After my last experience with relatively high power SET amplifier, I started thinking about upping the game and getting into power range comparable with push-pull tube amplifiers. There are two ways get there — either use high power tube like 833 or using tube in A2 class where grid gets positive in relation with cathode.
Tubes like 833 require even higher voltages than 845 or 211 and very high output impedance requires extraordinary effort to make output transformer compatible with it and still have enough bandwidth for full range audio. That will result in very expensive design, with a lot of full custom components. Example of that approach is WAVAC 833. But for the $350K price results ( https://www.stereophile.com/content/wavac-sh-833-monoblock-power-amplifier-measurements ) are not very impressive.
A2 class though can allow lower plate voltages with higher current, thus reducing requirements to power supply and transformer design. Load impedance 4-6K ohms is very much achievable without extraordinary effort. With output tube operating with grid current you need to have low impedance driver circuit. Easiest way to achieve that is using cathode follower with direct connection to grid of output tube. It is possible to use 845 or 211 tube that way, but there are also other tubes which were design to have grid current almost at all signal levels. One of examples is 805/838 tube. The only real difference between them is that original 805 had a plate cup, and 838 did not. Today China makes 838 equivalent, which for some reason they call 805. Thus when you buy a new production 805 tube, you are getting 838. Of cause neither of these tubes is really affordable as NOS — they almost all gone and were not manufactured outside of China for decades.
I didn’t want to do build from scratch and just like with 845 based amplifier I referenced above, I decided to get a modern Chinese amplifier and start from there. Line Magnetics is a mid-range Chinese brand which builds and markets tube amplifiers all over the world. They have amplifier based on 805 tube which is claimed to reach 48W of output power. Price (if you buy direct from China) is in relatively affordable range, even when one account for international shipping charges. And finally I made a move and placed on line order.
After 3 weeks of waiting UPS guy knock on my door, and when I opened it, large cartoon box was sitting next to it. The box has some signs of tear, which is explainable (it“s path was from mainland China to Honk Kong, to Japan, to Chicago and finally to Dallas). But when I opened it — everything inside was in pristine shape. Tripple boxing and attention to packing details did the job.
One note — when you buy direct from China, you likely getting the version that requires 220 volts power line. Some devices can be modified to use 120V, some are not. From what I read about Line Magnetics gear, I thought that I will be able to easily convert it with s bit of soldering around power transformer. But I also had a boost transformer, which I purchased many years ago, but never put in service. Thus I was ready to use amplifier out of the box too. As alternative, you can arrange 240V outlet in you music room — many audiophiles have it already. If you do not, getting step up transformer is much cheaper than hiring electrician. In the end I found that my version of amplifier (made some time in late 2019) does not allows easy voltage change, and I kept it running with boost transformer. But to use in my high-endish music room I bout another transformer, which simply looks more in place (UMI Step UP 110 to 220 Voltage Converter found on Amazon). It is rated for 1200VA which is well enough for amplifier that consumes 500W.
Line Magnetic LM-508ia has excellent fit and finish. Designers had clear attention to details and manufacturing quality was excellent. All chassis is covered with mirror like enamel probably ½ mm thick. Connectors for inputs and speaker outputs are of high quality. Amplifier came with pair of Line Magnetic branded Chinese 805 tubes, pair of also LM branded 300B tubes (used as drivers), pair of new production Electro-Harmonic 6SN7made in Russia, and a Chinese 6N9P (equivalent of 6SL7) tube of unknown brand. Later we will see that this amplifier works best with tubes that came with it from the factory.
LM-508ia has two adjustments — output tube current and hum balancing. When I turned it on, both of these adjustments were spot on. It only can be explained if each unit is tested and adjusted right before it is packed into the box.
>>>To be continued<<<
Tubes like 833 require even higher voltages than 845 or 211 and very high output impedance requires extraordinary effort to make output transformer compatible with it and still have enough bandwidth for full range audio. That will result in very expensive design, with a lot of full custom components. Example of that approach is WAVAC 833. But for the $350K price results ( https://www.stereophile.com/content/wavac-sh-833-monoblock-power-amplifier-measurements ) are not very impressive.
A2 class though can allow lower plate voltages with higher current, thus reducing requirements to power supply and transformer design. Load impedance 4-6K ohms is very much achievable without extraordinary effort. With output tube operating with grid current you need to have low impedance driver circuit. Easiest way to achieve that is using cathode follower with direct connection to grid of output tube. It is possible to use 845 or 211 tube that way, but there are also other tubes which were design to have grid current almost at all signal levels. One of examples is 805/838 tube. The only real difference between them is that original 805 had a plate cup, and 838 did not. Today China makes 838 equivalent, which for some reason they call 805. Thus when you buy a new production 805 tube, you are getting 838. Of cause neither of these tubes is really affordable as NOS — they almost all gone and were not manufactured outside of China for decades.
I didn’t want to do build from scratch and just like with 845 based amplifier I referenced above, I decided to get a modern Chinese amplifier and start from there. Line Magnetics is a mid-range Chinese brand which builds and markets tube amplifiers all over the world. They have amplifier based on 805 tube which is claimed to reach 48W of output power. Price (if you buy direct from China) is in relatively affordable range, even when one account for international shipping charges. And finally I made a move and placed on line order.
After 3 weeks of waiting UPS guy knock on my door, and when I opened it, large cartoon box was sitting next to it. The box has some signs of tear, which is explainable (it“s path was from mainland China to Honk Kong, to Japan, to Chicago and finally to Dallas). But when I opened it — everything inside was in pristine shape. Tripple boxing and attention to packing details did the job.
One note — when you buy direct from China, you likely getting the version that requires 220 volts power line. Some devices can be modified to use 120V, some are not. From what I read about Line Magnetics gear, I thought that I will be able to easily convert it with s bit of soldering around power transformer. But I also had a boost transformer, which I purchased many years ago, but never put in service. Thus I was ready to use amplifier out of the box too. As alternative, you can arrange 240V outlet in you music room — many audiophiles have it already. If you do not, getting step up transformer is much cheaper than hiring electrician. In the end I found that my version of amplifier (made some time in late 2019) does not allows easy voltage change, and I kept it running with boost transformer. But to use in my high-endish music room I bout another transformer, which simply looks more in place (UMI Step UP 110 to 220 Voltage Converter found on Amazon). It is rated for 1200VA which is well enough for amplifier that consumes 500W.
Line Magnetic LM-508ia has excellent fit and finish. Designers had clear attention to details and manufacturing quality was excellent. All chassis is covered with mirror like enamel probably ½ mm thick. Connectors for inputs and speaker outputs are of high quality. Amplifier came with pair of Line Magnetic branded Chinese 805 tubes, pair of also LM branded 300B tubes (used as drivers), pair of new production Electro-Harmonic 6SN7made in Russia, and a Chinese 6N9P (equivalent of 6SL7) tube of unknown brand. Later we will see that this amplifier works best with tubes that came with it from the factory.
LM-508ia has two adjustments — output tube current and hum balancing. When I turned it on, both of these adjustments were spot on. It only can be explained if each unit is tested and adjusted right before it is packed into the box.
>>>To be continued<<<
