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May 5, 2024

6SQ7/EL34 Headphone amp

Hey All,

Long time fan of the YouTube channel, first time poster. Anyway, I started this project a while ago. I finally have it up and running. It's a headphone amp version of the 6SQ7/EL34 amp. I worked with thermionic labs to get some custom output transformers to work with the original design. For the most part I stayed true to the original design but added/changed a few things.

-Lowered the bias of the power tubes to 50mA from 60 mA. The transformers could only be gaped for a max of 60mA, so I changed the cathode resistor from 500 ohm to 600 ohm to avoid running the output transformers at their max.

-The output transformers have the ability to run a couple different output impedance's on the secondary. I used a gold point switch to have the ability to select between a 32 0hm and 150 ohm tap. This makes it great for both low and high impedance headphones.

-Added a triode/UL switch.

-Added a switch to turn the plate to plate feedback on/off. I found triode sounds great with no feedback, while UL sounds better with it engaged.

Thermionic labs actually has the output transformers I used as a stock option now in case anyone has any interested in building a similar amp.

www.thermioniclab.com

Single Ended 3K5:32/75/150/300R 12W UL Headphone Transformers (Pair)

Pair of Single Ended Output Transformer for Headphone amplifiers ,12W ultralinear Designed for EL84-6BQ5-ECL82-ECL83 in triode mode or tubes with similar impedance, built in a massive core for lowish distortion in low frequencies. TECHNICAL FEATURES: Primary Impedance: 3k5 Secondary Impedance: 32/75/150/300 Ω Ultralinear tap:40% Primary Inductance Lp=25H @17Vrms/50hz Airgap set for : 60mA Primary Dc resistance: 223 Ohm Max Dc current :150mA Maximum output power : 12W Frequency Response: 20Hz-49Khz -3dB Core material: high quality annealed M6 silicon steel (19Kilogauss) Lamination thickness :0.35mm Core size: EI75/40 Dimensions :75mmx63mmx83mm Weight:1.45Kgr per transformer Winding Wire: Double coated Grade 2 enameled copper wire Wound with Precision , Multi section winding, for reduced leakage inductance and high dielectric strength. Secondaries are wound in bifilar method to ensure perfect load match and same bandwidth for all loads between 32/150 /300/600Ω. The user can rearrange the secondary taps for different load value following the enclosed connection diagram. Mounting: vertical placement

Big thanks to Steph for making all this info/projects available!