Re: 147 8ohm load resistor - a question...

[Randy Guttery <comcents@cybertron.com>]

Getter material deposits on the inside of the envelope in an area near 
the getter, which - depending on type, manuf. etc. can be anywhere - but 
is usually near the bottom of most signal tubes - (12AX7, 6AU6, etc.) 
and near the top of most power tubes (6L6, 6550, etc.) -- though again 
it just depends on where the manufacturer put the getter.  

Getter deposit area (i.e. the size of the getter "splotch" inside the 
envelope) is directly both to the size of the getter - and the amount of 
residual in the envelope when the getter is flashed. It is also true 
that the larger the tube the larger the getter deposit area - because 
the getter itself is larger - and since the volume of the envelope is 
also larger - the potential for residual (i.e. gasses not pumped out 
during the evacuation / sealing process) is greater. 

What is getter deposit?  That is the _silvery_ deposit you see.   Getter 
deposit is normal, and has no bearing on the (current) condition of the 
tube.  It is simply there - to be ignored.

Is that to say that there isn't visible indications of tube wear? Nope.  
There are several things:  And two are deposits on the inside of the 
envelope. A dull black deposit is common in tubes that are being run at 
or past their ratings.  This is actual metal being melted / blown / 
boiled off of the various elements (plate, for the most part, and some 
cathode material) and then settling on the envelope.  This is a sign of 
an abused (or very _very_ well used) tube.   Then there is the whitish 
kind of deposit.  This is _very_ bad... indicates oxygen is leaking in 
and the parts are literally buring up (the white stuff is fine ash!). 
There are more subtle signs of tube wear.  Look at the leads where they 
imerge from the glass seal (in the tubes where this can be seen).  
Actually you want to see the lead _within_ the glass seal portion.  On 
many new tubes - the wires will have a distinct red color to them.  Once 
the tubes start being used - the red fades to black.  This is esp. true 
with Western Electric tubes - and how those 300B collectors can tell if 
a tube is really new just by looking. Again - here is a high power tube 
with the getter high - leads in the seal visible - because the getter 
deposit is high in the envelope.   Other indications a tube has bee 
abused is if the plates look "funny" good plates will be smooth and 
uniform in color.  An abused tube may have warped plates discolored in 
rings or waves... 

While visual clues should certianly not be overlooked - there is nothing 
short of a good mutual dynamic conductance tester to tell a tube's tale!

Sorry to run on so - but I thought maybe some better getter info might 
help.  

best regards
randy