Although I have little interest in using vacuum tubes for RF amplification I

find tube type HI-FI audio amplifiers quite interesting. Here are some finished

projects. These amps are set up at my office at work. The speakers I listen

to are made by Klipsch they are RB35 large bookshelf speakers. For a source

I would use a CD player or my ipod ( encoded with AAC at 256kbs). This setup

is fine for the classic rock and roll I listen to.

This was my first tube audio project. It is a basic EL84/6BQ5 stereo amplifier

with the ultra-linear output configuration. The preamp and phase splitter tubes are

12AX7 or my favorite the 12AT7, both work fine. The output transformers are

Hammond 1650E. A fellow ham gave me an old Hammond organ

tone amplifier with 6 EL84 Amperex tubes in it for parts. I used the power

transformer, power supply choke, rectifier tubes and output tubes.

The power supply uses two 5U4 rectifier tubes in parallel.

This is overkill but that's the way it was in the organ amp.The amp is good for about

17 watts per channel into 8ohms. My future plans are to convert the output tubes to

the much cheaper 6GK6. I also want to try the 6AN8 for the preamp and phase splitter.

UPDATE-  Rewired the sockets for 6GK6 tubes and they work and sound exactly the

same as EL84s.

 

 

This amplifier came about because I was looking to build a project with tubes that are cheap.

The preamp stage uses a single twin triode a 12AX7 or 12AT7 tube. One triode is used for

the left channel and the other for the right. The phase splitter is also built this way.

By building the input circuits in this manner. I avoided the need for matched pairs of tubes.

Each triode half usually tests close to the other and even if they do not, should have the

same sonic properties. The phase splitter uses a single 12AU7 or similar type low mu twin triode

in a split load phase inverter. The output tubes are the ubiquitous 6AQ5 output pentode. Although

not produced anymore they can be found for about $6 each. In fact the 5AQ5 and the 12AQ5 are

the same tube with different heater voltages and they can be bought for about $2 each. This amp

can run 5,6, or 12 volts on the output tubes. A 3 position switch on the back selcts the voltage.

The rectifier tubes are 6CA4 twin diodes in parallel. There are 3 sets of inputs 2 on the back and 1

on the front. This amp will produce 8 watts per channel into 8 ohms with 250 volts on the output tubes.