The Infamous Tube Amp

I read a lot of article from the web forum regarding to 6AS7′s tubes. This is really an infamous tubes with a lot of negative comments likes non-linear, very low mu, hard to drive, unmatched dual triode and high heater current. But comments is just comment, How it sound? We might not know if you haven’t listen to it. Since infamous thing do not cost much why not try and build one.

Design Reference

I use the well known Liliput 6AS7 SE schematic as my design reference. It use one E88CC and one 6AS7. High Tension is only 200v and the OPT is 1K:8 ohm. Beside this, there are some other good design on 6AS7 SE in para-feed topology. Since para-feed design need additional two more choke, I don’t think it can fit into my small wooden box.

Voltage Doubler

This is the first time I use voltage doubler in my PSU design. Although it work but the current draw is doubled, so it increase the heat. All 5W resistor (in power supply) temperature increase to 70 – 75 degree within 5min.  In my prototype, I parallel two 10watt resistor and run it for four hour. What happen is, I successfully come out with a new electrical BBQ stove. It is hot enough to burn anything next to it, so don’t mess with the Zohan.

Magnetic Coupling

In wood chassis, you my not get the advantage of chassis grounding. Although RF noise can be solve by adding some metal plate under the chassis but it is still hard to shield the magnetic flux. A better care is needed here specially the layout and orientation of the transformer.

Output Transformer

I wound a 1K:7.3 ohm output transformer to match my fullrange loudspeaker. In my prototype, it sound a bit thin and sharp. My speaker is around 93dB which I think it is not the best match for Liliput amp, you may need something higher like Horn Loudspeaker.

Redesign

After first trial, I found that the performance is not so promising. It was time for a change. To  cold down the volcano, I replace the voltage doubler with a bridge rectifier, the high tension have been reduce from 300v to 160v but it still unable to reduce the heat. Later I get the answer from the Maurits 6AS7 power supply schematic. The current is draw from the first cap so it won’t pass through others resistor. This solve the problem pretty well. I replace the E88CC with a ECC83 to get more gain and voltage swing. The 6AS7 cathode bypass cap is reduce to 47uf. The value reduce is just base on my listening test. Increase it higher will give you a large booming base, and decrease it will lower the bass.

Schematic

This tube amplifier build with ShuGuang 6N5P drive by Tung-sol 12AX7. This fat lazy cat only bring me 2W each channel but it do “sound good”.

Sound Good

The high tension is only 160v, all cap is rated 200v instead of 400v. Only 2 tube are needed to build a stereo amp, so we save one tube. You only need a power transformer with 120v in secondary and a 2W rated OPT which is easily available. Finally, the cost is down but the quality is maintained. It do sound good to me, how about you?

This Amp perform very well with my Pawo Loudspeaker. Vocally nice and natural, stereo imaging good and accurate. Mid bass tight. Lower bass too soft,……. What you want to do? I want to c-u-t and s-t-y-l-e  h-a-i-r.

The Infamous 6AS7 Single Ended Tube Amplifier

Fantastic..!, she got the look, she can sing and she is adorable.

That is all for now and thank you for reading.

Sore Ja~matane,
(See you next time)