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Why not enjoy your favorite blues through tube amplification? This amp is built with three line inputs plus a preamp input for your mp3 player. The power supply rectifier is solid state with a CLC filter. There is a 6SL7 at the input; half is used for the preamp input, and the other half is used to drive the long-tail pair phase inverter with a constant current source that provides exactly equal out-of-phase signals to drive the push-pull 6L6's to 25 watts per channel. The finals are fixed bias with independent bias settings for each tube. The line input stage of the 6SL7 has a local feedback loop with a switch to provide 3 different levels of feedback, primarily to control the gain of the stage. There is also a global feedback loop with 10dB of feedback. Frequency response is flat from 15 Hz to 35k Hz.

The developement of this amp began as a desire to enjoy the sounds of tube-amplified audio in the home and was not built to audiophile standards with gold-plated connectors, exotic electrolytic capacitors, oil and film coupling caps, a custom power transformer, or expensive output transformers driven by rare and priceless power tubes. Now it is not that I wouldn't have liked all of that, but to be honest I just could not afford all of that. So it was built as most amps are built, with compromises. But regardless, I love the outcome and do not know if I really could have made it sound any better.

I started off by choosing the long-tail pair as my phase inverter, which was inspired by information gathered from the site "Fun With Tubes," which is full of circuit design information and is linked at the bottom of this page. I liked the idea of a phase inverter that guaranteed an equal drive signal to both power tubes, as the long-tail pair does, and the addition of the constant current source to keep them linear. The use of a transistor as a CCS, which did not bother me, is transparent to the audio output. I chose the 12AY7 because I already had two of them. The measured gain is 14x, which means a 3V signal will provide a 42V signal to the grids of the finals to drive them.

The finals are push-pull 6L6 operating in class AB1 with a fixed bias set at -22.5V and drawing 44mA. I chose this setup because it gave me the output power (25 watts) per channel without massive current draw. I went ahead and put in seperate controls for each tube's bias, which was not really necessary, but I did it anyway. The output transformers are 30 watt Hammond transformers at 6600 ohms of impedence to the plates of the 6L6's, which was ideal. The grid leak resistor is 330k, which was a compromise to provide a better load for the phase inverter.

In order to allow feedback, I needed some amplification on the front end. So I added the 6SL7; why the 6SL7 you ask? Just because I wanted an octal in my amp, and I needed the gain it provides; the fact that octals are so linear and lacking in distortion was just coincidental. The gain of this stage can be controlled by a switchable local feedback loop with three different levels of feedback. Only half of the tube is used.

The fourth input was wired to the other stage of the 6SL7 for the added gain that a mp3 player needs. An audiophile would cringe to see the bypass cap in the cathode, but the additional gain was needed, so I succumbed.

As stated earlier, there are 10dB of global feedback to provide linearity at 15 Hz to 35k Hz. There are grid stopper resistors installed to aid in controlling parasitic oscillations, along with a high frequency grounding cap installed across the phase inverter grid leak resistor. The RC coupling circuits utilize 0.33uF coupling caps at one stage and 0.01uF coupling caps at another to control low frequency oscillations. If you are planning to build an amp, be forewarned that these measures are necessary to prevent oscillations.

The power supply is sufficiently filtered to allow less 100mV of ripple, and the heater windings are center tap grounded. For all intensive purposes, there is no hum at the speakers.

How does it sound? Well I am not going to describe it in audiophile terms because I am not one and I do not own a reference recording, so suffice it to say that I LOVE IT and I do hear things that I've never heard before in the recordings that I have.

Inspiration for this project was fueled by the following great sites:

Fun With Tubes     Turner Audio     Audio Tropic

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