VACUUM TUBE HOME AUDIO
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, 1/2
is used for the preamp input and 1/2 used used to drive the longtail 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 final's are fixed bias with independant
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 promarily to control the gain
of the stage and there is a global feedback loop with 10db of feedback. Frequency response
is flat from 15hz to 35khz.
The developement of this amp began as a desire to enjoy the sounds of tube amplified audio
in home and is not built to audiophile standards with gold plated connectors exotic electrolytic
capacitors, oil and film coupling caps, custom power transformer and 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 better.
I started off by choosing the long-tail pair as my phase inverter which was inspired by info 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 gauranteed an equal
drive signal to both power tubes as the long-tail pair does and the addition of the constant current
source keeps them linear. The use of a transistor as a ccs does not bother me and 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, 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, ideal. The grid
leak resistor is 330k, 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 is just coincidental, lol. The gain of this stage can be controled by a
switchable local feedback loop with three different levels of feedback. Only 1/2 of the tube is used.
The fourth input is 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 is 10db of global feedback to provide lineararity at 15hz to 35khz. There are grid stopper
resistors installed to aid in controling parassitic oscillations along with a high frequency grounding cap installed
across the phase inverter grid leak resistor. The rc coupling circuits utilize .33uf coupling caps at one stage
.01uf coupling caps at another to control low frequency oscillations. If you are planning to build an amp be
fore warned 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 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