PA66-25 - PCB + Components + OT-25


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The London Power PA66-25 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 can use whatever tube types they wish to fit.

Extensive notes illustrate many options that the PA66-25 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. 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.

PA66-25 uses London Power's custom toroidal 25W output transformer that provides full power bandwidth over the audio range, making it suitable for hifi and musical instruments. Full power bandwidth assures consistent tone when used as a Power Scaled guitar amp dialled from full output down to milliwatts.

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 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, add another pair of output tubes with PX66 - Power Expander. You don't need four octals to achieve 25W, but you can have "100W of tone".

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

OT size: 4"D x 1.5"H (102mm x 38mm)

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, 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 Bias: -60Vdc - 3mA

Recommended Power Supply: PSU-25 mono, PSU-50 stereo

Kit requires assembly. If you would like to receive your PA66-10 with the PCB pre-assembled, add PA-BLT for each power amp requiring assembly. The assembled board is bulkier than the loose parts, so shipping will be slightly higher.

Tubes are not included.

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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