Tube Amplifier Project Page
 

After restoring a nice old 1938 Sparton Model 1568 console radio, I was so impressed with the quality of the sound that I started looking into building an amplifier using the same 2A3 output tubes that the big console uses.

Sparton 1568
After a lot of web research I found a few schematics that I liked from JE Labs http://members.myactv.net/~je2a3/welcome.htm  and Angela Instruments.

The first one was the 2001 version using a 6SL7 driver and the 2A3 output. Here is that schematic:

The above version only used one stage of amplification ahead of the 2A3, and the next version used both halves of the 6SL7 as a 2 stage preamplifier.

The above circuit shows the power supply as well as both channels of the stereo amplifier. Instead of the 76 tube, I used one half of the 6SL7 for the first preamp stage and the other half of the same 6SL7 for the second stage of the preamp, so the schematic is the same, it just uses one 6SL7 per channel, and no 76 tubes.

There were several modifications of this basic circuit including a SRPP version below.
Amplifier Section:

Amplifier
Power Supply Section

 

  Above is the shunt regulated push pull driver stage with single ended 2A3 output. A similar schematic uses the 6SL7 as a 2 stage driver, I might try it wired both ways.
Here is a longer dissertation of the amplifier theory that I wrote up:
Go To Tube Amp Theory Page
I found a variety of different amplifiers that homebrew builders made.

I liked the looks of this amp with the nice wood frame and black panel.
After deciding on the basic circuit, the SRPP version, I read a bunch of webpages about various amps and the different brands of components to use in the construction. Specifically I did Google searches on the parases: 2A3 schematic, 2A3 amp, 2A3 amplifier, and a few others.  I decided to use good quality parts but to try to keep the cost down as much as possible. It appears to me that the transformers, "iron," are probably the most critical components, followed by the signal capacitors, so I tried to find very good parts for those components but still not go crazy with the cost. Truth be told, I purchased a lot of the parts off of Ebay, such as the tubes, sockets, some teflon coated silver plated wire, and a dozen Russian 0.22 uf paper in oil (PIO) capacitors to use for the signal caps.

Here are some pieces of the lumber that I got at a local hardwood specialty store. I was just going to use something like cherry or walnut, but they had this beautiful piece of wood at the lumber shop that was called Brazilian cherry, or Jatoba wood. It had a beautiful color, nice grain, and was as dense as a rock. While I generally try to make earth friendly purchases and lifestyle choices, I went against my better moral principles and bought the piece of wood, feeling guilty al the way home envisioning it standing in a magnificent tropical rain forest. Anyway, here is what the pieces looked like after cutting them to the rough length. Fortunately my neighbor does a lot of woodworking so I took the pieces down the street and cut the 45 degree angles for the corners on his compound miter saw, and some grooves for the panel to set down in to.

I had to cut out an opening in the back panel for the switch and electrical connections so I went to work with a router and some borrowed bits.

At the same time I was purchasing various components and had the transformers together so I could make a layout on paper. I wish I had done the work myself but I took the template to a local plastics shop and had them cut the large holes and make the panel up from 3/16 inch thick phenolic XX, may be the Garolite XX material. That was chosen for its strength and durability. I will have to drill all the little holes later.

Well that is where I am right now, still putting the frame and panel together, but am eager to get started on the electrical construction.
OK, I have put the wood together for the base of the frame and finished it with spray lacquer. I put the back panel on recessed a little so it was not so deep in the opening from the back. The corners are glued with 2 biscuits in each corner to reinforce the joints, as well as a little wedge of wood glued in for additional reinforcement and to attach a bottom perforated panel.

The box is nice and square and very solid. I drilled pilot holes for all the screws since this wood is so dense. Shown above is the frame and some of the transformers that will be used.


This is a close up of the power transformer. It has two 2.5 V @ 3A windings for the 2A3 tube filaments,  one 5V @ 3A winding for the rectifier filament, one 6.3V @ 2A winding for the driver tube filaments, and a 280V- 0-280V @ 150 mA for the plate voltage. It is a Mars model LM-130D transformer specifically made for this circuit.

Next, I mounted the transformers and tube sockets on the panel and attached it to the wood frame. This is what it looked like before wiring it up. The output transformers are James 6113-HS purchased from Euphonia Audio. The sockets are vintage ceramic sockets.


This is the completed chassis showing the wiring underneath. The green capacitors are the signal caps connecting the drivers to the power tubes. The pots at the top are the volume controls. The pots under the 4 pin sockets are the hum balance pots. I fired it up and checked the voltages and all the voltages were right on the money according to the schematic, but there was a lot of hum especially when I touched the volume controls. I then connected a grounding wire to the housings of the voluime control pots and the hum went away. There is just a barely audible trace of hum with the volume controls full open and no signal input. My son provided his iPod for a test run and I hooked it up to a pair of Klipsch RB-35 speakers and the sound was loud clear and beautiful. No further adjustments needed. I also listened with a pair of Fostex single driver speakers I got from Fritz in California, who sells under fritzspeakers on Ebay and it sounded even better with a perfect tonal balance and huge volume.
Here is the finished amp. Not a very good photo so will get another one up soon.

