I am seeking advice from anyone have a solid and hand-on experience in RF power tubes operation.

I have recently built a CW transmitter using a German WWII tube (RS391 by Telefunken) as final amplifier.

The tube operating parameters match very well those shown in the datasheet, namely:
- plate voltage: 1500V
- plate current: 150 mA
- screen voltage: 400V
- screen current: 25mA
- suppressor voltage: 0
- grid voltage: -120V
- grid current 2.5 mA
- output power: 140W

I was very satisfied of the transmitter behaviour until I noted that one of the grids (I cannot determine which one of the three through the glass) surrounding the cathode becomes very bright on transmit (sign of very high temperature).

Please note the following:
- I tried three different tubes, all showing the same behaviour
- everything looks normal, no self-oscillations
- the dissipated powers, calculated on the basis of measured voltages and currents, a
* control grid: 0.25W against an allowable maximum of 0,4W
* screen grid: 10W against an allowable maximum of 20W
* suppressor grid: 0W
- reducing the control grid drive level causes a reduction in grid brightness, but if I reduce the grid current to almost zero I can still detect some glow.

At this point I am wondering whether having a grid working at high temperature is a normal condition and I am then worrying about a non-existent problem. That supposition comes from the fact that I can hardly believe that the screen grid, which has a very light structure, can dissipate 20W without reaching a very high temperature.

If you have some experience on the issue, please advise.

73

Tony I0JX
Rome, Italy