TU-8600S IS LIMITED PRODUCTION MODEL

TU-8600S is a third generation of TU-8600 series, a 300B single-ended tube amp kit released as a limited production model for 2017 and 2018. Receiving many requests from both domestic and oversea markets, we are again reproducing it as a revised version TU-8600S with some modification to the parts layout on the PCB from TU-8600R. And again it is a limited production model.

In 2019, TU-8600 was selected as the Stereophile Recommended Components, the first kit amp picked and reviewed by Sterophile. The new TU-8600S amp has a lot more improvements from its predecessors TU-8600 (2017) and TU-8600R 2018-19).

Some highlights:

• Mr Fujita, the famous amp designer, slightly modified the B+ power supply section, while keeping the same amplification circuit to further enhance the sound quality and the stability of the amp.
• Mr Lundahl from Lundahl Transformers, world-wide know designer and manufacturer of output transformers redesigned the OPTs with the know-how from LL2783C to make LL2785B/60mA for the new TU-8600S.
• The noise level of TU-8600 is very very low... amazing design indeed!
• TU-8600S 300B is not a HiFi amp. HiFi amps will only give you dramatic, high and low sound, missing the nuances and intricacies that TU-8600S can provide for you. Some people says that's the HiEnd...

TU-8600S FEATURES

Power circuit enhancement

A high efficiency, low magnetic flux leakage R-core is used. The rectifier consists of schottky barrier diodes that require shorter recovery time. This results in a reduction in even negligible noise spikes. The windings of the power transformer include individual windings for the filament heater (A-Power), high voltage (B-power), and bias to stabilize the Negative Feedback (NFB) circuitry (C-Power). In addition, all power is stabilized to reduce residual noise as much as possible.

Stable NFB circuit for great audio quality

To make the most of the 300B tubes, the characteristics of various parts and circuits, including OPT, have been improved. The NFB is applied to the cathode of the 300B by the third winding of the OPT as well as to the voltage amplifying stage. Consequently, the overall NFB is minimized as well as the chance for the transient response.

New active automatic bias adjustment system

For the 300B tube bias, the power supply is equipped separately, and a ”New active automatic bias” is used where the amp detects plate current and automatically adjust the grid voltage. This design earns TU-8600S the priviledges of both self-bias and fixed bias. In addition, current detecting circuit is moved from the cathode side to the B-power side resulting in even better sound quality.

Driving 300B tubes to full power

A very orthodox design is used for the voltage amplifying stage to bring out the outstanding characteristics of the famed 300B tubes. Of special note is that 12AU7(ECC82) is used in parallel for the 2nd stage to drive the 300B tubes to full power - resulting in 200Vp-p without clipping.

Tube-friendly design

Warm-up time for directly-heated and indirectly-heated tubes largely differ. In most designs, amp power-up starts heating both the directly heated 300B tubes (fast warm-up) as well as the indirectly-heated 12AU7 and 12AX7 (slower warm-up). In TU-8600S, the 300B tubes are heated after the indirectly-heated tubes have warmed up. A very tube-friendly design that will not overload tubes that have not warmed up fully.

Output level selection for headphones

The headphone volume settings and the color settings of the power LED can be changed by moving the jumper location on the PCB.

Ample space for larger coupling capacitors

Capacitors can have a big effect on sound. TU-8600S use high quality conductive polymer hybrid electrolytic caps and polypropylene film caps. For those who wish to use other higher quality caps, there is ample space provided on the PCB for mounting larger caps.