A Tale Of Two Tube Amps
    These projects started in September 2009. I had just finished the 6V6G stereo single ended amp pictured on the tube amp project page and decided to make 2 more amps that would be Christmas presents for special friends and relatives. I had some VT-25 tubes and have always wanted to try building a stereo amp with a class B push-pull cicuit. I know the purists would cringe at using elegant vintage directly heated triodes like the VT-25 / 10Y tubes for a class B amp rather than a single ended amp, but I had 4 of the VT-25's and I love the way they light up. Also the push pull output transformers can be smaller than the single ended ones and I laid out a design with only one transformer mounted on top of the panel and the rest underneath the panel, like my 6V6G stereo amp. Plus, I already had all the necessary iron on the shelf so I didn't have to buy any transformers for the project.

Above is the panel, made of Garolite, about 11 x 14 inches and 3/16 inch thick. I covered it with the blue masking tape for protection and drew the layout on the panel.

Camera broke with the photos of the panel layout, drilling the panel, and cutting and finishing the wood, Murphy's law in action.

These are the end pieces for the 2 amps. The top is for the 6V6 amp and is made of marblewood and the bottom one is made of Brazilian rosewood. Both are very dense but beautiful. I cut a bevel on the outside edges with a 45 degree bit, and cut a groove on the inside to slip the panel in to.

Here are the panels with the holes cut and the tape removed.

Next, I countersunk the screw holes so the screws would sit flush with the top of the panel. I used a hand held countersink deburring tool so I could control the depth of the cut.

These are the transformers for the filaments for the VT-25 tubes. They are actually 8.0 volts center tapped, so I had to put a couple of voltage dropping resistors in the circuit. I used a couple of 0.2 ohm 5 watt power resistors for each channel. The center tap then was connected to the cathode resistor and cathode bypass capacitor.

Here is what the wood end panels looked like after finishing and drilling the dowel holes and slots for the panel.

Detail of the inside face of the wood end panel.

This is the top of the 6V6 five tube amp after mounting the through panel hardware such as the power transformer, tube sockets, power switch, fuse holder, RCA input jacks, pilot light, and volume potentiometers.

Close up of the power transformer. It was flat black and a little rusty before the paint job using Rustoleum hammertome finish dark bronze paint.

Here is the top of the panel of the 5 tube amp that will use 12AU7 for preamp / driver tubes and the 6V6 for output and 5Y3G for rectifier.

These are the output transformers for the 6V6 amp. Just as generic pair of 8 watt vintage radio audio output transformers that are convertible for push-pull or single ended use, not really audiophile quality but sound good for general use.

Just finished the 6V6 amp, Nov, 2009. Tweaked up the voltages a little and put on the finishing touches. Here is what the finished product looked like.

I put some nice handles on it since it was so heavy, and used gold plated binding posts and knobs for the volume controls. The Marblewood looks beautiful and was a real hit with the new owner.

VT-25 Push Pull Stereo Amp

Here is the detail of the 7 tube amp using the 6SN7 drivers and the push pull pair of VT-25 tubes for the output, with 5U4G rectifier.

This was a pair of vintage Olson push pull audio transformers number T-383. They are pretty hefty and were new old stock 15 watt transformers that I got on Ebay a couple of years ago. I had never heard of the company before but I tested them on my breadboard mock up circuit and they sounded great so I used them instead of buying new transformers.

These are the signal coupling capacitors for the VT-25 tube amp. I am planning to use interstage transformers to couple the driver to the output stages, and am going to try using the parallel feed coupling so these vintage high quality caps should do the job. I had used a similar pair of Seal Pups on another amp and they sounded great with no leakage at all.

This is the 8 Hy 125 Ma filter choke for the VT-25 amp.

A monster 500 uf at 500 V JJ capacitor for the VT-25 amp.

These are 2 of the VT-25 tubes I had for the amp.

Finally got this amp done just before Christmas 2009. I made it for my daughter. The VT-25 tubes light up beautifully in a dimly lit room. The sound is quite beautiful too.

Here is the finished amp. The tubes all have brown Bakelite bases. The symmetry was possible since the choke, audio output transformers, and the filament transformers are all mounted under the chassis. I did have to put a few extra screws through the top panel, but I tried to keep the extraneous screw holes to a minimum. There is a pegboard panel on the bottom to conceal and protect the high voltage electronics, with some spaces between the dowels to allow for ventilation. The South American rosewood end panels were finished with about 8 coats of Deft lacquer, rubbed with 4-0 steel wool between coats. There is no stain, just the natural dark color of the wood with the beautiful grain pattern showing through.

This photo shows the VT-25 tubes with the filaments glowing. They are nearly as bright as a little 4 watt night light.

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Email to scottswim@aol.com