Blog Pages

11/21/18

PULTEC EQ PROTOTYPE

Revision "A" PCB's for 2 out of 3 sections of my Pultec EQP-1A are now completed.  The 3U enclosure is from Collective Cases.com.

6X4 POWER SUPPLY

The power supply PCB was the first section to be built and tested. The DC output from the 6X4 rectifier tube is good and clean. I purchased my 6X4's from Tube Depot.com. I purchased the Hammond 270FX power transformer from Mouser Electronics.


BALANCED AMPLIFIER

The output stage of a Pultec EQP-1A is a "Balanced Amplifier Topology". A balanced amplifier operates with a "balanced input signal" and the signal remains "balanced" throughout the entire amplifier. Theoretically, a "perfectly balanced" Balanced Amplifier can significantly reduce Even-Order distortion.


TRANSFORMERS

One of the challenges of building your own Pultec EQ is deciding which transformers you would like to use. Quality transformers are not cheap. So, getting it right the first time is important. For this build, I chose a Sowter 1475e input transformer and a Cinemag CM-S217D output transformer.


PASSIVE EQ STAGE

The passive EQ stage is currently being breadboarded on my bench. I will need to create a PCB layout for it once I am finished testing. Stay tuned.

Thanks for reading,
Trenton

11/11/18

SKRM AUDIO DSP PROTOTYPE

I just recently finished a prototype design for an audio circuit that integrates a SKRM DSP module with analog processing. If you've never messed with DSP audio effects before, the folks at SpinSemiconductor make it pretty easy. They offer small modules with built in DSP effects.
Trenton Blizzard Audio DSP SpinSemiconductor SKRM Module
For my design, the SKRM module will install on the bottom side of this prototype PCB. The header-socket isn't installed in this image. Getting DSP effects into your audio designs is a fairly straight forward process when using these modules. However, coding your own DSP will require a great commitment in time in order to master your own effects.

According to Experimental Noize, the FV-1 IC's are available individually and come pre-programmed with effects. You will need to include external memory if you want to create your own effects. Find out more at Experimental Noize.

SKRM MODULE

Trenton Blizzard SKRM DSP Module
Shown above is 1 of 3 SKRM modules available mounted in my BB-SK breadboard adapter. BB-SK adapters make it really easy to prototype audio circuits with these cool little DSP modules by bringing out all of the double row pins into a single row that you can plug into a standard electronics breadboard. All pin functions are labeled on the PCB for easy hookup.

Thanks for reading,
Trenton

11/3/18

How To Design A 12AU7 Gain Stage Part-2

Part 2 of my "How To Design A 12AU7 Gain Stage" video series on YouTube.
In this video I demonstrate:
  • Breadboard Assembly
  • DC Plate Voltage
  • AC Voltage Gain
  • Frequency Sweep



Thanks for watching,
Trenton

10/19/18

PULTEC PCB LAYOUT COMPLETE

I finally finished the PCB layout for the power supply and the make-up gain preamp circuits.  For Revision "A" I am soldering the wires directly to the PCB. Revision "B" will use wire-board connectors.

Pultec Power Supply PCB


Trenton Blizzard Pultec EQP-1A clone Power Supply PCB

Pultec Preamp PCB 

Just like the power supply, the Revision "A" Preamp PCB has wires soldered directly to the PCB. Revision "B" will incorporate wire-board connectors as well.


More to come as the boards get assembled and tested.
Thanks for reading,
Trenton


9/16/18

12AU7 Gain Stage - Part 1

In this video I demonstrate my "7-Step" process to designing a 12AU7 gain stage.



Thanks for watching,
Trenton

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