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

[John Schneider <jschneider@centuryinter.net>]

Theo Fokkema wrote:
> 
> >> >>Still on 147's: The 6550 tubes have a mirror-like enamelling on large 
> >> >>areas
> >> >>of the glass - is this normal? (Leslie is circa 1973).
> >>
> >> This is normal, this comes from parts of the glowing wires of the tubes
> >> coming loose and sticking to the glass, but it means that the tubes are
> >> wearing. But you can continue using them as long as they work.
> >>
> >
> >This is what is called 'A getter burn.'
> >
> >The following is from the Allied Radio Corporation 'Dictionary of Electronic
> >Terms' First Printing, Feb 1956
> >
> >getter.  An alkali or alkaline earth metal introduced into a vacuum tube 
> >during
> >manufacture and vaporized after the tube has been evacuated.  When vaporized 
> >it
> >absorbs any gases which may have been left by the vacuum pump.  The silvery
> >deposit in the glass envelope, vsually near the tube base, is the result of
> getter
> >action.
> >
> >John
> 
> As that dictionary says, the silvery deposit is visible near _the tube
> base_, even with new tubes since it came there in the manufacturing process.
> But here we were talking about large areas of the glass...

The newer tubes may have the 'getter' in the middle and/or at the top of the 
tube.
I'm looking at a 6550 tube and it has two getters in the middle and one on the 
top.
They are a round circle about 1/2 inch/1.25cm diameter mounted on the plate.  
The 
getter material has burned away and caused a mirror like black deposit, about 
1.25 
inches/3.2cm, on the inside of the tube.  This is normal.  On this particular 
tube, 
the outer edge of the getter burns in the middle of the tube have started to 
turn 
white on the outer edges.  This indicates that the tube has probably overheated 
at 
some time, which can be caused by a gassy condition in the tube or a leaky grid 
capacitor which caused the tube to lose some of its bias and it will 
overconduct.  
If this happens, the plates will turn cherry red and your volume drops.  The 
tube 
is going into self distruct.  If you ever see a 6550 or any other tube with 
cherry 
red plates, it is probably bad.  When you replace it, replace it in pairs, and 
watch them closely for about 10 minutes.  If one of the new tubes starts to 
turn 
red, turn it off quickly and call your tech, you have other problems.

Add on to this, questions and comments. You can write volumes about tubes.

John