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Wednesday, May 17, 2023

The integratred 300B Amplifier


The new integrated 300B amplifier for speakers and headphones is finished and passed all tests.

The amplifier uses a pair of 300B directly heated triodes as output tubes and a 6DN7 per channel for the input and driver section. The latter fits this application well and does a much better job than the ubiquitous 6SN7 which seems to be used in 90% of 300B amps. 

One half of a 6SN7 has a too high plate impedance to properly drive a 300B. Using both halves in parallel works well but will then require another tube to get sufficient gain for an integrated amp. Hence my choice of 6DN7 which has a strong system with low plate resistance which is used as driver and the other medium mu section as gain stage.

This build got three inputs but it can be equipped with any number of inputs. Volume control is a stepped attenuator with discrete resistors. The output can be switched between the speaker connectors on the back and the headphone jacks at the front. The speakers can stay connected when using headphones and are muted in that case. Any impedance headphone can be driven. There is an attenuator switch which lowers the output by about 18dB in case of very sensitive headphones. This way a good volume control range is always given.

Power output is about 6-7 W per channel on 8 Ohm speakers. The output impedance is rather low for a SE amp so that a 6 Ohm speaker could still be used.

The high voltage is tube rectified using a pair of 6AX4 TV Dampers or equivalent. These work into a choke input filter (LC) which is followed by separate LC sections per channel (3 chokes total) for decoupling and absolutely hum free operation. The latter also ensured by clean DC filament supplies. 

The dimensions are :
27cm wide, 40cm deep (plus knobs at the front), 30cm high (to the tip of the tubes)
Weight is 24kg.

The unit does not use any PCBs and is completely hand wired point to point.

Here a few photos from the assembly process. The parts are arranged in several layers in a 3D fashion to achieve the compactness.

Only top quality film caps are used in the high voltage supply. 

The amp is designed to work with any 300B compatible tube (No XLS types with increased filament current).

The standard option is with ELROG ER300B output tubes, two pairs NOS rectifiers and two pairs NOS 6DN7 driver tubes. This package comes at 7150,- Euros export price or 8500,- including VAT within the EU. Plus shipping. The amp can also be ordered without output tubes. In that case the price is 5900,- / 7000,-

Of course the amp can also be ordered with ELROG ER300B-Mo: 7900,- Euros export, 9400,- incl VAT.

Or go for the TM300B as output tube for 9000,- / 10700,-

These prices are valid in the introductory phase and will have to be raised by about 500,- due to the materials cost and labour which this amp requires. Also the free spare pairs rectifier and driver tubes are only included in the introductory phase. 

Since each unit is built to order many customisations are possible. For example no headphone outputs or headphone outputs only with selectable impedance. This concept can also be customised to use other tubes like 45, 46, 2A3, 71A.

Best regards


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