Tetrode_Preparationold

8/7/2019 Tetrode_Preparationold

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Preparation of old tetrodes GU-43b

& GU-78b before use

Revision 0: November 2006

by Dominique F1FRV & Roman US5WDX

Preparation of tubes which have not been used for years is necessary to

avoid flashes inside the tube. All tubes have built in "GETTER", but,

unfortunately for us the GU-43b's & GU-78b's do not have getters in

classical meaning, and gettering functions in these tubes are performed by

hot internal surface of the anode, heated by the electrons flow, which is

forced by the applying of nominal operating voltages to heater, grids andanode.

Getter definition from Wikipedia, the free encyclopedia

To prevent any remaining gases from remaining in a free state in a vacuum tube, modern tubes areconstructed with "getters", which are usually small, circular troughs filled with metals that oxidize


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quickly, barium being the most common. Once the tube envelope is evacuated and sealed, the getter is

heated to a high temperature (usually by means of RF induction heating) causing the material toevaporate, absorbing/reacting with any residual gases and usually leaving a silver-colored metallic

deposit on the inside of the envelope of the tube. If a tube develops a crack in the envelope, thisdeposit turns a white color when it reacts with atmospheric oxygen.Large transmitting and specialized

tubes often use more exotic getters.

Getter: un article de Wikipdia, l'encyclopdie libre

"Getter" est un lment composant la plupart des tubes lectroniques. Il vite l'apparition de gazrmanents dans le tube. Les tubes modernes sont souvent quips d'un getter de petite taille,

gnralement de forme circulaire, qui contient un mtal qui s'oxyde rapidement ; le baryum est le plus

courant. Une fois le vide effectu et l'enveloppe tanchifie, le getter est chauff haute

temprature (souvent par induction avec des radiofrquences) ce qui provoque l'vaporation du mtalcontenu par le getter. Le mtal absorbe alors les gaz rsiduels en crant dans la plupart des cas un

dpt de couleur argente sur l'enveloppe du tube. Si une fuite se dveloppe sur le tube, le dptdisparat par raction avec l'oxygne, laissant un dpt blanc. Les tubes spcialiss et les tubes de

fortes puissances utilisent souvent des getters plus exotiques.

Power tubes with conventional getter can be simply prepared for use, in

appliying heater voltage, with small air cooling on tube to evacuate heatercalories. One day of heating by year of storage is an average for a gassy

tube, and if you are lucky to find a tube with a good vacuum, one hour of

heating per year of storage is sufficient . . .

Tubes without conventional getter are more complex to prepair ...


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Here is a (not so) short description of the tubes conditioning process.

Procedure to be applied:Step 1 / Heater voltage

Apply heater voltage only for ~ 12 to 24 hours (more for very old tubes).Use start-up inrush current limiting resistor during the first 15 seconds.

Calculate resistor value for ~ heater nominal current on short circuit. Eg.

for heater 12 V @ 6 Amps, resistor selected is 2.2 Ohms, 20 Watts.

Check the heater voltage, and keep it at the lowest operating voltage givenin the tube datasheet. At the end of this pre-heating period, maintain

blower running at least 2 minutes after heater voltage removal.

Step 2: Check the vacuum of your tube.

Apply: heater voltage, variable Ug1 voltage (0 to -150 Volts) and no HV,

nor G2 voltages (and obviously no RF). Control grid 1 measured reverse

current should be less than 50 A @ -100 Volts.

Then, apply temporarely G2 variable voltage (0 to 300 Volts) with a push


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button, G1 reverse current should not increase, or at worse, no more than

25 %. There must be NO glitches during these operations.

If current is much higher than 100 A, be careful, vacuum is poor, or there

is some other internal contamination, or grids displacement toward cathode

due to manufacturing faults, cruel mishandling by the shipping service, or

improper use by its previous owner ...

If current is higher than 500 A, do not loose precious time with this tube.

Even with long time regeneration, it will never be safe to use, and most

probably never able to accept full rated voltages, or overloads. This tube is

good to be placed in a museum shelf ... (Except if you know an "expert" withgood experience on gassy tubes regeneration). But this tube will certainly

continue leaking and bringing you problems in use.


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Schematic of hot-pot bench, heater and G1 current test.


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Pictures of my UGLY hot-pot bench, heater and G1 current test.

With heater 0-15 V voltmeter and grid1 0-200 A meter.


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Heater soft start-up with 2.2 Ohms resistor.


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Small blower to evacuate heater calories


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1000 Ohms resistor will be too hot, so you go down to 500 Ohms, then 200

Ohms, just to keep them mildly warm, and leaving forever to 25-50 Ohms

50-200W to protect the tube against internal glitches in the future. All thiswill take at least 2-3 hours. When at nominal iddle current, maintain it for

2 to 12 hours, even more, if necessary, until the unwanted G1 leakage

current decrease down to the safe value (reverse current near to 0 A. The

lower the better ...).

