PA66-HP - Octal Push-Pull Power Amp PCB + Hard-to-Find Parts


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The London Power PA66-HP Octal Push-Pull tube power amplifier can be used with any of the common octal-based output tubes that use the standard pin-out, including 6CA7, 6L6, 6V6, 5881, 6550, EL-34, KT-66, KT-77, KT-88 and their variants. Each power tube has its own bias adjustment and monitoring jack, so that tubes can be mixed as well. Builders supplies whatever tubes they wish to fit.

Extensive notes illustrate many options that the PA66-HP accommodates, including: triode, pentode, ultralinear modes and/or switching; post-PI master volume; Drive Compensation in one of two places when Power Scaling is used; negative feedback loop; 'presence' control; fixed-bias or cathode biasing; fixed/cathode bias switching.

Switching between fixed-bias and cathode-bias can be "global" (both tubes) or independent. Independent bias switching allows one tube to be operated with fixed-bias while the other is cathode biased. Manual switches can be used (not supplied) or relays. PA66-CR is a kit of the relays, and its use greatly simplifies the switching. PA66-S1 includes the relays plus one panel switch for global bias switching. PA66-S2 provides the relays plus two panel switches for independent bias switching. The relays mount directly on the PA66 board.

Power outputs from 10W to 100W are possible depending on the tubes, power supply and output transformer used, with 10W to 80W being readily accommodated with common Hammond transformers. PA66-HP is optimised for use with London Power's super-low-noise power supply kits.

Single-ended amplifiers can be constructed using the PA66, with the output tubes wired in parallel and driven from a common drive-line. The wiring provisions allow the tones of each tube to be mixed together in a proportion set by the builder. One can even build a double-power "Champ-like" guitar amp.

The board has "reversible" component mounting positions allowing internal or external tubes. In both mountings, bias pots and meter jacks are accessed in the same fashion as the tubes themselves.

The board is also "splittable," where the front-end circuit and output stage can be separated physically to fit into different-sized spaces or for a different tube layout. Simply snap the board along the V-groove before or after mounting the components.

PCB size: 7.3" x 2.4" (186 x 61mm)

PA66-HP (PCB + "Hard Parts") includes the PCB, kit notes, tube sockets, bias pots and meter jacks. The remaining components, transformers and jacks can be selected by the builder, or they can obtain the recommended devices in the included parts list. Power Requirements: Varies with tube complement and power output: 450V maximum to W18; 650V maximum to W16. Kit requires assembly.


Other PA66 Packages:

PA66-PCB (PCB only) includes the PCB and kit notes. The builder can choose their own component types, transformers and jacks, or obtain the recommended devices listed in the parts list.
Power Requirements: Varies with tube complement and power output: 450V maximum to W18; 650V maximum to W16.

PA66-COMP (PCB + All Passive Components) includes the PCB, kit notes, meter jacks and all components that mount on the board. The builder can select their own transformer and tubes. Power Requirements: Varies with tube complement and power output: 450V maximum to W18; 650V maximum to W16. Kit requires assembly.

PA66-25 (PCB + Components + 25W Tx) includes the PCB, kit notes, all components that mount on the board, meter jacks, TS-1/4" input and output jacks, and a London Power 25W toroidal output transformer. The builder can select their own tubes. This option can be powered from our PSU-25 Power Supply Kit, or two amplifiers can be powered from PSU-50. Our PSU-20L Low-Profile Power Supply Kit for 20W Amp can also support this option to a 20W output, but the octal tubes will not fit into less than 2U vertical space. This version can be used as a hi-fi amp up to 25W. Power Requirements: Plate: 430Vdc at 175mA; Heaters: 12Vct at 600-1800mA depending on tubes used OR 6V at 1200-3600mA. OT diameter: approximately 4" (102mm). Kit requires assembly.


About London Power

London Power was founded in 1990 by Kevin O'Connor, after two decades of research into innovative audio amplification techniques. Kevin is an audio designer, author and speaker, known for his proprietary methodology called Power Scaling, and for his eleven books on audio subjects. Whether you seek amps, kits or information, you'll find useful answers here on not just the "how" but the "why" of audio amplifier design. Please enjoy the information on this site, and don't hesitate to contact us with questions.

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