Friday, August 30, 2019

Valve libary included in the update.

If you installed LTspice IV and the update you have a valve library waiting to be used. I will show where it is and how to access it.
Open a new schematic and the component library. Look for valves.
I opened the valve library and select triode.
Place the symbol on the screen.
Place the mouse pointer in the center of the symbol and click to open the dialog box.
Double clicking the box with triode in it turns it blue and displays an arrow.
clicking the arrow calls up a list of available valves. They are grouped. Notice the line above the triode blue bar says End tetrodes........ So scrolling up will show the tetrode list. The triodes will be above them.
Scroll the list and select the valve you want and click OK.
The valve type selected will be displayed on the schematic.
You have a large number of valves available.

Thursday, August 29, 2019

Wednesday, August 28, 2019

Tube amp to drive a speaker using power transformer for output transformer

 The Designer In Training gave me this circuit to consider. She intends to build one. Hopefully we will see a report on it.

This one is really cool in that it uses a power transformer which can be bought at any well stocked electronics supply. It serves in a communication receiver quite well. If you are and audiophile you might want to pass this one up. I priced the transformer at Mouser for about $10.

The transformer has 4 windings 2 - 115Vac primary and 2 - 6Vac secondary. I measured a filament transformer for the sim. The transformer used in the amp is a better choice. To simulate a coil we need the inductance and resistance. The resistance of the dual primary and secondary transformer is a good bit higher. Anywho, I used the data I had and expect the build to perform better than the sim.Notice the input is fed through an attenuator. R2, R3 and R4 drop 50% of the input voltage level. If R2 is raised to 100k the output will double. The amp is part of a receiver that I don't have the complete schematic for. The designer placed the attenuator there for a reason that is yet to be seen.
Driving a 4 Ohm speaker.
It could drive a headphone too.
Driving an 8Ohm speaker.

NOTE: In the sims I am using a green LED. The bias is effected by the LED voltage which varies with the color.

EDIT: oops! DIT pointed out I used 2 transformers. She intends to use 1 with 2 primaries and 2 secondaries.

mutual inductance

R2 is 1kOhm and R1 has no voltage dropped across it. The supply voltage is dropped across L2.
With R2 set at 1Ohm R1 receives 8volts. If R2 was a rheostat this circuit could be used as a light dimmer or motor speed control.

Tuesday, August 27, 2019

finding the inductance of an unkown coil

The short version is we put a resistor in series with the coil. We need our generator at the frequency we expect to operate the circuit. We can adjust the resistor or the generator to get the desired voltage across the resistor. What is that value? It would seem one half the generator voltage would be dropped across the resistor. Because the circuit has reactance and the voltage is shifted in phase this is not the case. The resistor will drop one half the voltage as the phase shift is 45 degrees. At 45 degrees the voltage will be 1.414 times the supply which will put .707 across the resistor. You can see the resistor and coil are dropping 700mv with a 1volt signal. To do a more accurate measurement you would need a Maxwell Bridge.
The Maxwell Bridge uses adjustable resistors and inductors to null the bridge. The detector D will read a voltage either side of balance.
The vector analysis shows that E is the vector sum when E1 = E2 and E3 = E4. This would be the point where D is null.

Sunday, August 25, 2019

putting a tube in a spice schematic

Go to misc to fine the symbol.
Select triode
Put the triode on the screen and select "spice directive"
Type in .inc and click ok.
This places .inc on the screen. Click it.

Click browse.
Locate your model and click it.
You now have the model and symbol on the screen.
Ground the cathode because spice does not like floating circuits. Hit run and enter a stop time. I usually use 10m.
If you installation has this directory you uae a different procedure to find the model bear with me on that.
The Valve directory offers a triode symbol too.
Look back and you will see TRIODE was displayed when the symbol was placed on the screen. Click triode and a box will open. If it has an arrow you can pull down a menu and select a tube otherwise you type your tube in. In this case 12ax7.
now the sim should run. In a basic LTSpice IV install you will not have as many options. We have tube libraries available from 4 sources (maybe more) so we have to match our symbol to our model. Some models include the filament which complicates matters further. Once you load and run your first sim just make a note which symbol you used. The models I sent out are "generic" meaning work with the spice model which came in the bundle. I hope this helps.

Wednesday, August 21, 2019

spice models valve, tube, jfet with link to calculator

The question asked was can we model valves in spice. The answer is yes. Next question, will it be like jfet? Let's take a look at a couple models of each.

 If you have a tube manual you can compare the curves and make changes to produce a better match. The better approach is to find a model made by someone else.

 I used the same jig but had to change the voltages and polarity to match the jfet. One of these I did create because I couldn't find a model on line.
For valves you can have a little fun, as playing a video arcade game with this program.

The top view is loaded with a diagram from the tube manual. The bottom is the spice response to the model. You get the number from the datasheet and enter them in the boxes. Click plot and hope for good results. a little tweaking of the numbers may make the response better or give you a reason to start over. This program can be downloaded here:
Valve model calculator

Monday, August 19, 2019

Dipping my toe into Maxwell's Law and FS Discriminator

The law states the E field is represented by a vector. The vector length represents the strength of the field and its has direction. The vector can be rotated 0 to 360 degrees to show the direction of the field.
Here we have some examples of vectors representing fields.
There is also a law for magnetic fields which says we can represent B (magnetic) fields with vectors. There is a little difference between E and B fields. E fields extend to infinity or until they are absorbed by a particle of opposite polarity. E fields are radiated by positive charged and absorbed by negative charged particles. Magnetic fields form closed loops. There is a three way relation between E fields, B fields, and motion. E fields in motion create B fields. In other words current in a wire will create an electromagnet. A conductor moving in a B field will cause a current to flow in the wire. These factors will always be present perpendicular to each other. We analyze their interactions using vectors. The arrow above the E says use a vector to represent it.
Some use FBI to remember the left hand rule. The thumb is F the direction the wire is Forced to travel. B is the direction of the magnetic field. (north to south) I is the direction of induced current.
The rule being applied when the wire is fed a current while resting in a magnetic field.
A current carrying conductor will have a field around it. This illustrates the field polarity. The E field will be radiating perpendicular to the wire surface. As I said before the E field, B field and motion. In this case the motion is current flow.

This brings us to the left hand rule for coils.
This is how to find the total inductance when coils are in series and parallel as long as the fields do not interact.

Take a look at Ohm's Law.
Put your thumb over the unknown and the formulas is what's left. Look at (A) two or more inductors 1/Lt = 1/L1 + 1/L2 + 1/L3 and assume E=1. Using I = E/R we find the current in each path. Adding give the total current and using the assumed 1 volt for 1/Lt gives our total. Anywho, I went through all that to get here.
Using vector analysis we can see how the discriminator works.

Sunday, August 18, 2019

magnetic properties

How many magnetic properties are there? I made a little oscillator using a pill bottle for the coil former. I tested the frequency with an air core for the reference. Inserting zinc, ferrite, copper and aluminum gave these results. Remember increasing the coil size will reduce frequency.

With air core it is 803.
The zinc is a roll of Zinc pennies. The frequency increased indicating the zinc lowered the coil's inductance.

The ferrite lowered the frequency because it increased the coil inductance.
The copper raised the frequency.
The aluminum raised the frequency too. The test demonstrates that we could use copper, aluminum, or zinc to tune a circuit and raise the frequency when inserting the core. The question is how can this be?
Here I have two disc suspended in magnetic fields. One is attracted the other repelled. One is aluminum and one is copper.