Hi!
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 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
Thomas
Absolutely gorgeous Thomas, congrats!
ReplyDeleteDear Thomas, I was working on similar design (employing 6336 instead 300b, but rest is, I guess, similar idea). I made LTSPICE model for 6DN7 and 6EM7. To my surprise looks like 6EM7 performed even better. Could you please comment on this?
ReplyDeleteHi! There are many ways to implement an amplifier. Choices of tubes depend on the specific goals you want to achieve. If the 6EM7 works well for you, use it
DeleteWell, I was counting on less high level comment. Discussing pros and cons, etc. For my learning and better understanding, to enrich knowledge
DeleteI am sorry if my reply was not what you expected. For a sensible exchange it requires much more than just throwing some tube types without context. Don't expect to enrich your knowledge this way. Best is to actually build, measure and listen, then build measure and listen again to something else. This is 1000 times more educational than a superfluous exchange in the comment section of a blog or doing some simulations.
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