[AMRadio] Old Tube Burn-in

[John Lyles]

The steps that K4KYV follows from old RCA documents are very 
conservative and good for bringing a tube out of hibernation. There are 
multiple types of getters inside RCA tubes, depending on the vintage and 
the size. Some tubes have getters that are activated by filament heat. 
Small glass tubes have a coating inside of the glass envelope. Others 
have anodes made from metal or with coatings that when heated, help 
adsorb gas ions. The very large tubes have chemical getters inside glass 
ampules that are sealed until the exhaust pinch off step of production 
where they are cracked open during a single mechanical step. They 
actually have long glass capillaries that reach through the pipe that 
gets nipped in a press to pinch off and seal the vacuum. At the same 
time it breaks the glass pipe and the chemical is exposed to the vacuum 
inside. Very clever construction.

When a tube sits a long time in storage, any small leaks that are 
inherent in construction (maybe on the order of 10^-8 Torr or better) 
will cause molecules of gas to deposit inside. I was told years ago to 
try and store tubes in places that don't have excessive temperature 
extremes. Heating the tube without HV or RF allows the elements near the 
filament (grid) to outgas and adsorb the ions in the getter. By running 
lower voltages, including RF voltage swing, there is less chance for 
destructive flash over, all the while giving the tube a chance to heat 
up quickly, as reduced bias voltage allows electron cloud to spread away 
from the filament towards the grid.  With no plate or screen voltage, 
the grid is heated by bombardment from the cathode/filament. Then the 
other voltages are applied in steps to again sweep the grid, the screen 
grid and the anode (plate) with electrons. This practice is equally 
applicable to Eimac direct heated TT tubes as well, but they don't 
specify in such detail that RCA did. These are all reasonable steps but 
are based on textbook ideas, and practice isn't so perfect.

In my workplace, a particle accelerator complex, we have a lot of 
multi-megawatt peak power tubes and hundreds of kW of average power is 
normal. We have specific conditioning practices for both tubes and for 
accelerator cavities that use similar steps. Get the average power up 
enough to outgas things, then slowly bring up the RF voltage swing so 
that voltage is applied gradually. Starting the other way, applying high 
peak voltage and low average power, is a recipe for problems, as the 
outgassing is happening simultaneously with large RF voltages. The last 
conditioning that we did in January took 12 days as the accelerating 
cavity had been open and exposed to air for over a week of troubleshooting.

John

K5PRO


> Date: Fri, 1 Mar 2019 02:24:52 +0000
> From: Donald Chester <k4kyv at hotmail.com>
> To: "amradio at mailman.qth.net" <amradio at mailman.qth.net>
> Subject: Re: [AMRadio] Old Tube burn-in
>
>   ARS W5OMR <ars.w5omr at gmail.com> wrote:
> I made a pretty decent deal at a recent hamfest - (6) 250TH'S. (4) of which
> are still sealed in their original boxes.
> I pulled the (2) out of their open boxes, and it appears as if they've
>> My concern for these tubes,  after resting for so long, is that they may
>> tend to be 'gassy'.
> That tends to be a problem with Eimac type tubes.  Graphite anode tubes seem to fare better sitting long time on the shelf.  I once "mined" about a dozen WWII era VT4-C/211s that were buried in the ground after the floor had rotted out in an old timer's storage building. Only two were bad; the rest tested as good as new.
>
> Here's what RCA recommends for breaking in thoriated tungsten filament tubes:
>
>
>   RCA's Recommended Break-In Procedure for Power Tubes
>
>  From Application Guide for RCA power tubes
> ?
> 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 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.
>
>
> For tubes that have been out of service for several years, I follow the above procedure, but with increased time for each step. I normally run the filament for about two hours (Step 2), then apply reduced grid drive for 30 minutes to a couple of hours (Step 3), reduced plate voltage (Step 4) for 1-2 hours, and finally let the tube run into a dummy load with full grid drive and full plate voltage for a couple of hours.
>
> With questionable tubes, I have been known to run the filament overnight before continuing with the procedure.
>
> Don k4kyv
>
> ?
>
>
>