INTRODUCTION

Last time I touched vacuum tubes was around 1967 when I was repairing radios and TV. I remember this old radio HIFI that was stripped from a console back in the early 60s There was a 12 inch speaker and a smaller speaker I think mid range and a small sealed back cone tweeter. There was a huge power transformer on the chases and a bunch of tubes. I do not remember what the output tubes were but I do remember them being in a push pull configuration. The sound was amazing and through out the years that followed I was never able to match the sound quality with all of the solid state amps I have built or heard. Recently with the hype on the vacuum tube single ended amp hoopla I started getting the bug and turned to the Internet for research.

The transistor brought a new theory in the amplifier and speaker industries. The current fad in solid state amps was to through a lot of current at a speaker in a small sealed cabinet. I will not debate with anyone about lots or little bit of power and how they sound.

As an attempt I wanted to build a tube power amp to see if I could find that good sound I heard when I was young. The following document contains the tube amp I chose as my first tube amp. Please note that I have never liked single ended power supplies for audio amps so it was not used.

DISCLAIMER
While this document describes a plan to build an electronic device which has about 360 volts DC under the chases caution must be followed. I will not be held responsible for any injuries incurred from some one following this document.

MSH MODEL-1 POWERAMP (Michael S. Holden) make no representations about the suitability of this information for any purpose. It is provided "as is"
MSH MODEL-1 POWER AMP (Michael S. Holden) disclaims all warranties with regard to this information, including all implied warranties of merchantability and fitness, in no event shall MSH MODEL-1 POWERAMP (Michael S. Holden) be liable or any special, indirect or consequential damages or any damages whatsoever resulting from loss of use, data or profits, whether in an action of contract, negligence or other tortuous action, arising out of or in connection with the use or performance of this information.

This information may include technical inaccuracies or typographical errors.

MSH MODEL-1 POWERAMP (Michael S. Holden) may make improvements and/or changes in the information at any time.

Step 1: Circuit Design History

I did not design the power amp circuit. However I did design the power supply.

The origin of the power amplifier design came from the schematic below, which came from the 1959 RCA RECEIVING TUBE MANUAL Tech Series RC-19

Step 2: Power Supply Schematic

The filter capacitors were sized to keep the amp supplied with power for 10 seconds after power-down while running at full output. This will assure me that there is enough reserve energy to supply any demand the input may supply. While this schematic is separate from the amp schematic both the amp and power supply are in a single chases.

Step 3: Power Amp Schematic

The amplifier schematic has been redrawn to reflect my preferences. The output transformer is a hamondP-T160 at 10W instead of the 25 watt transformer in the ordinal schematic. The components in this circuit will never produce 25 watts and is much more expensive than the 10 watt output transformer. If you decide to replace the 6V6 tubes with 6L6 tubes this transformer as well all other circuit components would need to be reevaluated.

Step 4: Parts List

All components are bran new and purchased from Tubes and More from the internet. https://secure.tubesandmore.com/ The parts listed uses their part number and 2009 prices. You may purchase the parts from anywhere you wish.

Step 5: Drilling Template

This drilling template is in the PDF file and was set up on an 11*17 page size which can be printed and will then give you a full sized template to mark the aluminum before drilling holes.

Step 6: Top Side Component Layout

Step 7: Bottom Side Component Layout

Step 8: Chassis Wood Sides & Paint

The chassis sides are constructed of 7/16 thick solid oak. The over all dimensions are: 12 1/4 by 8 1/4 by 2 1/2. The top of the oak box is rabbit out to fit the 8" by 12" aluminum plate. The corners are 1/4" box joint fit to make a strong and attractive joint. Four coats of hand rubbed Polly Urethane protect the wood sides of the chassis.

The chassis top is a piece of 20 gauge aluminum COVER PLATE, ALUMINUM, 12" x 8", HAMMOND P-H1434-22 To control corrosion the aluminum chassis top is painted with white dry powder paint and is backed at 400 D. F. This method of painting provides a stronger surface which is more scratch resistant than normal paint.

To give the project little flair all three transformers were dissembled and red power paint was placed on the outer skins.

This dry powder paint process is quick and more durable than normal spray paint. I researched it on the web and gave it a try. I will use it again.

Step 9: Chassis Wiring

The first thing to do is remove the aluminum chassis top from the chassis sides. This makes it much easier to wire. There is a ground loop which runs all around the chassis and is grounded to the chassis. Lay the components out while keeping in mind the wiring path and mechanical requirements. You will probably want to layout the components in some other format other than mine. I have shown my layout to give some ideas. I tried to balance the weight of the components and still have reasonable short wire length. The wire used was 22, 20, gage solid copper hookup wire with insulation of 600vdc.

Solder lugs were placed at different locations on the wire side of the chassis. A drill was used to start the tube sockets holes and finished of with a step drill. The hole sizes are listed on the layout sheet in the Chassis Construction chapter.

Rubber grummets �e uses any time a wire pass through the top side to the bottom side of aluminum top.

Step 10: Testing & Evaluating

The frequency response chart was created using a small VB program I created. If any one wants a copy please email me at MS.Holden@comcast.net and I will email you a copy of the program. The amp seams to suffer performance below 30hz. This depends on the level of the input voltage so I picked an arbitrary level.

Step 11: Lower End Frequency Response

The top graph below shown at 20hZ shows a great deal of distortion and crossover issues. By bumping it up to 30hZ most all of the problems went away. I would want to start the lower end frequency response at 30hZ. Wonder what is wrong?

Step 12: Tooles Used

In the picture below are some of the main tools used in this project. Also these tools were used: Soldering iron, Multi meter, power drill, wire strippers, diagonal cutter, assorted screwdrivers, nut drivers, and others.

Step 13: In Conclusion

I am sure that there are better tube power amps out there. This one works fine for me. A 10W tube amp with a high SPL speaker of 90 dB or better is a good combination and delivers the sound that I heard when I was a boy.

Below is the mono HIFI preamplifier I built to go with the Model-1 Power Amp.