Antonio Vernucci wrote:
pleased to read from you after a long time.

Thank you, it is good to see you here!

Just for fun I am building a transmitter using German tubes and components.
The Telefunken RS391 is a plain power pentode optimized for suppressor grid=
modulation, but is not a special tube. It is similar to an RCA 803, just s=
omewhat less powerful than it (110W plate dissipation against 125W of an 80=
3).

With an 803, I would not expect to see that behaviour... but this is not an
803. The grid structures are likely different.

The transmitter works beautifully, no self-oscillation or other instability=
signs. All currents and voltages, including the screen current and voltage=
, are as they should be, no overload whatsoever. The only oddity is that I =
see a glowing grid on transmit (not sure if it is the control or the screen=
grid). I tried three different tuvbes, all showing the same effect.=20

If the currents are in the ballpark, and it's not oscillating, and the load
is good, then you have done everything you can do, I think. It is likely
a design issue with the tube and it might shorten the tube lifetime, but
that might just be how that tube is.

An easy conclusion would be that having a glowing grid is normal. But I hav=
e some difficulty to accept that (I have never seen a glowing grid). =20

Yes, I don't like it.

I have seen glowing grids before, but only when something was wrong. There
are a lot of tubes that will draw excessive screen current and have a very
obvious screen glow... but they won't do that for very long until the screen
melts and sags.

The issue I am raising is not specific to the RS391, but it is of more gene=
ral applicability. For istance the "typical operation" section of the RCA 8=
03 datasheet reports as normal a screen voltage of 500V and a screen curren=
t of 33mA, corresponding to a dissipated power of 16.5W

How is it possible - I am asking myself - for a tiny grid immersed in betwe=
en two high temperature elements (namely the plate and the cathode) to diss=
ipate 16.5W (which is a significant amount of power) without reaching a ver=
y high temperature? Does the 803 screen grid also glow?=20

The 803 screen grid does not glow, or at least not to the point where it is
obvious and visible. But it does dissipate a considerable amount of screen
power, and the screen is designed for that.

If I were you, I wouldn't worry about it because I don't think there is
anything you can do about it that you haven't done already. But if I were
you, I'd be using an 833. Or maybe a 6AQ5.... I have 90 DXCC countries now
with the 6AQ5 as a final... but those last 10 are proving to be difficult...
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."