Sometimes we get questions if we can make a 6SN7, or if we plan it. At the moment the answer is negative. Still here is a nice idea, how you can use 20B-V4 to replace 6SN7 in an existing schematic. Though this does take some understanding of circuit design.
The 20B is a directly heated tube, and 6SN7 is indirectly heated. However the 20B-V4 is a nice alternative, as the V4 Version tubes have a physical cathode connection indeed.
So here is how to fit 20B-V4 in a 6SN7 circuit. You do need a pair 20B-V4 to replace one 6SN7. For the rest, you just connect the tube as it is. So connect the plate and the grid. Then, the cathode of the 20B-V4 (also called the center tap) you just connect to the cathode of the schematic, and the tube is connected.
Now comes the question how to heat the 20B-V4, this is the part which is different. You need a stabilized floating 5V supply to the heater, and that is all. This basically will make the 20B-V4 fit into the 6SN7 schematic. We have tested this with a 20B-V4, and indeed, the bias was the same. In case you receive a pair of tubes where it is not fully the same, you can change the cathode resistor somewhat. As a rule of thumb, to increase the bias current by a desired value (in percent), you decrease the cathode resistor by the same percentage.
To get a a 5V DC heater supply, there are a few solutions.
- To replace one 6SN7 you need two 20B-V4 tubes, and two 5V Power supplies.
- Use a Heater Choke from Lundahl. Believe it or not, but this will give lower hum a stabilizer IC. Reason is not so much the Chokes value itself, though they do have many Ohms impedance still at 100Hz. (or 120Hz even for the USA). Their great function comes with a C-L-C configuration. The capacitor after the choke, this is the device that does the work. As can choose it really big, and price will stil be low. That gives a 100Hz impedance of only miliohms. With a filter like this, The AC residue will be lower than any stabilizer IC.
- 5V modules from Tent Labs.
- 5V USB power supplies from a good brand. For intance from Apple, the 5V 2.1 Ampere t Versions go for a very low price on Ebay.
- What we do not support, is self made 5V stabilised circuits. Of course these will work, but diffciculties may come when you overestimate the simplicity of making a really hum-free 5V circuit yourself.
- Consider a USB power bank for real battery operation. These are small size, and some can supply the needed 1.4 Ampere for 20 hours, and cost less than 20 Euro. This gives you a dead-quiet driver tube, battery operated. Make sure the connection cable to the tube is not too long or too thin. When you switch on and off the amplfier with a switch of this type, you can connect it such, that the battery unit sees the mains voltage, when the amplfier is off, and vice versa. Like this the batteries are always charged automatically when the amplifier is off, and the battery is detached from the mains when the amplifier is on.
The gain of 20B is going to be the same. (20x). The transconductance of 20B is higher, and plate impedance is lower, which is always a good thing.
Please mind, this example is only for the classical working point 250V, 9mA, at -8V Grid. You do need to check the 20B curves for any other working point, and be aware the EML curves are made with DC heated filament. So to compare this with the AC heated method as for the 6SN7 you need to increase the grid voltage with 2.5V. For instance you find -11Volt in the 20B curves, that would correcpond to -13.5V for the 6SN7. If so, bias will be the same.
Test results of EML20B-V4 at 6SN7 Bias (250V-9mA)