© COPYRIGHT NOTICE All Rights Reserved

(Last updated: 05-Apr-2018 0:14 )


We often gets questions like how to build a GM70 or 2A3 amplifier, or what is the best sounding 300B. Well, most of the sonic results depends using the right tubes, the right way, and not save on transformer quality. .

The thing is, most users do not know where to start. So they search the internet for information, and usually ends up in the forums, getting stuck in confusing discussions by people who pretend to know a lot about GM70 tubes, or 845 tubes, and in real life never seen one. What you want to know is probably this:

  1. What is a good circuit. (Good sound, and problem-free ciruit)
  2. What components are used, and where can I get them
  3. Who will help you when there is a technical problem. (free phone support, Email, etc)

We started this page just new, and the intention is to offer some circuits here that are tested, and guaranteed to work and build without problems, and we offer you free technical support when building them.

If you are interested in building those kits we have two "goodies" to offer:

  1. You get 10% discount on the total order. Just be aware we have no "ready packed" box as a kit, but you need to select all components yourself, based on the schematics. So you can choose low cost Chinese tube sockets, or the better Yamamoto. Choose silver-oil capacitors, or the lower cost types. Etc. When we prepare the quote, please ask this 10% discount, and mention which of the schematics you are building.
  2. When you have a picture of the ready build amplifier, and we choose to place it here, we reward you with a 100 Euro free order from the jacmusic website.

Schematics by Dipl. Ing. Leven

  • 300B-XLS PSE or SE
  • KT120
  • EML1605

These design aim for practically lowest noise, and lowest distortion, yet not overstressing the tubes.

These are found on 4tubes.com -> Go totube schematics with Lundahl Transformers and then choose: LEVEN.

Parts list

AD1, NF2 Amplifier. K. Anzai, JAPAN. Also used by Hiraga.

The original schematic of this amplifier I received with French text, long ago. The book it was copied from looks to me from the 1980's but there is no reference.

I am not able to find much about Mr. K. Anzai, Japan, other than that he is a circuit designer from Japan. If someone can find some more, please let me know, so I can place it here. Some people report this schematic was also rebuild by Jean Hiraga.

Also the schematic shows great resemblance with the Yamamoto AD1 amplifier, first generation. The resemblance is very large, probably there are some connections.

Interesting, Lundahl has the right transformers in the program, we re-build the AD1 at Emission Labs, and I have a small stock of NOS Telefunken NF2, some in sealed boxes even. NF2 has a top cap, but this is for the grid1. So there is no DC voltage on the top cap. It is advised you use an coaxial cable to connect to the grid 1, and ground the cable shielding. Yamamoto has very nice Teflon plate caps. The NF2 is a glass tube, but it has conductive (bronze) painting from the outside. It has the same socket as AD1, and is from the same period. So all on all this is a very nice combination.

I have not tested the schematic, but is a tested design by others. This should would work problem free, and is easy to build. The Filament voltage of the AD1 must be adjusted to 4V. Note, the EML AD1 draws a more filament current than Telefunken, and some other kind of tubes exist too, which can be used instead of AD1. So use the variable resistors in the filament circuit to get exactly 4V regardless what tube you put in. The transformer has enough AC voltage to use stabilizer ICs, or much better use filament chokes from Lundahl. Anyway, you can do that as you whish. Perhaps try it with AC heating first, as shown here with very little effort, only variable resistor is used. If not totally hum free, add DC heating. (This may be needed above 100dB speakers)

The only change I made, is use the rectifier from the American 5AR4, to the European AZ4, to be all in-line with the European tubes from the 1940's.


  • Input Sensitivity: 0.5V
  • Output Power 4.1 Watt per Channel
  • Output impedance 8 Ohms
  • Frequency range at full power 5...80KHz @ 3dB (with Lundahl transformer)


300B Classical Amplifier short description:

This is a classical stereo 300B amplifier, without savings on a good concept. All transformers are the high quality Lundahl. This amplifier is working fully hum free, and has automatic bias. The only function of the meter is to check good operation of the tubes. Also the mains voltage can be controlled by the meter.


  • Input Sensitivity: 0.7V
  • Output Power 7.1 Watt per Channel
  • Output impedance 8 Ohms
  • Frequency range at full power 5...80KHz @ 3dB (with Lundahl transformer)

Read more about it here

GM70 monoblock circuit diagram

GM70 Dual monoblock amplifier. Parafeed and Sakuma principle.

in preparation. The Japanese Designer SAKUMA, is famous for driving an 845 with an 845, to fully cancel harmonics. We do so here, driving a GM70 with a GM70.


The picture you see here, is by Philippe M. from Switzerland.

Left monoblock
Right monoblock

in preparation
Type "50" amplifier. Sakuma principle.

© COPYRIGHT NOTICE All Rights Reserved

(Last updated: 05-Apr-2018 0:14 )