Stereo Tube Amp Built Into a Vintage Cashbox

This Instructable shows how to build a stereo tube amplifier using a vintage cashbox and the parts from an old tube radio.

This Instructable requires the following:

• familiarity with high voltage circuits is a must have, as the circuit has 300+ Volts in it, and these voltages can kill.
• a donor stereo tube amplifier. Most of my donor amps were bought at hamfests, estate sales and from online classified websites. The donor amplifier provides the power and output transformers needed, and can also provide many of the passive components if you choose to use the original circuit.
• a circuit to implement if you choose something other than the original circuit.
• a cashbox to serve as the chassis for the amp.
• soldering iron and solder.
• electric drill, step bits and drill bits.
• a Dremel or similar tool to cut square holes in chassis.
• all passive components required for the circuit chosen. This includes all resistors, capacitors and inductors needed.
• RCA inputs, speaker outputs, a volume potentiometer and volume knob.
• a power inlet with integrated fuse and a power switch.

Remove the two output transformers and the power transformer from the donor chassis.

Mark your chassis for tube sockets, power transformer hole, power inlet hole, RCA inputs, speaker outputs, output transformer mounting screws and power switch. Make all required holes and install all components on the chassis. The amp now looks complete...

Install your chosen circuit into the chassis. I chose a different circuit from the one originally in the amp. The original amp used a pair of ECL86 tubes which have a triode and a pentode in one tube. The new amp uses a 6N1P-EV, 6922 or 6DJ8 preamp tube and 6P15P-EV or EL84 output tube on each channel.

Make sure that when the box is closed, there is no shorting of components due to the two halves coming together!

Test the amplifier using a dim bulb tester. A dim bulb tester is a must have for all electronics hobbyists. If you don't have one, look up the plans for one online and build one. In the event of a short, the bulb gets bright and saves the circuit from damage. The test showed no shorts, so I plugged the amp directly into a regular power outlet and took some measurements. The measured voltages were very close to those predicted by my PSUD II simulations. Please see the attached schematic for details. I plugged the amp into my living room system, and it sounds very good.