...check out this response about tube swapping for an NTK directly from RODE:
From Darren Rose at RODE
"The NTK circuit was based on the 6922 tube specifications and that’s the one we recommend for field replacement when not purchased from Rode Microphones. Obviously you know that some valves sound better than others, like the (NOS) Telefunkens. When you make thousands of microphones, like we do, NOS are not an option as we could never obtain enough stock.
As I am sure you will understand it is the designers’ job to make sure that every microphone leaves the factory with the same sound quality (tone). We source tubes from all over the world and like all the components that Rode buys don’t rely on markings. We have found two identically labeled valves to be totally different in specifications.
We test every tube before it leaves the factory ensuring that they meet their specifications. A good tube is one that has good specifications, but because they are essentially an electro-mechanical device it still must be tested in house by Rode staff. I don’t have a view when it comes to the manufactures as long as they meet specifications: (Whether they be Siemens, Amperex, Sovtek or Svetlana)
The 6922 tube is basically electrically equivalent to the 6N11, 6DJ8, ECC88 and the 6N1P (Obviously the 6N1P uses 635mA max heater current which is twice as much than the other tubes.)
Now the NTK power supply was designed to supply 350mA to the tubes heater elements. The NTK power supply can supply up to 650mA heater currents which means the 6N1P will work in the NTK although I would want to have a listen. The power supply will maintain its 6.3Volt output even at 650mA heater load current.
The 6922, 6N11, 6DJ8, ECC88 and the 6N1P are all plug in replacements to each other. The manufacture datasheets for all these part suggest that you will not have any problem with plate voltages or other specification when used in the NTK. How they all perform and how well they operate is another question all together.
Just incase you were wondering about plate voltages in the tube, here is some brief technical information.
The nominal grid voltage (pin 2 on the tube) is: -77V
The anode (pin 1 on the tube) is connected to ground via a 43K ohm resistor. The anode voltage can swing between 0 Volts to -50 Volts.
The cathode (pin 3 on the tube) and the internal shield (pin 9 on the tube) can swing in voltage from 0 Volts to -125 Volts.
You can try any of these tubes quote in this email. Remember, even thou two tubes have the same markings they may sound very different, I guess this is the fun with NOS. It really is a case of personal flavor so try them and make up your own mind."
...and then there's this thread:
https://homerecording.com/bbs/showthread.php?t=134225&highlight=NTK+upgrade
"Oh little Grasshopper he says..."
