Blog Pages

2/24/18

PULTEC EQP-1A POWER SUPPLY

TUBE POWER SUPPLY

Here's a shot of my prototype Pultec power supply using my 7 pin breadboard adapter. It worked perfectly for the 6X4 dual diode vacuum tube. The rectifier circuit of the Pultec EQP-1A is called a full-wave center-tap configuration.

trenton blizzard pultec power supply

POWER TRANSFORMER

The power transformer is the Hammond 270FX. I chose the 270FX because the 6.3VAC heater winding has a center tap. This center tap is important because the circuit puts about 60VDC on the center tap to help reduce the heater-cathode potential and helps extends tube life.

The 270FX also has a separate 5VAC winding that is typically used for 5V heaters. However, in this application it is only used to power the indicator light. This is a complete waste of a good low voltage tap. I guess you could add USB charging to your Pultec if you desired.

CAPACITOR WORKING VOLTAGE

You need to be careful when prototyping the supply by itself because you run the risk of exceeding the MWV (maximum working voltage) of the electrolytic capacitors.

The brown resistors on the right are my load current to help drop some voltage across the 470 and 1k Ohm series resistors. I'm not pulling the 22mA that would be typical of a Pultec circuit. So, the DC at my first capacitor is almost 400V. That's really close to the 450V maximum rating so I don't operate it for very long.

trenton blizzard pultec power supply voltage

Next in line for this project will be the make-up gain amplifier. However, I'm currently waiting for my output transformer to be built by Cinemag. I'll post after it arrives.

Thanks for reading,
Trenton

2/19/18

BJT Common Emitter Circuit #2

BJT Gain Stage #2


Here is the second common emitter gain stage for this series. It operates on a 9V battery or 9V DC supply and uses a 2N5089 NPN transistor. This circuit has a fairly low input impedance so you won't be plugging your electric guitar directly into this stage unless it has active pickups with low output impedance.


BJT STAGE SPECIFICATIONS

This circuit has been bench tested to verify that the design works. Use a good trim pot (see "Adjusting Bias) for R1 to bias the circuit. SPICE simulation calculated R1 to be about 34k Ohms.

The gain AV for this circuit, unloaded, is about 19.4db or 9.34 volts per volt. Is is the current draw in microAmps at 513 uA. Voltages for collector, emitter and base, VCVB, and VE, are all given. Resistor values are given with notes.

Zin is pretty low at 4.7 kOhm. Zout is about 15 kOhm. RL = open represents my oscilloscope probe resistance at 1 Mohm. (Not truly "Open")

Specifications for this stage:


AC GAIN

AC gain for a common emitter stage can be calculated approximately by:

Av ≈ RC total / RE total

EX-1: Av ≈ 15 kohm / 1.5 kohm = 15,000 / 1,500 = 10 V/V


Notice that we neglected the load resistance of 1 Mohm. But, you can't neglect the resistance value for lower load resistances. Let's calculate the gain with Rl = 5kohm.

EX-2  Av ≈ (RC total / RE total)

RC total = 15 kohm || 5 kohm = (15 kohm x 5 kohm) / (15kohm+5kohm)

RC total = 3.75 kohm

Av ≈ (RC total / RE total) = 3.75 kohm / 1.5 kohm = 2.5V/V

Not bad considering the actual measured gain was 2.32 V/V. Remember, take all resistances connected to the collector( Rc in parallel with your load) and divide that by all of the resistances connected to the emitter (In this case just the emitter resistor RE). Gain vs. load resistance is given below.



HIGH BETA TRANSISTOR

The transistor used in this example is an NPN 2N5089 with a Beta greater than 300. Almost any transistor with a Beta greater than 300 should bias up pretty close this this circuit with a little adjustment of the trim pot R1.

Note: Transistor "E-B-C" (Emitter, Base, Collector) pinouts for transistors can vary. All 2N5089's will have the pinout given below. But, other transistors might be B-C-E, or C-B-E. Always verify device pinout before building.



ADJUSTING BIAS

Adjusting bias is easy when you use a quality potentiometer. I use Bourns pots for prototyping.


Thanks for reading,
Trenton

2/2/18

Raspberry Pi 3 GUI - Part 7B - 3 LED's In Action

Below is an image of the three LED's turned "ON" from code in Part 7A.

blizzard python code led

Here's a short video of the program in operation:


Next up I will be hooking up the Raspberry Pi-3 GPIO pins to turn on phantom power, switch phase, and activate an input pad on a preamplifier.

Thanks for reading,
Trenton