Vacuum Tubes

Last updated: 2012/02/02

1Ж17Б Amplifier

The 1j17b is a miniature pentode that was used by the Russian military for their communication equipment. I was thinking of using two, along with a step-up DC-DC converter, for 60V HT, to make a small headphone amplifier. My iPod just doesn't have enough kick to drive a pair of big Sennheiser headphones. This tube is unusual in that it uses parallel rods instead of grids as control, screen and suppressor elements.

iPod and miniature vacuum tubes, the perfect combination

The Millett Hybrid Headphone Amplifier, BUF634 SMD version

The 12AE6A triode/double diode

I love the boxes

I recently (July 2010) bought some 12AE6A vacuum tubes on e-bay. Why? To built a Millett Hybrid amp of course. The Millett is a low voltage vacuum tube headphone amp. Gain is provided by two 12AE6A space charge tubes together with a semiconductor output stage to drive a low impedance load. This has the advantage of not needing an expensive output transformer. I almost exclusively listen to music on headphones, I love tubes and I'm nervous of high voltages so the Millett is perfect for me. I also managed to get free samples of the Burr-Brown BUF634 which is used in the output stage. The BUF634 is used to match the very high output impedance of the tubes to the low impedance of a pair of headphones. It has a gain of <1 and is only there to provide current to the load. The BUF634 is R80 each from RS Components! Score! The BUF634's are surface mount so I had to adapt the board in Eagle CAD.

Tweaked layout for SMD BUF634.The large pads for the BUF634 is to help with heat dissipation. They actually don't get hot.

CLICK image to download EAGLE CAD file

Thank you Gregory for double-checking my work!

Millett boards etched by my good friend, Richard Smuts. The rest of the assembly I did myself.

The Super-Simple 12AU7 Hybrid Headphone amp

My improved PCB design with thicker tracks and neater layout. Version 2, 2011

There is also another hybrid tube design I found here. This design uses a single 12AU7 tube. This double triode makes it possible to have a stereo headphone amp with just one tube. Very simple to construct and devoid of high voltages, the 12AU7 (ECC82) / IRF510 Headphone Amp is a perfect beginners project. I redesigned the PCB artwork to make the layout neater and the tracks thicker. I no longer make PCBs for sale. I outsource those to a fabrication house. I only make prototypes myself.

To power the Super-Simple 12AU7 headphone amplifier I recommend you use a linear power supply as opposed to a switched-mode power supply. Experience has shown switched-mode power supplies to introduce too much noise, audible as a pronounced hiss during quiet passages. This circuit will reward a clean, well regulated power supply. I also recommend you use a power supply of 12.6V. This is the rated voltage for the 12AU7 filaments. Any lower and I'm concerned one may not get sufficient electron emission. This amplifier draws ~598mA at 12.62V. The LM317s and IRF610s (in my case) get quite hot. I measured just under 50C with a temperature probe with the heatsinks on.

I'm driving a pair of Sennheiser HD500s with mine. The sound is clear, detailed and punchy but never harsh. Overall the sound is very pleasant and open. It can easily drive my headphones to uncomfortable levels. I have it connected to a Monica2 NOS DAC. To eliminate switch-on pops I'm using a soft-start power supply based on the following from the National Semiconductor LM317 datasheet:

Recently when assembling an amp I had a real problem with a humming/buzzing noise I couldn't trace. Turned out it was RFI from the DECT cordless phone base station on my desk. Moved it away and the amp went quiet.

The Power supply. Use R₂ = 2180 Ω for 12.6 V.

Boards etched by my fabricator and good friend, Richard Smuts

*All photographs are copyright AJ BREDEKAMP