Luxman A3000 Mono Tube Amplifer

A tube-type monaural power amplifier kit that takes full advantage of the development of new materials such as output tubes, driver tubes, and output transformers, as well as circuit technology that takes advantage of these materials.

The newly developed high output triode output tube 8045G is used as the output tube.
This output tube has a plate loss of 45W due to the large plate electrode with high heat dissipation effect using 5-layer laminated metal plates and the electrode structure with heat dissipation fins.

The output transformer uses GX100-3.6, a newly developed transformer developed for 8045G.
The GX100-3.6 uses a special winding method called quadrafilar winding, a multi-layer split structure transformer technology. This quadrafilar winding means that the primary winding of the output transformer is divided into the plate side (first winding) and the cathode side (second winding), and a bootstrap winding (third winding). This is a method in which a minor NF winding (fourth winding) is provided, and the windings of these four systems are wound at the same time.
The appearance is inherited from the OY36 type, but the iron core size has been increased to increase handling capacity and reduce loss as much as possible, making it possible to extract up to 100W even at 35Hz.

A cross-shunt push-pull circuit is adopted as the output circuit.
This output circuit has a configuration in which the load of the upper and lower output tubes is divided into two parts, one on the plate side and one on the cathode side, and these are cross-connected to each other. In this circuit, 6dB negative feedback is applied to the output tube, which reduces distortion and internal resistance, which has a great effect on bare characteristics.
To implement this circuit, an output transformer with a special winding method is required.

The cathode follower direct connection circuit is adopted as the drive stage, and the output impedance is suppressed low to obtain the drive voltage without waveform distortion. Moreover, since 100% negative feedback is applied to this stage, there is little distortion and phase change, and conditions are convenient for direct connection with the output tube.
This drive stage uses the newly developed high-voltage bi-triode drive tube 6240G.

Since the cathode follower direct drive circuit cannot generate gain, a predrive circuit also using 6240G is provided to compensate for the gain. The drive voltage of this circuit is 200Vrms.
In addition, about 5dB minor loop negative feedback is applied to the cathode of this stage from the fourth winding of the output transformer in a balanced manner, but various characteristics after the predrive stage are improved by this.

The first stage of the A3000 employs a differential amplification method using a 12AX7 double triode, which also serves as a phase inverter.
Moreover, the differential amplification was similarly arranged in the second stage directly connected to the plate of the first stage, and the signal whose phase was inverted at the first stage is voltage amplified and sent to the pre-drive stage. In this configuration, the balance of the push-pull operation is automatically corrected, so the distortion characteristics are also very excellent.

The power supply circuit uses a power transformer with good regulation (S-1934), a choke coil with low DC resistance (C-1744), and a large-capacity electrolytic capacitor (220μFx2).
Power is supplied to the output stage from a bridge rectifier circuit to minimize voltage fluctuations.

There was a tube kit TU3000 sold separately.
8045Gx2, 6240Gx2, 12AU7x1, 12AX7x1.

Specifications

https://audio-heritage.jp/LUXMAN/kit/a3000.html