After I had obtained a few surplus Russian tubes from Dr Alex,
sent me an e-mail entitled "Starting Procedure". The subject of this message certainly
piqued my curiosity! As I read it, I realized I had forgotten a very important aspect
of using tubes which have spent a significant amount of time, often years, "on the shelf"
Due to the nature of physical materials, a certain amount of gases are always
trapped inside the metals and ceramics, etc., used to construct each tube. A vacuum tube
depends on a relatively hard vacuum to function without arcs and other undesired, often
disasterously destructive, internal current flow.
While in storage, a certain amount of the
gases trapped in its materials is "leached" out into the vacuum of the tube. If one were to
plug such a tube into an amplifier and apply all voltages and drive, the small amount of gas
within the tube would ionize and provide said undesired internal conductive paths; such
conduction often reduces an otherwise useful tube to trash.
It is, however, possible to
prevent these events from occurring by taking some rather simple measures to prepare such
tubes for use!
Fortunately, tube manufacturers considered this situation, and inside each vacuum
tube is a metal surface called a "getter" - a surface made from some particularly reactive
metal like barium. Being very reactive, it will react with more or less any molecule that
hits its surface, and form a non-volatile reaction product.
In a small glass tube, the getter
is the shiny surface on the inside of the glass wall. The trick is to get the atoms of gas to
move around enough to strike the getter and become absorbed. This is what UR4LL's e-mail
addressed. I also did some reading on the internet and found an applicable Technical Bulletin
Svetlana and a very nice treatise by
SM5BSZ, including input from SM6EHY, which put it all in perspective. I include
three excerpts from SM5BSZ:
"As amateurs, we often use old tubes. They may be unused, but may have spent a very long time
stored away. A high power tube should work at very high voltages without arcing so it has to
have a very good vacuum. When the tube is stored, vacuum gradually deteriorates over time and
the procedures below will help to restore a good vacuum.
The cathode is the "heart" of the tube. There are several types of cathodes, and they have
different properties. Read in the manufacturers data book how to treat the cathode of the
tube you are going to use. A pure tungsten cathode will give maximum tube life if the heater
voltage is made as low as possible for the desired output power, while a thoriated tungsten
cathode may be damaged if it is underheated.
When the heater is switched on for the first time, it may be important to allow it to reach
its final temperature slowly. I have been told that a slow heating, raising the voltage from
zero to nominal gradually, over a time of several hours, will increase tube life considerably
in normal professional operation. I guess this has something to do with a rechrystalisation
that takes place when the heater wire is heated for the first time, but I do not really know."
PREPARING SURPLUS TUBES FOR USE
1. Only Ufil during 12 hours min.
2. To ground all grids.
3. 25% of Ua during 4 hours.
4. 50% of Ua during 4 hours.
5. 100% of Ua during 4 hours.
COMMENTS: We have found that some of the larger tubes do a lot better if we bring filament
voltage up, either over a period of a minute or so using a variac, or using the procedure in
"Step 1", below, and leave the power on in this manner (with cooling fan on or liquid coolant
flowing) for three to five days. We have found, for example, that GS-23B operation can be
SIGNIFICANTLY enhanced, especially for those which work at 23cM, by a minimum of five (5) days
with the filament on as described!
Subject to adjustments as outlined above, the steps shown below (prepared 16 June 2000) will
change as further experience dictates:
Step 1: Per SM5BSZ, I'm going to bring the filament voltage (Ufil) up
slowly; probably 20% Ufil for the 1st hour, 40% during the 2nd hour, 60% in the 3rd
hour, 80% for the 4th hour and, finally, apply 100% Ufil after four hours, leaving
it applied until 12 hours has elapsed. I will turn the forced air cooling on at the start of
the second hour.
Steps 2 & 3: I have a variac on the input to my HV supply and will vary the
plate voltage (Ua, or anode voltage) as required to obtain the specified percentage
for the specified period. If you don't have a means to vary your HV, SM6EHY says, "Connect
the anode through a 50 to 100 kiloohm resistor. Leave the tube with the plate voltage through
this resistor for 2 hours. If the tube has been unused several years, leave it for 24 hours."
Most of my tubes are from surplus "spares stock" and are several years old. Based on this, I
will have to err on the side of too much time at each level, rather than trying to shorten the
process in any way! UR4LL says to ground all grids before applying Ua; SM6EHY says
nothing about the grids until his equivalent of step 4.
Step 4: Again, I will vary my HV using the power supply variac. SM6EHY suggests,
"Now the vacuum is improved, and you can replace the 50 to 100 k ohm resistor by 1 k ohm and
apply the screen grid voltage. Start with a reduced screen grid voltage for half an hour and
then apply full voltage and wait for half an hour again. In both cases, the control grid
voltage should be adjusted for negligible plate current." If I see plate current starting to
creep up, I will unground the control grid and apply bias per SM6EHY.
Step 5: I will follow UR4LL's procedure. After step five, he says, "Tube is ready
for use." Step five per SM5EHY: "Now it is time to remove the plate resistor and start to
use the tube with some RF." Click
here To read SM5EHY's comments within the context of the SM5BSZ