Magnasync Moviola URS Guitar Amp Conversion (2015)
The Magnasync/Moviola URS is a 5W 'combo' amp, which was originally used for film editing. The circuit shows similarities to that of the Fender Tweed Champ 5E1. A friend had recently purchased one and asked me to service it to work as a guitar amplifier. This project allowed me to find a solution using the knowledge I have gained while studying vacuum tube theory.
In order to make this transition I replaced the capacitors, reinforced the ground, and added a 1/4" jack with adequate input impedance. Old capacitors are prone to leakage and leakage causes undesirable audio hum, since the amp was first introduced in 1955, replacing the capacitors is a simple fix for this problem, it also increase the attack response of the amp.
Another big cause of hum is the grounding circuit. The original ground layout was done in a way to disperse the relative ground across the entire chassis, which helps to decrease hum. However, looking at the schematic, the heater filaments for all the tubes were connected in not the best manner. The power transformer supplies 6.3VAC for the heater filaments, but the 6.3VAC secondary winding is not center-tapped. Rather than creating an artificial ground, one side was grounded. This induced hum into the DC ground via the AC heater supply. I lifted the grounded side and added a 220 ohm resistor on each lead connecting the opposite side of the resistors to ground, creating an artificial ground for the heater supply.
The final adjusted was to the input impedance, adding a 1M ohm resistor made the input impedance sufficiently high enough to avoid any signal loss. The 33k resistor is called the grid-stopper. This resistor is important because it acts as a RC low-pass filter with the input capacitance of the tube and stops any radio frequency interference. From the original circuit, I disconnected the external lamp from the opposite side of the 1ohm 1W resistor (R13) and fit the 1/4" input into its place. I also disconnected from the power supply the 470k ohm resistor (R4) and all components that were connected to the 4-pin input jack.
Magnasync/Moviola was a manufacturer of film projection and editing equipment based out of North Hollywood, CA. To aid in the editing process, they also made a few audio products which supported the larger upright and flatbed Moviola film-editing machines. One of these was the URS 5-watt tube amplifier with built-in speaker, which was debuted around 1955.
This small tube amp was originally intended to allow personal to hear the movie soundtrack while editing 24 FPS movie film. It came with a volume / power switch, built in 4" speaker, 1/4" external speaker out, and 4-pin optical / magnetic input. The circuit operates under a three stage amplification, with a single ended output stage, and tube rectification. There are three vacuum tubes: one 6X4 twin diode, one 6AQ5A pentode, and one 12AX7 twin triode. The 6X4 operates as the rectifier tube after the power transformer. One half of the 12AX7 is the input amplifier which boosts the input signal and adjusts the output impedance to drive the volume control.
The second half of the 12AX7 is the second amplifier stage and output stage driver. This acts as a suitable load for the volume control while also conditioning the signal for the output stage. Last, the 6AQ5A is part of the output stage. The 6AQ5A converts the signal from low-current and high-voltage to high-current and high-voltage. It then feeds it directly into the output transformer which converts the signal into high-current and low-voltage for the speaker.
Fender Twin Reverb Clone (2010)
I decided that I wanted to build a Fender Twin Reverb, so I order a kit from Mojotone.com. This amp is what I primarily use now and building this amp taught me a lot and deepened my desire to understand how to design tube amplifiers. Through the building process, I was excited, but overwhelmed. The kit only included the components, a schematic, and a component layout diagram; I was expecting some sort of step-by-step instructions. I spent many hours examining the schematic and see how it related to the layout.
My education on how schematics looked as actual circuits was very minimal due to only being a first year engineering student, and this project serves as a challenging introduction. There were a few questions about how components actually went, and kinks to work out, but I feel this project was one of the first that jump started my personal projects. I have continued my research in understanding how to design tube circuits and I am currently working on my own designs.
My education on how schematics looked as actual circuits was very minimal due to only being a first year engineering student, and this project serves as a challenging introduction. There were a few questions about how components actually went, and kinks to work out, but I feel this project was one of the first that jump started my personal projects. I have continued my research in understanding how to design tube circuits and I am currently working on my own designs.