I was recently asked to build a line stage and power amp for someone. The request was for single chassis designs at reasonable cost which would still offer outstanding sound quality. The natural choice for that is a design which does not use any of the overly hyped and expensive tubes, so I proposed a SE 6CB5A amp.
The 6CB5A still is among the most underrated tubes. I chose a similar circuit as already discussded in this single ended amplifier concept article. With some minor changes. Instead of the Lundahl LL1663 or LL1664 I decided to use the LL1682/70ma as output transformer. This transformer has a slightly higher primary impedance and a 5 Ohm secondary. This makes the amplifier usable with a wider range of speakers. It has a better damping factor for good low frequency control at a minor loss in maximum output power. As in the original design the LL1660 interstage transformer is used.
Here is the complete schematic of the new design, showing both channels which share a common power supply:
There are also some changes to the original power supply design. As in almost all my recent power suppies, I opted for a full wave Graetz bridge rectifier scheme. Since everything is supposed to fit into a single chassis, the available space for tube sockets is limited. Therefor the decision was to go for a hybrid bridge. Two 6AX4 TV damper tubes for the upper part of the bridge supplying the B+ and UF1007 low noise silicon diodes for the ground legs. Separate filter chokes provide isolation between the channels to avoid any interaction through the common supply.
As in all my designs, a ground lift switch is used for convenience. This offers the possibility to break ground loops in case the signal ground is connected to earth in several places in the system. Of course all metal parts are always connected to safety earth, no matter if signal ground is lifted or not.
The amp will be constrcted in my 'classic' style, everything mounted on a metal plate which will be inserted into a wooden frame. The metal plate is already manufactured:
The back side of the plate:
The detailed construction will be covered in a second article.
A 6AH4 line stage is the perfect preamp for the SE 6CB5A. Circuit and assembly of the linestage will also be shown in separate posts. Stay tuned!
Best regards
Thomas
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