With RCA 245 tubes
With EML 45 tubes
From historical production all 45 can be used when they have sufficient emission. This can be seen
from the plate current on the meter, it must be approximately 32mA, and not drop at high loudness.
End-of-Life with 45 tubes.
It is hard to say with only the amplifier at hand, if this point has been
reached. Weak tubes will give at first a less quality sound, a more fuzzy sound, and loose
dynamics.This can be heard by experienced listeners, but is hard to describe here. The tubes can be in
that condition for some time, and/or start to loose Bias on the meter already at that moment, or later.
Allways, what you should do, before starting to use an 45 tubes, write down the initial value on the
base of the tube, with a sticker. Later, if you want to know if a tube has lost emission, it is very good to
compare with that initial value. Of course, the absolute value is also imporant, but most important is a
change from the initial value, because only the CHANGE will indicate something. Note, that some
lowering of the bias current is normal when the tubes get older, and is not allways a bad sign. So
absolute values can not be given here. Example: A new tube gives 35mA and after a long time you
see it starts to drop, and will go down steadily to a value like 29mA. So some part of the life time has
been used up. How much that is, can not be said. However if new tubes give 29mA they have full
100% lifetime ahead of them. So, the main function of the meter is to write down the initial value for
each tube, and compare with that later. The number itself is less important unless it gets very low, or
unless is drops at high sound levels.
Any problems with sound disappearing for 0.5 seconds after a tone with high loudness, means the 45
tubes are at the end of life definitely.
Tube burn in.
This is necessary with new 45 tubes, and also with used tubes that have been stored
for more than 2 years. Though normal function will appear, the initial sound is harsh and unpleasant
sometimes. Just use the tubes normally for a period of 50…100 hours and good sound will develop
surprizingly at the end of the burn in period.
717A tubes.
These are in the family of the famous Western Electric 408A tube, though not the same
tube. Because of the metal shielded sides, 717A is very suited for HiFi. With 717A there are many
used and weak tubes for sale, and we can only say, never buy those from people who can not tell you
if the tubes are good, have no microphonic, and no crack noise. Also NOS tubes come out of the boxes
“weak” sometimes from too long storage. It appears these can not be recovered to good condition if so.
However such 717A that appear to be good, will stay so for many years longer. The only way to test
717A is on a tube tester giving results in Mili Ampere. Any “Good/bad” scale testers can not test this
tube well. Always be sure that if you buy them, there has been a microphonics test done as well. Last
warning is for auctions, there are many so called new tubes, that are sold with very low test values.
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