“Omega” Single-Ended Amplifier with Reverb

Mike Udelhofen

Phys 498 Spring 2011

5/12/2011 The scope of my semester project was to design and build a high quality, vacuum tube- technology guitar amplifier in order to learn about the operation of guitar amplifiers and tube- based electronics on the whole. In doing so, I was able to create an amp custom-built to my specifications for a very reasonable cost. Furthermore, from drafting and implementing the design, specifically ordering all components, and keeping record of every step of the process, I will be able to service the amplifier on my own should it encounter any problems.

My initial desire to build the amp stemmed from previous experience with music equipment, such as building guitar pedals and modifying guitars, and the desire for a great sounding tube amp of my own on a limited budget. I researched several vintage guitar amp schematics among the likes of the Gibson GA-9, Vox AC-15, and several amp kits before settling on the Fender Princeton as a base for my design. The Princeton 5F2-A model, produced between 1955 and 1960 in the “Tweed” section of Fender’s design history, was a low-output, class A amplifier with a single tone and single volume control. It used a singe 12AX7 tube for the preamp section and a single 6V6 tube for the power section, along with a 5Y3 rectifier tube.

It output about 5 Watts of power at the speaker. The schematic is shown below.

Figure 1 - Fender Princeton Amp, circa 1955-1960

For my design, with gratuitous assistance from Professor Errede, I implemented additional tone controls and a reverb tank, causing it to require and additional 12AX7 and 12AT7 tube. We also stepped up the power and output transformers to allow it to deliver up to 25 Watts at the speaker. The more power-demanding operation of my amp caused it to require a 6550 tube, which can handle 25 watts of power, instead of the 6V6 in the power section. In the end, my schematic hardly resembles the original 5F2-A Princeton and includes controls for volume, bass, treble, presence, reverb, and master volume. The initial circuit design for my amp is shown below.

Figure 2 - Preamp Circuit

Figure 3 - Power Circuit The next step of the process was to translate the schematic to a tangible, understandable

layout to simplify the soldering process and placement of components. This included design characteristics such as careful consideration of components that may create or be susceptible to stray magnetic fields, as they can, for instance, greatly change the operation of vacuum tubes.

From here, I had to order the components from several sites across the internet and in town. I made a detailed spreadsheet listing each component, the site selling it, and the price to

come up with an optimal total cost of components while considering shipping charges. I ordered

most components and transformers from Antique Electronic Supply, but had a few from Weber

Speakers, such as a copper cap rectifier to replace the standard 5Y3GT tube. My total estimate

was almost perfectly correct, coming up a few cents short of $434 aside from a few hardware

components from stores in town.

Instead of purchasing a chassis online, I opted to salvage one from an existing amp. I

bought a Univox U65RN, which is tall enough to withstand severe effects of magnetic fields

from the back of the 12” speaker it accommodates. I removed the entire circuit from the chassis

of this amp and had to prepare it for my own layout by drilling new holes for jacks in the back of

the chassis (e.g. reverb in/out, line out) and in the bottom of the chassis for tube sockets and

transformers.

Next, I began the actual build process, which involved mounting all tube sockets,

transformers, jacks, switches, and potentiometers. I had to devise a way to mount my terminal

board, which involved insulating tape underneath the circuit board, all of which was bolted down

by three sets of plastic nuts and bolts. I then soldered all components together. As a rule of

thumb, long leads in the preamp section were to be implemented using shielded coaxial cable to

resist stray magnetic fields skewing their operation. However, these wires were unsuitable for high voltages and thus were not used for any connections in the power section. I implemented a

star grounding technique, soldering each ground connection in a given section together and

grounding them via one wire to the main star ground point, attached to the chassis. The star

ground sections were determined by the frequency at which the section operated at, thus each

section started and ended at a capacitor. The final result is shown below from two different angles.

Figure 4 - Completed amp, top view

Figure 5 - Completed amp, rear-right angle view

Once the soldering was completed, I took measurements with Professor Errede to determine if operation was safe. The voltage measurements taken are shown below.

@ Operate (V) @ Standby (V)

Vac line 120.7 120.7

Vac main secondary 358, 358 360, 360

Vac 6.3Vac 6.44 6.46

VB+ 459 502

VA=VB+ 459

VB=Vchoke 458

VC=V6550 scr=V1K 440 VD=VB+rvb=V4.7K #1 381

VE=V4.7K #2 372

All voltages are in V Grids Cathodes Plates

V1A (12AX7) 0.00 2.038 210.7

V1B (12AX7) 121.5 (G-K tie pt.) 124.0 371

V2A (12AX7) 0.000 1.468 215.8

V2B (12AX7) 0.000 1.996 246.8

V3 (12AT7) 0.063 6.10 373

V4 (6550) 0.000 48.6 457

We also obtained the RMS voltage at the output at full power as 7.70V. With the measurements taken, it was deemed safe to operate. The amp, with new tone circuit, allows for much greater overdrive than the original Fender circuit. I feel I have a much greater understanding for electronics and circuitry as a whole, in addition to their specific correlation with audio equipment. I have learned priceless information regarding the safety of high-voltage circuit design and operation, as well as proper star grounding techniques to keep the circuit noise and 60-cycle hum at an absolute minimum. The building of my tube guitar amp has been not only a valuable learning experience for me, but I also get to come away from the project with a high-quality, all tube-driven amplifier for my use in the endless search for tonal perfection.