Blog Pages

5/6/18

PULTEC EQP-1A POWER SUPPLY SCHEMATIC EXPLAINED

The Pultec EQP-1A uses a simple tube rectifier and RC filter networks to create high voltage DC. Follow along as I explain each stage.

PULTEC EQP-1A POWER SUPPLY SCHEMATIC



HIGH VOLTAGE AC
The high voltage transformer I am using is the Hammond 270FX. High voltage AC (alternating current) comes into the supply circuit by connecting the red transformer wires to (J6) and (J8) labeled "RED". Note: Either of the red wires can connect to J6 or J8. But, don't forget to connect the "RED/YELLOW" wire to (J22) labeled RED/YEL or your supply will not work. The red/yellow wire is the center-tap for your high voltage winding and is required with this type of dual rectifier circuit.


TURNING RECTIFIED AC INTO DC
When you send AC current through a diode one half of the sine wave current gets blocked by the diode and hence 'removed' from the total circuit current flow. This process is called rectification.

Although a DC voltage is created with rectified AC, you still need to 'filter' the rectified AC to get it smoother and closer to what a high voltage battery would typically supply. If you try to power your Pultec without filters, the result is a whole bunch of "HUM" in your circuit. For older Pultec EQ's, excess hum could be a good indicator that the electrolytic capacitors have dried up.


6X4 DUAL RECTIFIER
The Pultec EQP-1A uses a 6X4 miniature 7-pin dual rectifier tube connected as a full-wave-center-tap rectifier. 'Dual' means there are two rectifier diodes inside the tube. In order for any full-wave-center-tap rectifier circuit to work, tube or solid state, you must connect the 'center-tap' of the AC winding to your circuit ground. The high voltage winding center tap is the RED/YEL wire on the Hammond 270FX. Note: Your transformer might be different. Always check before connecting!

6X4 pin-out
Here are the pin assignments for the 6X4 tube.
 
FULL WAVE RECTIFIER CENTER-TAP CONNECTION
Below is a schematic showing how the high voltage AC is connected to the 6X4 tube. Note: center-tap RED/YEL connected to signal ground.
RC FILTERS
The rectified AC, from the 6X4 cathode (pin-7), is filtered by 4 simple RC filters made up of (R7-C2), (R6-C1), (R1-C?) and R3-C?). The question marks are because those capacitors are located on the make-up-gain-stage PCB and I don't have it completed at the moment. Also, note the wattage rating of resistors R1, R3, R6 and R7 in this build.

The original design used 40uF electrolytics rated at 450VDC. Although you can still purchase 40uF capacitors, they tend to be more expensive than the more common 47uF value. I went with 47uF for cost and availability purposes.

6X4 HEATERS
The 6X4 heaters operate on 6.3VAC or DC with a current draw of 600mA. However, for the Pultec EQP-1A you must use AC if you are going by the original design schematic. I use solder pads rather than PCB traces because I like the convenience of printed circuit board assembly, but the connection quality and flexibility of wire.

 HEATER CONNECTIONS

Green heater wires connect to (J9) and (J10) labeled GRN. Wire is used to connect all tube heaters. The green/yellow transformer wire (heater winding center-tap) connects to a 60VDC node created by a simple voltage divider that feeds from the high voltage DC output. The purpose of the 60V bias is to help reduce the voltage stress from heater to cathode. The 6X4 heater-cathode maximum voltage is 450V.




POWER INDICATOR
The power indicator lamp is powered by the 5VAC transformer secondary winding (yellow wires). You connect the yellow wires to (J18) and (J19) labeled YEL and your lamp to (J20) and (J21).

GROUNDS
There are plenty of ground connections on the power supply PCB. The RED/YEL high voltage center-tap wire can connect to any of them. However, J22 is centered in the middle on the PCB.


That's about it for now. I'll follow up once I have the real PCB's in my hands.
Thanks for reading,
Trenton

5/4/18

PULTEC EQP-1A MAKE-UP GAIN STAGE SCHEMATIC

My Pultec power supply PCB layout is nearing completion. Here's a look at my schematic for the makeup gain stage. This PCB will be able to accommodate either the 9600T or the S217D output transformers by Cinemag.


I am still in the process of making the component footprints for all of the parts. I will keep you updated.

Thanks for reading,
Trenton



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