6CY7 Headphone Amplifier

 

Doing a quick and dirty breadboard did not afford the luxury of adding a 300V output to the breadboard power supply so that was taken to the existing 200V. As a result, distortion was a bit higher than predicted with a full 300V.

Component values were dictated by what was at hand and while the breadboard did not exhibit motor boating a complete re-evaluation would be needed for a proper implementation.

 

Power switch not shown and filaments were run in series off the switched 12VAC wallwart power with the "center" grounded. The STP2NK60Z circuit is a cap multiplier and is set for a roughly 10V drop to block ripple and sag.

Note that when running a transformer backwards, as if the case with the 12.6VAC-240VAC unit, the output is lower than the winding rating by twice the "regulation" specification and it will be further limited by the wallwart DCR.

 

It may be sloppy but it works.

The snap track came from junked equipment, as did the tube breadboard PCB. Originally a field termination strip the 'middle' components were removed, traces ground off, and tube wafer sockets added. The socket pins were then wired to the existing terminal blocks along the board edge. I made a couple of 'two tube' boards, for push pull circuits, and some 'single tube' boards that sport additional terminal strips down the middle on either side of the single socket.

The power supply is more 'breadboard stock', as is the separate small cap multiplier board.

This is the smaller board and the other has more snap track space, with a larger power supply (300V, 100VA), for bigger projects.

 

 

Bipolar Supply Version

A 100V offset on the output requires relatively high voltage coupling capacitors and a bipolar supply version would allow lower ratings at the expense of a more complex power supply. And balanced bipolar supplies should also reduce hum susceptibility of the output stage.

Note, in both spice simulations a 6EM7 model was used and the increased frequency response in this one is due to an alteration of the that model to reflect the lower inter-electrode capacitances of the 6CY7. The other circuit should have the same response but I left the original numbers in to illustrate the difference between the 6EM7 and 6CY7 in this regard.

The back to back output zeners provide voltage protection for readily available 35V capacitors but note that the capacitors must now be non-polarized. The output tubes will go into positive grid drive before the zener breakdown is reached.

B+ to the output section should be fused to protect the zeners in the event of a tube fault.

In both cases 20mA idle current was chosen to stay comfortably within the ratings of the reversed 20VA transformer and total wallwart power (which includes heaters).

 

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