Wednesday, February 4, 2009

"Gettering" GU74b / 4CX800A

There is no such thing as a perfect seal! Vacuum tubes (especially high-power transmitting tubes) not used for a few years might exhibit serious problems if put into service without a prior conditioning of the vacuum. With time, gas molecules leak inside and/or are released by the tube's internal components. With years and years of storage, the vacuum could deteriorate to a dangerous level and once the tube is used for the first time it could "flash-over" - the gas molecules inside will become ionized by the electron flow and this will create a flash of high-temperature plasma between the cathode and anode, damaging the grid(s) and other internal components. A chemical composition, called "getter" is factory deposited inside the tube to complete and maintain the quality of the vacuum (the getter is visible as the shiny, black-metallic area on the inside wall of the glass envelope (in smaller tubes)). In power tubes, the activation of the getter is done by heat. Therefore, it is recommended, before putting into service a power tube with very long on-the-shelf life (more than a couple of years) to condition the vacuum first. This is done by applying power to the filament (cathode heater) only and leaving it on for a period of time. The hot filament will heat up the getter and also will improve the vacuum by itself (gas molecules will react with the hot tungsten filament and the cathode surface, forming chemical bonds and effectively extracting them).

Here is a simple fixture I used to "getter" my newly acquired GU-74b tubes (NOS, manufactured in 1990-92).

The "chimney" is made by cutting the top portion of a plastic soda bottle ("Classic Seltzer Water" sold at Costco to be more specific :). The cooling fan is from computer power supply. The tube MUST (!) be cooled with forced air while being "gettered" or the high temperature will damage the metal-ceramic seals and destroy the tube. The fan is raised about an inch from the surface to allow for air intake. I slowly raised the filament voltage from 3v to 12.5V (12.6 is the nominal voltage) over a period of 5 hours in 5 steps (3v, 5v, 7.5v, 10v, 12.5v) using variable power supply. I, then left the tube running with the nominal filament voltage for about another 8 hours. The fan should run continuously, powered by a separate 12V supply. The current drawn by the heater is around 3.6A (maximum allowable is 3.9A). Absolute maximum voltage for the filament is 13.3V but it should never be reached! Recommended operational voltage for the heater is 12.6v and exceeding this voltage is not healthy for the tube. Measures must be taken to avoid short in the power leads (at the tube's pins) - best is to use proper tube socket but wire-wrapping with solid copper wire (AWG 18 or 16) and heat-shrink tubing insulation could work too.
At the end of the "gettering" procedure (after power is disconnected) is also a good time to conduct a few electrical checks for possible internal short between various tube components - using just a simple ohm-meter, while the tube is still hot (! be careful handling it to avoid burns - use gloves) and one more time when the tube completely cools down. Check for short between the heater and the Cathode, between Cathode and 1st Grid (G1), between G1 and G2 and finally G2 and Anode. While the Cathode is still hot, it is normal to see somewhat lower resistance between the Cathode and the grids or the anode - it is a vacuum tube after all and it will conduct current if electrons are emitted by the hot cathode. This resistance will gradually increase as the tube is cooling down.
After installation in the amplifier it is recommended to start using it at low power and low duty cycle (maybe just starting in SSB mode) and gradually increase the power output.

13 comments:

Tobias said...

Thanks for the instructions. I'll proceed as explained with my NOS GU74b.

André Luiz Osternack said...

Hi Friend,
I have 2 of these electron tube, they are new in the box, and would like to sell them. But as I know nothing about them and would like more information about them. I live in Brazil and have not found anything like that here.
thank you very much for your attention
thanks
Andre Osternack

Andrey E. Stoev said...

Gu74b is a Russian made (by the Svetlana company) high-power ceramic tetrode. The "western", "Eniac like" is designation is 4CX800A. It was originally manufactured for the Russian military and used in the PA of tanks and submarines radios.
Nowadays the most popular practical application is in amateur radio power amplifiers (like the Bulgarian made ACOM 2000). The price vary and it also depends on the date code - the newly manufactured tubes are more expensive - most of the ones on the market are from the 80s and 90s. Used ones are cheap as it is difficult to determine the amount of wear on the cathode etc.
Check on eBay for pricing.

Anonymous said...

Please tell me what two pins you use for the filament(cathode heater).

George Pattinson said...

Thanks for the post. Very helpful and effective tips!

George Pattinson

Anonymous said...

Pins 3 and 7 are the ones to use.

Ernesto Mantegazza said...

Dear Guys,

I need to do gettering procedure on a NOS GU74B just bought from Russia.
The procedure described is clear. Only one question :

1. Current measured on filament (3,6A) need to be limited using a proper power supply with current regulation or this current need only to be measured at the and of the procedure in order to check if within 3,5A limit at 12,6V ?
Looking the procedure described it seems that power supply need only to be limited in Voltage.

Many thanks for the answer.

Regards

Ernesto

Andrey E. Stoev said...

You only need to monitor the current while controlling/limiting the voltage. If your power supply has a current meter (ammeter), just keep an eye on it while going thru the different voltage steps so you don't exceed the max allowed current.
The resistance of the filament will be changing as it heats up and this can cause a current variation if a fixed voltage is used.
So the short answer is - if your PS doesn't have a current limiter, you must put in-line an ammeter and enforce the maximum allowed current threshold.
Varying the voltage while monitoring the current is the easiest way to do it.

Bill B said...

Just obtained two GU74B tubes that I thought were recently tested. Put them into my HF2500DX and warmed the tubes for a few hours before attempting to draw any plate current. When the amp was placed into transmit, without any RF drive, there was a loud flash over. Have you had this experience and what is the next move?
Thanks;
Bill W2PKY

Andrey E. Stoev said...

Sorry to hear that. The tube(s) are likely destroyed and you might have some serious damage to the amp. Often, the HV choke at the Anode supply is the first to suffer.
The flash over can occur as Anode voltage is applied even without drive.
Heating up old surplus tube for a few hours is not enough. I normally keep them heated at least 8 to 15 hrs. Gettering is a slow process and the older the tube the longer it takes. Sometimes the tube is just defective and shorts - Russian quality control in the 80s was not something they were famous with..

Bill B said...

Hello Andy-
Do you know what the typical bias voltage would be on the GU74B?
After the flash over can not draw any plate current even on a good tube.
Have the proper screen voltage but the grid bias is -180. Ray says should be -160.
Is -180 enough to cause cut off.
Thanks for any ideas.
Bill W2PKY

Bill B said...

Bias is actually -80V when in transmit mode.

Bill B said...

Found the problem, [Cathode circuit] top to bottom jumper on the bias board had a bad solder joint. Looks like the solder was crystallized from the mega-amp current drawn by the tube during the flash over.