If tube is too gassy, it will flash many times during its conditioning process.

Every time the anode fuse will blow, and most of the times the screen fuse

will too. Fast HV fuses are expensive and not always easy to find. Cheap,

quick but still effective solution is a homebuilt fuse, made of a very thinwire (breaks at 1-2 A), soldered between two copper holders on a few

centimetre piece of insulating material. Just remember to solder that wire

about one cm above the surface of that insulator, so during the glitch the

breaking wire will not burn it by the high temperature arc. When there is nofuse or no time to make one, unsolder the HV wire going from PS to the

plate choke, bend it 2 cm away from its holder in HV safe direction, and

make a 2 cm jumper made of copper wire of 0.05-0.1 mm of diameter. This

simple fuse may save the tube during a strong glitch.


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STRONG WARNING ! ! !

You are dealing with lethal voltages.

Be very careful during every operation, andremember about high energy stored in power

supplies capacitors. If you are neither confident

nor experienced enough, ask someone authorisedand experienced.

Step 3b: if vacuum is good

On the amplifier, with adequate air cooling blower running, apply 100% of

heater, grid 1, plate HV and G2 screen voltages. Note: You MUST have in

the +HV line a 25 to 50 Ohms permanently installed resistor of 50-200 W,

and a temporary 200 ohm 100 W resistor to protect the tube against


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internal glitches. Fast fuses shall be also placed on HV & G2 lines.

Then, still with NO RF, slowly decrease G1 voltage, to run the tube on itsiddle bias plate current. Starting from 50 mA and going up 100-150-200-

250-300 mA, up to nominal iddle current (200-300 mA for GU-43b, 400-

600 mA for GU-78b), by 10-20 minutes steps. This will take at least 1-2

hours.

Fastest and most effective conditioning of the tube is when anode is very

hot, typically when tube idle bias current reaches 50% of nominal operating

value, and higher (at full HV). Do not exceed maximum plate power

dissipation level, if you dont want to kill the tube. When at nominal iddlecurrent, and time and resources permit (electricity isnt cheap), maintain

heating of the anode for a few more hours. The longer the better...

Very often no glitches occurs during the whole conditioning process,

everything goes smooth and quiet, but one have to be prepaired for a worsescenario. For example a short voltage spike in your AC supply network can

make a good tube flash if it is not yet ready for say 50% voltage overload.

But lets hope it doesnt happen and we made it succesfully ....


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Adequate cooling should be provided during the complete process, and should

be kept running ~5 minutes after all voltages were removed.

Now: your tube is OK for RF !!!

If in a hurry, when there is no time to heat the tube under its nominal iddle

current for another few hours, start a small RF drive, starting from 5

Watts and after few minutes slowly going up by increasing TRX output in 5

Watts increments. Be very careful each time to tune the amplifier

correctly, because new untrained tubes, often do not tolerate big load

mismatch, they may flash again... For that reason a good idea is to stillkeep 200 Ohms 100 W resistor in anode for the first hours of operating

with RF, since the peak plate voltages now far exceeding the anode HV

power supply voltage, and 200 Ohms is a bit more effective in glitch effects

reduction than 25 or 50 Ohms. Of course with 200 Ohms you can not run PA

at maximal power output, especially in RTTY, but i wouldnt suggest doingthis, with just installed tubes, untested yet by heavy load. Give them some

time, in most cases few hours will be enough.

This is a very time and electricity consuming procedure, but it is done toprovide a long and healthy life for the tube.


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After this procedure done, you will begin to love your GU-43b or GU-78b,

sadly known as a big glitchers. It is so because in most cases people just

didnt took time to train them properly for 3-4 kV HV they put on them.

That's all fox !!!!

Thanks to Roman US5WDX for his help.These procedures were written, following the recommendations of the

engineers working at the tube manufacturing companies LAMINA (now owned

by Thomson), and POLYARON. Somewhat different, but similar in the idea,

of other recommendations who can be found on other power tubesmanufacturers web resources...


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In amateur and professional world is also known a much faster technique of

high voltage inter-electrode burning process of cleaning internal

contamination and high vacuum recovery. In general, it is done by applyingcurrent limited high voltages between tube electrodes, which 3 to 10 times

exceeding nominal operating values, so arc is appearing and burns inside. It

can be done fast, but it is important to have a correct control on arcing,

otherwise it can make more harm than good, so it is another article todescribe that process, and extra equipment is necessary.

[email protected] OR

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