[Boatanchors] TUBE TEST

[w8au at sssnet.com]

At 06:54 PM 7/1/2009, Magoo wrote:
>I just did some tests on a bunch 14) of NOS Russian 6550 tubes.  On 
>my Hickok 605, every tube showed very good, in most cases exceeding 
>the basic minimum for that tube.  HOWEVER, every
>tube reached a peak and then began to slide back down, often below 
>acceptable minimum.  Bad sign...I used to see 6146 tubes and similar 
>from FM radios with the same characteristic and
>consequent low power output.
>
>Has anyone else had this same issue?  I know the tubes are brand new and
>about 10+ years old.  Is this a characteristic of the older Russian tubes on
>the whole?  How about the new tubes?

Having done maintenance years ago on commercial two-way gear (6146s, etc) you
have to consider that when you push the "test" button and the big tube draws
plate current, the input line voltage drops a tad. (evidenced by the drop at
the AC "cal" point, the first setting prior to testing the tube.)  This causes
a small drop in filament voltage.  A weaker tube that has many hours on its
filament will show a drop in emission and consequently a drop in plate current.
Cathode type tubes also suffered from a progressive condition called, "cathode
interface."  This does not mean it won't provide high output under 
normal voltage
conditions, but that its capability to do so will drop off sooner than later.

A new tube will sustain a small drop in filament voltage without losing
detectable emission.

This used to be the test for "replace" or "don't replace."  To cut down on
future "trouble calls" we usually replaced the ones showing drop off.
I was always appreciative of this particular test.  A test that Hickok never
planned, no doubt.

If any NEW tube suffers this way it must be a manufacturing 
problem.  What else?

Perry   w8au