Il giorno martedì 8 novembre 2016 19:12:21 UTC+1, Scott Dorsey ha scritto:
Antonio Vernucci wrote:
I have recently built a CW transmitter using a German WWII tube (RS391 by T=
elefunken) 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) su=
rrounding the cathode becomes very bright on transmit (sign of very high te=
mperature).

I am not familiar with the tube. However, if you have grid stoppers in place
and a plate choke and you've looked on a wideband scope or spectrum analyzer
to see that there's no parasitic oscillation, and you've verified that the
screen current is good, it's likely okay.

Excessive screen current combined with a hot screen is a big big red flag,
but you say your screen current measures okay. What happens if you drop the
screen potential a bit?

At this point I am wondering whether having a grid working at high temperat=
ure is a normal condition and I am then worrying about a non-existent probl=
em. 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 r=
eaching a very high temperature.

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

Does the tube datasheet mention this at all? This is a tube that you will
find few people familiar with.
--scott

73

Tony I0JX
Rome, Italy


--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Hi Scott,
pleased to read from you after a long time.
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 somewhat less powerful than it (110W plate dissipation against 125W of an 803).

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.

As you suggested I tried to decrease the screen voltage, and the grid brightness also decreases. But I do not see why I should decrease the screen voltage if it is well within the specifications limit.

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

The issue I am raising is not specific to the RS391, but it is of more general applicability. For istance the "typical operation" section of the RCA 803 datasheet reports as normal a screen voltage of 500V and a screen current of 33mA, corresponding to a dissipated power of 16.5W

How is it possible - I am asking myself - for a tiny grid immersed in between two high temperature elements (namely the plate and the cathode) to dissipate 16.5W (which is a significant amount of power) without reaching a very high temperature? Does the 803 screen grid also glow?

73

Tony I0JX
Rome, Italy