The London Power PA66 Octal Tube Push-Pull 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.
Extensive notes illustrate many options that the PA66 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; Body control; 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. 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 is optimised for use with London Power’s super low-noise power supplies.
The PA66 versions with output transformers use London Power‘s custom toroidal OTs for outstanding fidelity so good they can be used for hifi.
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 is also “splittable” where the front-end circuit and output stage can be separated physically to fit into different sized space or for a different tube layout. Simply snap the board along the V-groove before or after mounting the components.
For even more tonal options, more power, or both, add pairs of output tubes with our PX66 Power Expander, one PX66 per push-pull tube pair.
PCB size: 7.3″ x 2.4″ (186 x 61mm)
PA66-COMP (Standard DIY Kit) – requires assembly and 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: 500V maximum to W18; 650V maximum to W16.
PA66-INT (Built Kit for Internal Tubes) – PCB portion of the PA66 kit is pre-assembled, configured for internal tubes. The assembled boards are bulkier than the loose parts, so shipping is slightly higher.
PA66-EXT (Built Kit for External Tubes) – PCB portion of the PA66 kit is pre-assembled, configured for external tubes. The assembled boards are bulkier than the loose parts, so shipping is slightly higher.
PA66-HP (PCB + “Hard-to-find Parts” Only) 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 the builder can obtain the recommended devices in the parts list. Power Requirements: Varies with tube complement and power output: 500V maximum to W18; 650V maximum to W16. Kit requires assembly.
PA66-PCB (PCB Only) – PA66 printed circuit board only, with notes.
PA66-50 (Standard DIY Kit + 50W 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 50W toroidal output transformer. The builder can select their own tubes. Use London Power’s super quiet PSU-50 to support PA66-50 plus preamps, effects loops, etc. Power Requirements: Plate: 430Vdc at 350mA; Heaters: 12Vct at 600-1800mA depending on tubes used OR 6V at 1200-3600mA. OT diameter: approximately 4.5″ (1115mm). Kit requires assembly.
PA66-50-INT (Built Kit + 50W Tx) – this is the PA66-INT plus the 50W transformer. Transformer fly-leads will not be connected.
PA66-EXT (Built Kit + 50W Tx) – this is the PA66-EXT plus the 50W transformer. Transformer fly-leads will not be connected.
You are welcome to contact London Power to assist you with your choice, at 807-473-0952 or [email protected]