Will add a list of the parts used next update.
  Parts:
Transformers:
Mars LM-130-D power transformer 120 VAC primary, Sec 2.5V @ 3A, 2.5V @  3A, 5V @ 3A, 6.3V @ 2A, 280-0-280V @ 150 mA.
The Mars transformers were purchased from VT4C.com in Hong Kong, and they also have tubes, knobs, wood frames, panels, sockets and just about everything you might need, but the shipping is a bit high to get the parts half way around the world.
James 6113-HS output transformers
Hammond 159Q choke 7 H @ 150 mA DC, 100 ohms DCR, 500 V test

Tubes:
5U4G  rectifier
6SL7  drivers (can also use 6SU7, or 5691 tubes)
2A3  power output tubes

Resistors: 2 watt metal oxide for the power supply and carbon composition for the others

Capacitors: Sprague Atom for the power supply filter caps and Elna for the bypasses, Russian paper in oil .22 uf 500V for the signal caps

Tube Sockets: Ceramic body modern made sockets
Fuse Holder: panel mount screw top holder from Radio Daze
Switch: lighted rocker switch
RCA phono input jacks, gold plated from an internet dealer in Hong Kong
Speaker binding posts, gold plated from Hong Kong internet vendor
Wire: various gauges of stranded silver plated teflon insulated hook up wire 18 GA for filaments, 20 or 22 GA for others
AC power inlet:   generic unit ordered from Radio Daze
Terminal boards: Four post terminal boards from Handmade Electronics
Potentiometers:
    100K audio taper Clarostat A-47  half watt
    50 ohm hum balance potentiometers, 4 watt vintage wirewound units
Knobs:    Machined solid aluiminum knobs
Wood:   Jatoba wood cut to 3/4 inch thick, and about 3.25 inches width also called Brazilian Cherry
Various hardware 6-32 and 8-32 screws, washers, nuts, solder lugs

More photos of other amps I have built recently:
This one just finished in October 2009. It uses a vintage power transformer and vintage single ended Zenith OT-1 audio output transformers. The rectifier is a 5Y3G, the preamp/drivers are 12AX7's and the output tubes are 6V6G's. I found a pair of vintage Zenith 6V6G tall tubes that give the amp a more classic appearance than the short 6V6GT style tubes.

Front view. The power switch is on the right and the RCA input jacks are on the left.


Rear view. The wood is wolid walnut with lacquer finish and the 2 end pieces are held together with the 2 walnut dowels.

    Below are some photos of a second amp that I made. This one used a vintage power transformer and a vintage 12 H choke that I got from Gary Schneider. I did need to get a separate 2.5 V filament transformer for the 2A3's since the old power transformer did not have a 2.5 volt winding. I used a slightly different circuit but same basic components, it was called the 2001 version of the 2A3 stereo transformer from Angela Instruments. This one used Transcendar output transformers and all the "iron" is black. I repainted the vintage transformers with gloss black Rustoleum paint and it looks great. The sound is awesome.



I used a bid JJ 500 uf at 500 V filter capacitor on this one and that plus the 12 H choke makes for zero hum and great volume and super fidelity with the Fostex speakers. The underneath of the chassis looks a little messy, but not too crowded as the whole unit is about 12 x 18 inches overall.

OK, here is one more that I made with trasnformers from Mars products, excellent output transformers and the same power transformer as the first project above.

Top view, flash made some wierd light pattern on the panel but shows the general layout.

Inside view. It started out neat but got into a bit of a jumble as things went on. I have a better idea for the ground strip layout on the next project. I have to use a brass ground bus strip since the chassis panel is nonconductive and it needs a central grounding point.

Guitar Amp Project
This is pretty far from what I usually do, but it is my first attempt to build a vintage looking guitar tube amp for a friend. Fortunately, I managed to salvage a chassis from an old 1960's stereo that had good power and output transformers and had similar tube line up to what I was going to build. I wanted to use a Fender 5E5 Pro-Amp circuit and make it in a cabinet with the old tweed and some custom touches. It is still in the works but here are a couple of photos of the construction.

This is the underside of the chassis after most of the rewiring and component replacement.


This is the cabinet mostly assembled with the tweed Tolex applied and the birds eye mable accent panel on the top.

Finally finished the guitar amp. Here are the photos. It works great with the 12 inch Peavey 6505 guitar speaker.

Front view shows grille cloth and matching color handle.


Rear view shows the speaker and you can see the chassis mounted from the top.

Second Guitar Amp Project

This amp is nearly finished. It uses more or less the same chassis as the above, with push pull 6L6 output tubes, a 5U4G rectifier, 12AY7 input and 12AX7 driver/phase inverter. It has a 16 ohm Eminence 12 inch diameter speaker and will shake the walls. I still have to make some trim parts and the  front grille. The wood is solid mahogany, and is used the same Fender 5E5 Pro Amp schematic.

Finally finished this guitar amp and had a friend come over and play it. Sounds great, awesome volume.


Tha mahogany finish doesn't show up too well in the photos, but it looks like fine furniture.
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