>
>I noticed some discussion about the availability of 3-500zg tubes.
>I can honestly say that I have sold many 100's of the 3-500zg tubes made in
>China, and they are a great tube......now.
>When they first started making the 3-500zg tubes in China, they were real
>DOGS! I mean it was not unusual to have 4 out of 10 fail.....but several
>thousand tubes later, and a few factory manager be-headings later (those
>Chinese take product control seriously) I can say without reservation that
>the present 3-500zg tubes made by Taylor, are as good as you will
>find.....period. It is very unusual to have a failure in the present day
>3-500zg tubes from China.
>On the other hand, I have heard several horror stories from the guys that
>are buying new amps with the 3-500zg tubes which are made in Europe
>installed in them new from the factory. I personally know of 4 people who
>have bought brand new amps with the European made 3-500zg tubes in them, and
>they have had to have their tubes replaced under warranty. One guy is on his
>3rd tube in 12 months....I wonder if maybe he is not a little rough on his
>tubes, but he says he is not.
? Not all cases of tube failure can be blamed on the station operator.
For instance, 3-500Z filament to grid shorts (lateral movement of the
filament helices) can not be caused by operator error. In order for such
to occur, the tube must have enough VHF gain to sustain an oscillation
condition. I know of a case in Santa Monica where two pairs of new, out
of the box, 3-500Zs filament to grid shorted within one-hour in a Henry
3K-A -- an amplifier that has high anode supply voltage and high VHF-Rp
parasitic suppressors. .This, combined with 0.3pF of feedback C, is
somewhat risky in my opinion. When Eimac tells us in their ratings
that a tube is for "Oscillator and Amplifier Service", they aren't
fooling. Like Eimac's W. B. Foote told me in 1986, it's the
responsibility of the amplifier designer - not the tube manufacturer - to
make an amplifier unconditionally stable. Or, from another angle --
Tubes that can never oscillate do not amplify.
cheerz
>...
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