Bringing old Tx tubes up to operational status after decades of storage

[w1vtp]

Any one have comments - experience with this?  I have a dozen 813s I'd like to safely bring back to operational status and this approach seems to be the best one yet

http://www.nd2x.net/tube-prep.html

Al

[KA0HCP]

Taking hours or days to bring the filament/tube up to temperature does not seem logical to me.  Either get it running or don't.  It needs to be hot to function.

Numerous sources point out that running a filament under voltage/under temp can result in 'poisoning' the getter material.   

[Jim, W5JO]

http://www.bext.com/filament.htm

[K5UJ]

I only had time to skim the eimac sheet (thanks Jim) but I got that: 1.  cool the heck out of the tube, 2.  meter the filament v. and fix it so the v. can be adjusted.

I need to do no. 2

I've only heard of special procedures for old tubes when they are either m.v. rectifiers or high power tubes where the gettering is done by getting the anode to show color.   

[Jim, W5JO]

Rob when I was fooling with high power tubes there was a sheet included with the tube that gave directions on how to bring it up to service.  Too bad someone doesn't have a copy of those directions for each type. Tubes are different depending on construction but I have never seen one lose vacuum nor have "partial' vacuum. 

The only problems I have ever seen is when tecs would pull tubes with a glass envelope.  They would rock it back and forth which can break or crack the glass at the pin seal.  If it is just cracked, when it heats the vacuum turns into a pressure vessel and the filament goes real bright.

[N0BST]

Taking hours or days to bring the filament/tube up to temperature does not seem logical to me.  Either get it running or don't.  It needs to be hot to function.

Numerous sources point out that running a filament under voltage/under temp can result in 'poisoning' the getter material. 

I think the instructions were to run the tube filament only under these conditions.  There are some tubes, usually big ones for FM broadcast, which can indeed be run well below rated voltage in service.  You run the tube initially at rated voltage for 200 hours or so, then drop the filament voltage until you see a drop in the output meter.  Then bring it back up a couple tenths of a volt above the "knee."

Scott Todd

[KM1H]

An 813 should be run right out of the box with filament voltage only to see if the seals are holding. If it looks OK then hit with HV; if no gas glow try RF bringing it up slowly still watching for gas.

A little gas may not bother anything as long as the DC and audio hasnt reached the breakdown point where it arcs.

If there is no getter flash its likely a version where the anode is coated with the material and it needs to be brought up to color to work.

I recently went thru a box of about a dozen unknown 813's and fired them right off on AM one at a time at 1200V. They ranged from weak to excellent but not a one had seal or gas problems.

[k4kyv]

Here's what RCA says:

The following "break-in" treatment is recommended for new or used tubes which have been in storage for an extended period, before placing such tubes in service.  This "break-in" treatment preferably should be in equipment in which the tube is to be used when new circuits are tested or when adjustments are made.

Step 1: Make sure that the cooling system and protective devices are functioning properly.

Step 2: With no other voltages on the tube, apply voltage to the filament or heater in the normal manner and operate at the prescribed typical operating voltage for 15 minutes.

Step 3: Apply reduced value of rf drive power and grid-No.1 voltage (approximately three-quarters normal drive power) for 15 minutes.

Step 4: Apply reduced value of plate voltage and grid-No.2 voltage (approximately one-half normal values) until stable performance is obtained.

Step 5: Increase rf drive power and grid-No.1 voltage to normal.

Step 6: Increase plate voltage and grid-No.2 voltage to normal, gradually or in steps.  Operate the tube until stable performance is obtained at each voltage level.

After the tube is given the above treatment and is operating normally to give the desired output, it is suggested that the readings of the meters and the control settings be recorded for future reference.

From Application Guide for RCA POWER TUBES
ICE-300 4-64

[k4kyv]

The purpose of the break-in  period is to activate the getter to remove residual gas. It is not so much to prevent arcing, as it is to prevent damage to the surface of the cathode or filament.  With thoriated tungsten tubes, the coating of thorium oxide on the tungsten filament is no more than a few molecules thick.  Once that coating is removed or depleted, the cathode emission is gone for good.  This is most often the reason tubes become "weak" and lose emission. 

With many hours of normal usage, the thorium coating is gradually depleted, an inevitable natural process.  That's why there is a finite limit to the lifetime of a tube, usually thousands or even tens of thousands of hours.  But if full plate current is abruptly pulled through a tube that has long lain idle on the shelf, without the proper break-in procedure, the stray molecules of gas that have accumulated inside the envelope get bombarded with electrons, and scattering of electrons results, somewhat like what happens with billiard balls.  The scattering of electrons in random directions effectively "sand-blasts" the filament and damages the thorium coating, thus shortening the life of the tube if not destroying it immediately.

Operating the tube with too high OR TOO LOW filament voltage is another way to prematurely deplete the thorium coating. 

Yet another way is to run the tube for many hours with nothing but the filament on. Without any plate or positive grid voltage, energetic electrons escaping from the hot filament in the vacuum form a cloud around the filament, but there is no near-by positive charge to attract them away.  These electrons remain in a cloud in close vicinity to the filament, and the magnetic field from the a.c. filament current agitates them, causing them to mechanically vibrate to and fro at the a.c. frequency.  These vibrating electrons likewise have enough kinetic energy to sand-blast and damage the thorium coating on the filament.

Again from Application Guide for RCA POWER TUBES:

Standby Operation

During standby periods, the tube may be operated at decreased filament or heater voltage to conserve life. 
It is recommended that the filament or heater voltage be reduced to no less than 80 per cent of normal during standby periods of up to 2 hours.  For longer periods, the filament or heater voltage should be turned off.

[W7TFO]

Active de-gassing was the topic of an earlier discussion here on the forum.

Filament cooking can help, but it only does so much in that some of the gasses found do not respond to the getter due to the chemical compositions.

I have a complete article by the engineers at WOR radio and Machlett Labs years ago, using ion-pumping via high voltage AC under no-emission conditions.

E-mail me and I will forward same to any needing the info.

I have done this with a neon sign transformer & Variac, and it works as outlined in the article.

It also works with gassy (yes, they get gassy, too) MV rectifiers that arc-back for no good reason.

73DG

[WU2D]

813 = tough. Put the boots to them.

[N0BST]

I have done this with a neon sign transformer & Variac, and it works as outlined in the article.

It also works with gassy (yes, they get gassy, too) MV rectifiers that arc-back for no good reason.

73DG

I remember when I used to work at the KSTP (AM) transmitter.  My co-worker Ray Brown told me about this, but I had since forgotten the details.  I would be very interested in the article- PM sent.

Scott Todd

[KM1H]

There is a big difference in procedure between tubes with handles and 813 or even 4-1000A size.

If there is a seperate internal getter and visible getter flash on the glass no amount of wasting time on "burning in" is going to help a bit.

Filament tubes can sometimes be rejuevenated by running the filament voltage about 20% high for around 15 seconds.

Indirectly heated tubes can often benefit by running just the filaments for up to 24 hours (at normal voltage) or longer to recondition the cathode. This often reportedly works with the external anode tubes such as 4CX150 and up. The problem here is that you dont know if it helped but many seem to have near religious belief in unknowns. I'll admit to doing this with 8122's and other expensive tubes with the end result the tube is full output, weak, or blows the fuse.