I have always loved tubes. My first amplifiers were a couple of home brewed things built after the schematics in Electronics November 1953 issue and was called Linear standard. They used a 12AX7A, a couple of 6AU6 as drivers and a pair of KT66. I got them from a friend of my father back in the late 60's. They were scrapped after a while, I was too young to realise the value of them. I still have a couple of the KT's and one of the output transformers from them. The KT's are the real stuff and still strong.
Around 1989 I returned to tubes and designed my own tube amplifier. It's still under development but sounds like the better amplifiers in the good old days. Tube selection was easy since I used what I had available at the time, both NOS and used. Used tubes are useful when designing, replacing them with NOS when it's working. For some reason I like low gain triodes like 6J5, 6SN7 and 12AU7/ECC82. The preamplifier is totally feedbackless and the power amplifier has a very low feedback. It's a compromise between dynamic sound and distortion.
Nothing is new under the Sun, all feasible tube circuits has already been invented. My amplifier is not an exception, the configuration has its roots back in the 50's. Single tube input, long tailed pair phase splitter and a ultralinear output with cathode bias.
The power supply is ridicolous. I had a couple of huge 600VA irons with a center tapped 64V secondary in the junk box. I connected the secondaries in series feeding a voltage doubling rectifier connection with a pair of 2200µF capacitors. Gives 350V at God knows how many amps. To avoid surge currents I added a couple of 2.5 ohms NTC resistors between the irons and the rectifier. Believe me, it's only a temporary solution... and have been for 8 years.
The preamplifier is powered from an Oltronix 500V regulated adjustable lab power supply and is also a temporary solution. Gee, seven EL34's in operation for 2x22W...
Power output is around 22W into 8 ohms. Low feedback gives a dynamic sound but also some harmonic distortion but the low impedance plate load keeps the annoying odd harmonics down. The amp is under modification, it may motorboat with a less stiff power supply than the monster mentioned above.
During the modification I listen to my pair of Quad II which I have restored. Sounds good but a small signal pentode is a small signal pentode and thus have its less pleasant sound signature. Still much better than silicon thoo...
I have also built a small amplifier using 3 6DJ8 and 4 6BQ5. The output irons are small, too small, but with very interesting properties. I bought these back in '89, they are unmarked but reliable sources told me that they are wound by Partridge. The sound is very promising but the cores saturate at the extreme lows. The tubes are cathode biased into pure class A. I'm looking for a quad of matched 6BQ5's, the current set is not very well balanced, probably causing the saturation.
Update Dec 2003:
This page is my oldest alive, unmodified since 1996 until now.
Still using my Quad II’s. I have redrawn the schematics above to make them more readable. The EL34 does not like being fed from a single 350V source so it will get a separate 400V supply for the prestages.
The links below are old and may not work.
Last modified 2003-12-21