The difference between tubes is to be expected in 1296MHz PAs since even a very small mechanical change will de-tune a cavity at this frequency. Anyone using GI7B, GS9B, 2C39, or 7289 triodes, all rated for 3000 MHz, experience the same problem. YL1050 and TH327 PAs also exhibit the same variations at 23cm and it is considered normal. Everyone should be aware of this fact before they get started. In fact, after testing over 20 tubes, It appears that only one of somewhere from 5 to 7 are good for 23cm, so testing GS-23b tubes for 23cm suitability is a necessity. Even so, those tubes which do not work at 1.3GHz (see discussion below) should operate at 432MHz, no problem!

Why Tubes Vary in 23cm Performance

The problem is that radiated heat from the plate plus the dissipated power in the screen will mechanically change the screen, resulting in a change in plate/screen capacitance. The ONLY way to minimize this problem is to have GOOD (preferably water) cooling to eliminate the heating from the plate to the screen. It all has to do with the mechanical design and assembly of the tube. If the grid cage and the screen cage windows are perfectly lined up the resulting screen current will be very low and stability improved. A tube that shows high grid and screen current doesn't have the grid & screen cage windows perfectly lined up and some accelerated electrons hit the screen instead of passing through the window.

Many tubes have the cathode in the bottom and then a flat grid above it and then a flat screen above the grid and so on. The GS23B has all the elements inside the tube in a coaxial configuration. The cathode on GS23B is a cylinder in the middle and then the grid is another cylinder outside the cathode and so on. This makes for a very compact tube with large element surfaces and low inductance. This is a very common design in large tetrodes.

A tube that responds well to drive and has normal grid and screen current but low efficiency probably has a small variation in the distance between the elements around the circle. Any variation in this distance will accelerate the electrons in a non-symmetrical fashion around the circle and they will arrive out of phase. There is nothing we can do about that from outside the tube. This behavior is frequency dependent since the phase error gets smaller at lower frequency. This is why a tube like this would work well at 70cm but not at 23cm.

Testing Conditions

Overall, maximum plate dissipation is a function of cooling, so it should not be a problem to exceed it as long as the temperature is kept low.

         Tube max data:
         Max Screen dissipation  =  12W
         Max Control dissipation  =  1.5W
         Max Plate current  =  1.2Amps
         Max Cathode current  =  1.8Amps

None of the above critical parameters were exceded. These parameters are based on continuous carrier so it might be possible to get even more output in intermittent ham use, but I am not going to test it.

I have stress-tested the amp with tube #1 at 1200W output and everything looks good. I increased the drive-power to 100W and the plate current went from 1020mA to 1120mA. The efficiency increased. I got 200W more output for 300W more plate input. This is to be expected since I am getting further away from the idle current of 200mA. I set my CW-keyer to 20 seconds of code and 60 seconds pause with 100W drive. I operated the amp for more than 96 hours like this with no problems occurring. I am exceeding the screen dissipation by 10% under these conditions, but I expect that the ratings on this tube are very conservative.

PA Construction - Materials