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FS: "Shiny Eyes" 2W Mini SE 4P1L direct heated tube amplifier

"Shiny Eyes" 2W Mini SE 4P1L direct heated tube amplifier

"Shiny Eyes" Mini SE 4P1L is a compromise between size and output power of a single ended, direct heated tube amplifier. If one wants to design single ended amplifiers using direct heated triodes ( in this case a well praised direct heated pentode in triode mode : the 4P1L) there is no free lunch; either smaller size but some hum and lower quality sound, or bigger and heavier and no hum, better sound. My goal was to make a hum free single ended low power, very compact so you can call it a desktop amplifier. I built it for my nephew who is overseas now and he does not need it anymore. "Shiny Eyes" Mini SE 4P1L in a compact design, measures about 13"x13"x10" (LXWXH) and weights about 32 lbs. It consumes about 85W.
The power output is delivered on either 4, 8 ohms speaker output or 32 ohms headphone output.
Description of the schematics:
• The power supply has 3 transformers and two chokes. The main transformer (Hammond 270EX) provides the high voltage, the filament supply for the tube rectifier and for the driver tubes. The high voltage is full-wave rectified by the 6BY5GA tube followed by a CLCLC configuration of 20uF film-20H-57uF(47uF Mlytic Mundorf + Obbligatto oil 10uF)-20H-47uF (Mlytic Mundorf + Obbligatto oil 10uF). The chokes are Hammond 193Cs. This configuration provides an extremely low ripple. The 5V secondary is used to provide DC to the 76 driver tube ( a voltage doubler with 2x10mF followed by LDO LD1117 adjusted at 6.3V dc). There are two transformers that supplies for the final tubes filaments. These are also Hammond, the 167L12. Each has 12.6V CT secondary used to provide 8V to the Coleman filament regulators. The 650mA output of the Coleman regulators is used to power the filaments of the 4P1L direct heated tubes. The complexity and the quality of the parts used in the power supply is very important for a very good sounding amplifier.
• The main amplifier has two stages ( a configuration that I found to be better sounding to my ears than three or more stages). First stage, which is also the driver, is using the 76 type of tube. The driver is self biased and has a Ale's gen2. gyrator load. It is capacitor coupled to the final stage. The reason I chose a IHT is for complexity reasons. The final stage is using the very good sounding tube. It is a direct heated pentode in triode mode, the 4P1L. It is self biased and loaded with the James JS-6115HSE with the secondaries at 4, 8 and 32 ohms. The measurements mentioned bellow were taken using the 5Kohms primary tap. However, I connected the 4P1Ls anodes to the 7Kohms primary which gives a little less power but better distortions. It is no problem for me to connect them back at 5K if desired.
• This is an integrated amp, so it is provided with volume control, separate for each channel, using Alps RK27 100kohms.
• The tubes used are available to replace at decent low cost ( less than $50 for all of them)
The amplifier measures as follows on 8 ohms output:
• input signal at 0.83 Vrms, output power at 1 W with THD at 1.62%
• input signal at 1 Vrms, output power at 1.47 W with THD at 2.16%
• input signal at 1.08 Vrms, output power at 1.65 W with THD at 2.61%
• input signal at 1.22 Vrms, output power at 2 W with THD at 4.65%
The frequency response is dominated by the FR of the output transformer which is better than 28Hz-35kHz @ -2db.
This amplifier has a Neutrik high quality, locking type, 1/4" headphone jack. This is connected to the 32 ohms output.
The sound of this mini single ended tube amplifier is astounding: impressive tonality, great soundstage, amazing mids, easy to listen to for hours. During the 100 hours of breaking in, I listened to my Alpair 12Ps ( 92dB sensitivity). The sound is extremely good and for such a small power amp it can sound loud enough. Sure enough this amplifier is not for everyone. One needs a full range driver with sensitivity higher than 92dB depending on what SPL level one wishes to listen music at.
Note that, despite the very low ripple of the power supply, there is a low hum when listen to very sensitive headphones ( such as 112dB that I own). However, the sound is rewarding. It is known that is very hard to build hum free headphone amps using direct heated tubes, unless one builds a separate enclosure power supply. In this case the scope of work was defined differently. There is no hum whatsoever in the speakers.
If you like to listen at low levels and care about tube sound, then you will fall in love with this little gem. There is a nice synergy between 4P1L and 76.
There is room for small improvements, such as upgrading the coupling capacitor, upgrading the volume controls.
I am proving the amp with a set of extra tubes except the rectifier.
I am asking $1,300 that includes shipping to UScon.


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How to describe the sound of this amp:
- fine details at all sound levels
- well defined bass at low distortion
- amazing mids ( my son was astonished by the sound of his songs played through this amp. He said :" This is an unbelievable good sound")
- crisp highs, very detailed
- overall the tonality of the 4p1l surpasses, in my opinion, the 300B
I was very well pleased with the sound of James output transformers.
I wish I can hold to this amp myself. However, I need to sell in order to finance other projects.