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(Last updated: 22-Jun-2018 13:07 )


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.

New made AC Meter seen on Ebay for 10€

Mains Balancer + Adaptation from 230-240-250 to 230V.

This here is not a kit, just a good schematic, and all we have for sale for you, is the special Lundahl transformer, at the regular price. All other parts are standard off the shelf material which you may already have even.

Hammond B3 OrganThis example is not about HiFi equipment, but about my 1954 Hammond B3 organ, which own since 1990. However it was my ultimate experience with unbalanced power lines, and it took me 20 years to solve this problem! As it appeared later, the problem was the electric field, and not the magnetic field. The B3 is build entirely with tubes, and has a mechanical tone wheel generator, which supplies in parallel all frequencies needed. By pressing a single key, you connect this key to the appropriate tones. Up to 6 harmonics and 2 sub harmonics are mixed in an adjustable way, and the result is a tone which is remarkably programmable in sound. The difficulty with this method lies in the very low signal coming from the tone wheel generators, in the range of 15 millivolt. The generator works completely passive. It uses rotating wheels of iron, with a wave pattern on the edge. A magnetic needle is positioned near to the edge of the tone wheel, and the result is a changing magnetic field in the gap between the tone wheel and the magnetic needle. All we need to do now, is pick up this field with a coil, and we have a sound signal. So far so good. However this method is suffering very much from external field radiation, simply because the electrical signal is so low. When playing with 10 fingers, you pick up hum from perhaps 50 single tones, so from 50 of those coils all together. This adds up to an audible signal. I can tell you, my search for a good solution was fruitless ever since, and also discussions with members of the German Hammond club didn't get me the tiniest bit further. Then I wondered why the mains transformer is arranged at an angle of 45 degrees to the chassis. That looks so odd. At it's outside has a fat, copper layer around it. This layer acts a a short circuit winding to radiated fields. So this transformer obviously was intended to radiate as little as possible. However, some residue bypassed the steel of the whole amplifier chassis, and also the steel of the tone wheel chassis which is positioned underneath the amplifier chassis. I removed the transformer from the chassis, and connected it with longer wires, so I could lift it out a little bit, and rotate it too. What I found out, didn't come as a real surprise. The Hammond engineers obviously have choosen the best position already, up to the centimeter, and also the best angle to mount it. But I did find out, the radiation field of this transformer was the trouble cause. Now you need to know, the B3 was designed in the USA, and the mains was only 110V there at the time. (So not even 115V). However in Germany, we connect the whole organ with an auto transformer, and we have this unbalanced power line of 230V entering the machine, instead of 110V as intended.

So what can I improve? Coming back to the original problem, the mains outlets as a matter of principle are always unbalanced. So one wire of the mains cable has 230V AC on it, and the other wire has zero volt. My solution is shown in the schematic on the left. I discovered this way to use an isolation transformer, after searching for 20 years, what caused the hum in my Hammond B3. And I can say: This solution works! I think I got rid of 2/3 of the hum signal. There is now an LL1662 in my B3, and it will stay in there. The B3 is large size, and inside is enough space. For HiFi purposes, you need to build an external box, and connect all equipment to this box. The box can even be build from wood, because the Lundahl LL1662 is a very low radiation isolation transformer for HiFi purposes.

The solution:

The explanation is valid for any equipment, not just a Hammond B3. I suppose at some moment, we all have experiences how mains cables can be a nasty source of hum, due to field radiation. At some point, all the search ends, and a residue may remain, not matter what you do or try. In such a case, I recommend to BALANCE the mains voltage directly at the mains outlet. In all cable theory, and practice too of course, we learn that balanced wiring is not susceptive to hum, and also not radiating any signal, because sending or receiving of hum works by the same mechanism. Well, the mains outlet must be called unbalanced wiring, so you have 230Volts signal on one wire, and zero Volts on the other. There are many cases, where an unbalanced input can cause hum, but we talk about 1V signal. With a mains cable, we have 230V unbalanced. That is really very much! Unbalanced and unshielded, and like that, it passes the record player, CD player, signal inputs, the cables are like spaghetti wire laying around everywhere behind your equipment. Before you pay a fortune for a "black mamba" or "anaconda" mains cable, consider pay just a fraction of it, and build this nice box, and I am sure it will help you more than voodoo cable.

So if we say there is "230V" on the mains, there is actually that voltage only on one of the two wires. We call it this line "hot" or "live". The other one is the neutral, and it has zero volts on it, or a few single volts only. Using an isolation transformer with a secondary center tap, we can connect the center tap to the primary "neutral". (Not meaning the electrical ground). So we loose the isolation transformer property, but we get back something else: We get a balanced mains voltage! So no more 230V, but 2x 115V, which are in series, and in anti phase. For the equipment it looks like 230V, but in fact one line is at 115Volt, and the other at 115V with 180° phase shift. Of course, the balancing point must be connected to the "Neutral" and not to the "live" . For this you just need to plug in the primary the right way. As we do not know what is the right way, there is a relay used in such that is becomes active, when you connect the mains wrong. In that case, the relay comes up, and switches off the primary. It gives a "clack" and a red lamp burns. So you can't go wrong. When you connect the mains plug the right way, the relay will be off, the green lamp will burn, and the voltmeter indicates the voltage to your equipment.

Another big problem today, is variations on the primary voltage. The transformer used, gives us elegantly the option to adapt the voltage is steps of 10V. Moreover, the transformer is still giving the filtering it is intended for, as well as it removes the (small) DC components from the mains, the output is floating, given by the way it is connected. All in all, this is a very useful box, to connect your equipment, offering many advantages, by just using the Lundahl LL1662 at the best of it's possibilities. LL1662 is an AIR GAPPED transformer, making this possible.

Please read the text on the schematic as well, for more explanation.

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 -> 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.

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© COPYRIGHT NOTICE All Rights Reserved

(Last updated: 22-Jun-2018 13:07 )