[Amps] Tube wear out phenomena

[Jim Thomson]

Date: Sun, 20 Dec 2015 23:17:49 -0700
From: John Lyles <jtml at losalamos.com>
To: amps at contesting.com
Subject: [Amps] Tube wear out phenomena

Thorium is added at the 1% level to pure tungsten, to improve efficiency 
of electron emission from the filament of direct heated tubes. It allows 
a reduction of the temperature of a filament and reduced heating power, 
which all contribute to longer life over tungsten filaments. It is not 
added to the other metal parts of tubes.

It is true that the number of turn-on events is a factor in the lifetime 
of large tubes. Without overloading or misusing the tube,
filament emission lifetime should be the primary end of life event. When 
the filament is energized and deenergized repeatedly, even with careful 
current limiting and ramping of voltage, mechanical changes occur to the 
'basket' and lead to sagging and growth in diameter at the bottom. Given 
enough of these events, there is a finite possibility of shorting 
filament to control grid in large modern high gain tubes (where the 
spacing is already very close). Hence, it is better to leave them 
operating, and reduce the voltage when going to standby, but keeping 
some heating, sometimes called black heat mode.

Operating a high power tube below it's rating doesn't have an effect on 
lifetime, unless the filament is operated far below design which can 
result in damage. On the contrary, operating below rating usually does 
contribute to lifetime extension, especially if the filament voltage is 
managed so that emission is not excessive, far beyond the peak plate and 
cathode current desired in the operating point.

I have some amplifiers that run over 100 kW at HF, using modern 
tetrodes, and they routinely exceed 30,000 hours life before 
replacement. They are used for peak current, but the average power is 
quite lower than their rating.

73
John
K5PRO

##  folks who have used fil xfmrs that are rated for much greater current than required tell me
they have ended up with grid to fil shorts.  IE:  using a 30A  fil  xfmr  for a single 3-500Z etc. 
These were cases  where no fil step start was used.   Typ fil xfmrs will  usually, but not always,  
limit the peak inrush current to a value  equal to  double the normal  operating current. 

##  some will  use  a small rheostat in one leg of the fil xfmr primary.... to reduce the fil V a bit...and 
it also offers some inrush current protection.  Some will use a high Z type fil xfmr to limit max inrush. 
Other methods include manually operating a varaiac.   or  motor driven variac.  Other schemes  
include  a  SS  ramp up of the fil voltage.   Another method... a buddy used on his hb  3CX-6000A7
was a step start resistor in the fil xfmr pri  and a variac.  Variac used to fine tweak the  fil V.  Once set,
it was not touched.  Then the step start resistor  + timer was used to do the step start.   Direct heated
tubes  for the above. 

##  You would be surprised at how low a fil V you can use  for normal operation on a new or  rebuilt tube. 
This is all for  direct heated  GG triodes.    Both  eimac and svetlana say to run the fil v at normal rated voltage,
measured at the socket,  for  the 1st 150-200 hrs.   During that time, the emission increases  a bunch, then reaches 
a peak, then level off.   At that point, ( we use 200 hrs total fil on time), the  fil V can be decreased.   The procedure 
we used was to reduce the fil V  till the PO  just started to drop off by a few watts.  Then the fil v is increased by  .1 volt. 

##  If  just  ssb / cw is used,  we just pulse tuned the amp...and reduced fil V till the pep watt meter dropped off a few watts.
If  rtty /FM used, then the full cxr is used,  but same procedure used.  Its all checked every few months.  So far after several
years of use, and several thousand hrs of fil on time, the  fil V has not had to be increased.   The  3x6  requires  7.0 vac..and
its been sitting at 6.1 volts  for years now.   Since the getters  are fil heat activated, the fil V is brought up to normal 7.0 vac
twice a year for a day or two..then  reduced back down to 6.1 vac.  

## Fil V  is measured by installing small 100 uh  chokes in each leg with some test leads..with chokes  at the socket end.
Other end of the test leads is  wired to the panel to  a pair of banana jacks. Small  value resistor, like 100 ohms, is wired into each
leg of the test leads, just prior to the pair of test jacks.  This ensures that in the  case of an accidental short at the test jack end, nothing 
bad will happen.  The pair of resistors  will  not affect the external meter reading.   A  fluke  87 is plugged into the pair of test jacks.
The pair of chokes  keeps  drive RF  out of the DVM.  A .01 uf is wired form each  test jack to chassis. 

##   A  surplus sola constant V xfmr is  used to feed  a  steady   120 /240  to the input of the variac  / step start resistor. 
Output of that mess  feeds the pri of the fil xfmr.    A  110-120 vac  @ 60 hz, hr meter is wired  b4 the variac, usually
between one hot leg and  neutral.  

##  b4 the tube was put into operation, it was run at rated  7.0 vac fil V for a full 48 hrs..with just the  blower on..and no
B+  and no drive.   That was to fully getter the tube, b4 B+ and drive applied.  After  48 hrs, tube put into normal operation.
After 200 hrs,  fil V was reduced.  Same concept was also used on a 3CX-3000A7 tube.   I don’t bother  with the 3-500Z tubes,
except to use step start, fil V is left alone. 

##  I investigated  the black heat concept, and came to the conclusion for typ ham use, even if the tube was used for
2-8 hrs per day, its  probably not worth the effort to used black heat.   Which would involve running the fil V at aprx
3-5 volts during the times it was not in use. The blower would still be required, albeit its speed reduced. 
I can see the  merit for the use of black heat for large  SW broadcast  tubes etc...where the  total off time  isnt  very much,
and  or when the broadcast tube is used for say  4 hrs  on,  2 hrs off, then on again for 3 hrs, then off for a few more hrs  etc.
At that point you may as well leave the fil on in the black heat mode during the off periods.   

##  You can read detailed reports of the black heat results for SW broadcast use in the various literature, esp from the UK.    

Jim   VE7RF