RoeTest Computer - Röhrenprüfgerät - Röhrenmessgerät - professional tube-testing-system (c) Helmut Weigl
Eimac transmitter tubes|
If you experiment with extreme ranges you risk damage of the
Eimac (and other manufacturers) produce power transmitter tubes. There are many different constructions using
glass/ceramic/metal with air and water cooling. Those tubes have large power at high frequencies.
The very powerful types are anyway excluded from testing with the RoeTest
Only the smaller tube types can be tested with the RoeTest
When testing/measuring those tubes there are a number of problems:
The base of these tubes fits into a standard loctal socket. If you think you can operate the tube with that you are wrong.
The tubes are getting really hot. I tested that. After a short time you could use these tubes as a soldering iron.
This obviously must not happen. For examle the data sheet for the 4X150A specifies that the maximum base temperature
must not exceed 175 °C and the anode temperature is limited to 250°C.
To ensure sufficient cooling there are special sockets. The socket has the name "SK-600". This special socket has
air gaps between the pins. The tube is put into a thermal pipe:
The pipe is constructed using heat stable teflon. Its task is to guide an air flow coming from the bottom along the
base plate and then through the plates of the anode heat sink. The air flow must be generated by a fan built into
the case. According to Eimac's description it should look like the following:
I built a special socket box with a fan for the RoeTest
Following the socket box from the inside:
The case of the socket is directly connected to the ground at the 12 pole multiple connector. The heater connections and
G2 have bypass capacitors to ground. There is a UKW choke inserted into the G2 line (inside a heat shrink tube). A resistor of
1 kohm is connected in series with G1. The anode is connected externally to the banana jack for pin 10 of the RoeTest
This wire also needs a UKW choke connected in series.
The voltages required for operating at nominal values are not present in the RoeTest
For this reason measuring is done using lower voltages. As a matter of safety I only used the 300V anode voltage range.
With this circuit a used 4X150A could be measured. With a 4CX250 the experiment failed. This tube soon started to
oscillate very heavily.
These tubes tend to oscillate very heavily. With the 4CX250B (a new one) very soon high frequency oscillations
at high power were observed. It is essential to avoid oscillation anyway. Not only that measuring is impossible but
also the RoeTest
Eimac states 1951 in "The care and feeding of power-tetrodes" that the user has to take care of suppressing oscillations
and that there is no panacea for that (simplified and summed up). So you can only try to use different circuit designs
to suppress oscillations (capacitors, chokes and so forth...)
Further Eimac writes that a combination of a resistor and a choke (in parallel) in series with the anode connection
(eventually also in series with the screen grid connection) in most cases can remedy the oscillation problem.
Eimac recommends to use a low inductance resistor of 50-100 ohm and some turns of wire wound around it
(see paragraph at the end of this page). I used a resistor of 68 ohm / 4W and some turns of enameled wire.
As a precaution I monitor the anode voltage at the anode connection with my oscilloscope for oscillations.
Using this circuit I could measure a new 4CX250B successfully. I only had to find appropriate measuring settings
for low voltages. The following settings were practical:
For recording the characteristic curve I created an own paramterer file. The UG2 curve was recorded at
-15, -20 and -25V.
At last I mounted a transparent bottom of Plexiglas at the socket holder. So the tube can be watched during insertion (you can easily bend the contact springs of the socket if the tube is inserted a little bit twisted).
In the following pictures of the static data and the recorded characteristic curves are given:
Excerpts from "The care and feeding of power-tetrodes", 1951, Eimac: