GM70 Page

Before we talk about the GM70, here is some elementary information about the 845: The 845 can do 100Watt plate dissipation in class C (as a transmitter tube) and only 75 Watt in Class A (as audio tube). This 75 Watt is the fatal limit, which means it is possible as a working point for the 845, but really not 1Watt higher. Though this 75 Watt or CLASS B, is really easy to find in the 845 data sheet, it is amazing how many designers think Class B and C makes no difference, and "safely" design their amplifiers at 80% of maximum which they think is 80% of 100 Watt, so 80 Watt bias. Then, another thing comes, even so unknown, it amazes me. All tubes loose additional energy in the plate resistance (Rp), which is a dynamical parameter. So plate AC signal flows through it. (whereas DC signal does NOT flow through it). However since Rp is thermally located in the Anode, it heats that part up. And this is not very little. So when you are getting 16 Watt out of an 845 stage, you may as well be dumping another 6 Watt in the anode, additional. So the tube runs at 86 Watt instead of maximum 80, and the maximum was 75Watt anyway. It is totally amazing what "great" names in HiFi industry make this mistake. To save money with the power supply they operate the the around 1000Volts, and draw 100mA in order to get the power out still. The 845 has not the full efficiency any more at 1000 Volts, so bias the tube at 100Watt, is the only way to get enough power out of it, and they specify the amplifier at 25 Watt output power, which is a plain lie. (ALLWAYS). You get perhaps 22 Watt at 10% distortion, but that needs 1300 Volts. At Hifi , low distortion level, he 845 gives only 16 Watt provided you use it not below 75 Watt dissipation.

As a user, all you can do is wonder why your 845 tubes develop this whisker noise sounding like "shhhhh...." and it comes and goes.

It is better to use the GM70 when designing a new amplifier, it is a many times better tube. Please don't ask how to modify an existing amplifier for GM70, it is too late for that once it is made for 845.

The Chinese 845, is unreliable because it gets mistreated during shipment. As a dealer we receive them poor packed sometimes, fully unacceptable. After the end-users gets it, the tube has already micro cracks in the Carbon, and that develops a whisker noise sooner or later.

Like the 845, the GM70 has Thorium (bright yellow burning) filaments, but the GM70 has much higher filament power. This makes the GM70 can work at lower voltage as the 845, if you want it. The 845 stops working good below 800V. The GM70 works still fine at this voltage.

GM70 has the latest Russian technology in, and with this can outdo the 845 in terms of:

  • Reliability. GM70 develops no whisker noise.
  • Plate dissipation (845 in Class A: 75 Watt, 845 Class C 100Watt, GM70: 125 Watt CLass A)
  • Output power (845 gives 22 Watt at 1200V, GM70 gives 43 Watt at 1200Volt)
  • Minimum required plate voltage. GM70 works fine at 800V

The GM70 will play in any 845 circuit when you adapt the heater to 20Volt DC, 3Ampere. So it does have almost twice the filament dissipation, and produces twice as much light when it glows. As you may know, higher filament power will make any tube work better. This is also what makes the GM70 work so much better and easier to use than the 845.

Expected output power is 43Watt class A when you work at maximum settings class-A. SE transformers from Lundahl are available, as well as nice Teflon sockets from Yamamoto via our website. The tube has bright Tungsten heater. Imagine the looks of this tube, it has 60Watt heater power, and tungsten tubes burn brightly yellow. Then, with the very reasonable price of this tube, this is what makes the ultimate triode amplifier! Datasheet available via the link "tech corner" of this web site.

For low hum it is recommended to run the filament from a simple unstabilized rectifier circuit, and still add a hum potentiometer as you would do with AC heating. Like this the tube will be dead silent.

Look at that! These are rare exceptions to buy NOS like this, in original packaging. I took apart one of them to make these pictures.
Most GN70 found, have Carbon Plates. The plate of this GM70 pictured here, is made from copper. This process can be found with some very high quality tubes from England, such as Mullard ECC81, or Cossor GZ37 rectifiers. As most 845 users have experienced or heard from others, the 845 can have some whisker noise after bad transport conditions. This comes from the electrical connection to the anode, which is (and only can be..) a shrink connection. With this GM70 version, that problem was overcome with the copper anode, to which you can normally weld something. The bars, on which the anode is connected were hard-metal, I can tell so from cutting them off. They are from Molybdenum. This is so hard, it damaged the cutter tool somewhat.
Another picture, after removing the plates from the tube. Note there are three cooling ribs on it on each side. The basic idea for this plate shape is from the Western Electric 300B, which has one cooling rib on each side, because of this construction. The GM70 has two extra "folds" on each plate side, which form a cooling Rib. Since copper is an excellent conductor, this tube is a nice construction.
The grid is wound on softer nickel support bars.
The getters are most probably Zirconium getters.

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