on top, but the actual device has the drain as the middle leg. Just the way it is. So, in order to understand how the device operates you need to refer to the schematic and hope the draftsman followed a logical convention. But when you need to actually wire up the circuit, you must follow the layout diagram which will show the physical connections for a device.

So, bottom line... schematic is for understanding, layout is for building. Learn to relate what you see on one drawing with what you see on the other. I've attached a screen capture from a datasheet for the IFR820 which shows schematic symbol and actual device layout. Memorize it!   

[2deaf]

Sluckey's got you covered pretty good, but I already drew this so I'll go ahead and attach it.

The flange on the IRF820 is connected to the drain which is connected to B+, so beware.

Heat sink those legs when soldering.

[Bugman3183]

Makes perfect sense to me now.   I really appreciate you guys taking the time to explain these types of things for people like myself that may not be in the know. Clearly I was confused.   Much appreciation!

[tubenit]

I drew that schematic and layout 8 years ago (2011)  & this was one of my first efforts in using a mosfet as a cathode follower.

Since I had some reasonable knowledge of wiring up tubes, I was attempting to think of "how is this mosfet like a tube?".

So, I drew this up in a manner that made sense to me at the time and helped me understand it.  Unfortunately, my "private" logic and labeling was confusing to you.  I am appreciative of PRR, 2deaf and especially Sluckey helping you walk thru it to understand it better. Sluckey has helped me resolve more issues then anyone on the forum & I have certainly made my share of bone headed mistakes over the years.  Reread and memorize Sluckey's reply #14 on the thread.

This attachment is the way I think about this.  It may not be technically accurate, but it helps me think about how to wire up a mosfet on a layout board.

With respect, Tubenit 

[Bugman3183]

I do believe it's my lack of knowledge that was confusing more than anything else. I've always been very impressed with your drive to be creative and innovative, and also, your willingness to share your creations and help others. I built one if your amps in the past.  I believe you labeled it "Ryan's amp", which was essentially a clean channel only tweed bluesmaster with active fx, and you were kind enough to help me through some issues with that one.  I've always been grateful for that.   Thanks again.

[Bugman3183]

Great sounding amp btw!