Re: Expensive faulty amps



Guncho wrote:
On May 9, 3:38 am, Benj <bjac...@xxxxxxxxxxx> wrote:
Guncho wrote:
On May 8, 3:19 pm, "Elvis Kabong" <ampscie...@xxxxxxxxxxxx> wrote:
Ex. Most contemporarily manufactured tubes are good for about a
year, but if you play 5 days a week, about 4 hours a day, ideally
you should replace your output tubes every 6 months. The rock
stars usually have them replaced before each gig!
Your logic makes sense but so does saying "If you buy brand new
tires every year, your odds of getting a blowout are decreased".
But how common is it that tubes blow?? I've never had it happen.

The logic only makes sense if the quality of the tubes is high
enough! One continuing problem has been that the making of decent
tubes has become sort of a lost art like the making of a decent
mummy. Thus if a certain percentage of new tubes come with defects
that can cause them to suddenly fail, you are actually better off
with your old time-tested tubes that have proven themselves
defect-free rather than popping in new ones every gig!

Generally, the major defect in tubes is loss of emission. When this
happens the power output and gain of the tube drops. This problem
comes from the fact that the cathode in the tube is coated with a mix
of stuff that (believe me) is truly black magic. The whole idea is to
get electrons to boil off of a cathode that is really too cold for
this to happen efficiently. The purpose of the "magic" coating is to
get around the problem. Some transmitter tubes get around the problem
by simply cranking the cathode temperature and then emission loss
isn't such a problem. Modern tube amps are considerably better than
vintage tube amps because modern circuits are much better at
maintaining operating conditions over a range of emission loss. If
the parameters are set by hand, someone already noted the importance
of setting them every time a tube is changed.

But tubes can and do go bad in other ways. One way is a short. These
are usually mechanical problems often related to the vibration of the
tube from the sound or the shocks of hauling the gear around. In this
case well-built (expensive) tubes are going to have WAY fewer
problems than tubes just sort slapped together with a spot welder.
Also the electrical connections in a tube can "open up" also usually
due to a mechanical failure. Another problem that can suddenly
develop is gas. A poorly constructed tube can have dirt or other
chemicals that get sealed inside and at a most inopportune moment
will suddenly barf a burst of gas into the tube causing it to
generate a nice glow and probably quite working. Quality tubes are
made with "getters" inside designed to chemically combine with the
excreted gas and keep the vacuum, cheaper tubes will have cheaper
getters and hence problems. Also a tube can become "microphonic"
which is again a mechanical problem. It's usually due to some spot
weld coming loose and letting the tube parts vibrate which comes out
in the signal. A microphonic tube should be replaced because if it
vibrates enough a short is probably around the corner!

A lot of these problems are why NOS tubes are popular and why one has
to be a bit skeptical of tubes from third world countries. If I were
a rock star tech, I think you'd do better to TEST the tubes for every
gig changing them if they show signs of weakening rather than just
slapping new ones in there hoping the new ones are as good as the
ones you just took out. Of course I do get the idea that presumably
new tubes were tested at the factory so in that sense you ARE
testing them by replacing them! But then year ago I worked for an
appliance dealer who had rows and rows of tube TVs "burning in" for
a week or so due to the very high failure rate of supposedly new and
tested tubes. And those weren't third world tubes either!
Unfortunately if you want reliability it translates into quality
construction which unfortunately translates into $$$.

So you would agree that old age is not a common cause of tubes
suddenly "blowing"?

Chris

What are you asking, really? What's your point?

Don


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