Blog Pages

3/10/18

PULTEC EQP-1A POWER SUPPLY SCHEMATIC

The Pultec EQP-1A circuit draws approximately 20mA from the power supply. The total circuit current is the combination of the 12AX7 plate currents, 12AU7 plate currents, 12AU7 grid bias current, and the Heater DC network current. Back when I taught electronics I sometimes used this schematic example as a test question.

trenton blizzard pultec eq power supply schematic

My Pultec EQP-1A power supply schematic is shown next.  Note: heater connections are not shown. This schematic shows how I wired the Hammond 270FX transformer. The two green (GRN) heater wires connect to all heaters.

12AX7 / 12AU7 HEATERS

Since the 270FX heater voltage winding is 6.3VAC, you need to connect the heaters on the 12AX7 and the 12AU7 in parallel by connecting pins 4 & 5 together on each tube. You then connect one green wire to pin 9 (center-tap) and the other green wire to pins 4 & 5.

6X4 HEATERS

For the 6X4 tube, you connect one green wire to pin 3 and the other green wire connects to pin 4. The 6X4 tube does not have a heater "center-tap" pin. The +60V on the green-yellow (GRN-YEL) wire floats 60 volts on the heaters and helps reduce cathode to heater potentials which can extend tube life.

trenton blizzard pultec eq power supply schematic

Thanks for reading,
Trenton

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

1/25/18

BJT Common Emitter Stage - #1

I've received a few emails over time requesting that I post some simple transistor gain stages that could be used for various audio projects. So, I will be posting a series of transistor circuits starting with this one. The first series of circuits will all operate on +9 VDC.

This single stage BJT (bi-polar junction transistor) stage has been bench tested to verify that the design works. Specifications are given below. Use a trim pot for R1 to bias the circuit.


BJT STAGE SPECIFICATIONS

The gain AV for this circuit, unloaded, is about 16db or 6.3 volts per volt. Is is the current draw in microAmps. Voltages for collector, emitter and base, VC, VB, and VE, are all given. Resistor values are given with notes. Input impedance, Zin, and output impedance, Zout, are also given. Notice that the input impedance, is pretty low at 4.8 kohm. RL = open represents my oscilloscope probe resistance at 1 Mohm.

Specifications for this stage:

AC GAIN

Also notice that the AC gain AV V/V drops quickly once a real load has been placed for "RL". Notice that with a load of 5 kohms you only get about 6db of gain. Load value matters.


SPICE DC VOLTS

The circuit draws about 700 micro amps from the supply. Notice that the collector voltage is set to 1/2 VCC, or 4.5 VDC. SPICE simulation agreed fairly well with the real bench tested circuit.



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.



ADJUSTING BIAS

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


Thanks for reading,
Trenton

1/17/18

Raspberry Pi 3 GUI - Part 7A - 3 LED Control Buttons with Tkinter

Controlling 3 LED's with Tkinter on Raspberry Pi 3. This code adds some color to the status text so it's easier to tell when the LED's are on.

(1) WINDOW WITH 3 BUTTONS - LED's "ON"




(2) WINDOW - LED's "OFF"




(3) CODE
#! /usr/bin/env python

#import Tkinter module, GPIO and atexit
from Tkinter import*
import RPi.GPIO as GPIO
import atexit

# setup GPIO 
GPIO.setmode(GPIO.BOARD)
GPIO.setup(7,GPIO.OUT)#red led
GPIO.setup(11,GPIO.OUT)#yellow led
GPIO.setup(15,GPIO.OUT)#green led

# create window, window title, window size
window = Tk()
window.title( 'tkinter7.PY')
window.geometry('300x200')

# define dynamic properties for led button
def red_led_on():
    GPIO.output(7, True)
    red_led_status.configure(text='ON',bg='red')
    button_red_led.configure(command=red_led_off)
    
def red_led_off():
    GPIO.output(7, False)
    red_led_status.configure(text='OFF',bg='white')
    button_red_led.configure(command=red_led_on)

def yel_led_on():
    GPIO.output(11, True)
    yel_led_status.configure(text='ON',bg='yellow')
    button_yel_led.configure(command=yel_led_off)

def yel_led_off():
    GPIO.output(11, False)
    yel_led_status.configure(text='OFF',bg='white')
    button_yel_led.configure(command=yel_led_on)

def grn_led_on():
    GPIO.output(15, True)
    grn_led_status.configure(text='ON',bg='green')
    button_grn_led.configure(command=grn_led_off)

def grn_led_off():
    GPIO.output(15, False)
    grn_led_status.configure(text='OFF',bg='white')
    button_grn_led.configure(command=grn_led_on)

def cleanup():
        GPIO.output(7, False)
        GPIO.output(11, False)
        GPIO.output(15, False)
        GPIO.cleanup()

# create text box 
red_led_status = Label(window, relief='flat', width=4)
yel_led_status = Label(window, relief='flat', width=4)
grn_led_status = Label(window, relief='flat', width=4)

# create button
button_red_led = Button(window)
button_yel_led = Button(window)
button_grn_led = Button(window)

# set button text
button_red_led.configure(text='RED LED',command=red_led_on)
button_yel_led.configure(text='YEL LED',command=yel_led_on)
button_grn_led.configure(text='GRN LED',command=grn_led_on)

# set initial status text
red_led_status.configure(text='OFF',bg='white')                     
yel_led_status.configure(text='OFF',bg='white') 
grn_led_status.configure(text='OFF',bg='white') 

# set location of button and text
button_red_led.grid(row=1,column=1,columnspan=1)
red_led_status.grid(row=1,column=2,padx=10)
button_yel_led.grid(row=2,column=1,columnspan=1)
yel_led_status.grid(row=2,column=2,padx=10)
button_grn_led.grid(row=3,column=1,columnspan=1)
grn_led_status.grid(row=3,column=2,padx=10)

atexit.register(cleanup)
window.mainloop()






1/16/18

Raspberry Pi 3 GUI - Part 6B - LED Control Button with Tkinter

Controlling an LED with RPi.GPIO - Part 6B


This demonstrates one way to control functions of a microphone preamplifier, such as phantom power, using a software interface. Code can be found in Part 6A.

(1) LED CONTROL PANEL "LED = OFF"


(2) LED ON BREADBOARD = "OFF"


(3) LED CONTROL PANEL "LED = ON"


(4) LED ON BREADBOARD "LED = ON" 


NEXT UP - ADDING TWO MORE CONTROLS

Thanks for reading,
Trenton