> >Barnacles are described in many texts. Copper actually grows
> >barnacles when in a vacuum.
>
> [chortle] these are long copper filaments that burn away easily. Are
> 3-500Z anodes made from copper? > >This entire topic has been discussed
> here before.
3-500's primarily fail from gas or mechanical problems like bad
welds.
Several tubes out of the last batch of 100 3-500Z's from Varian in
Salt Lake City before the power grid facility was closed all had long
single strands of grid wire sticking out of the top of the grid.
All 100 had to be replaced, due to arcing.
> ? We have heard taurine feculance here before..
It seems to follow you.
> I have opened dozens of kaput 3-500Zs from amps that reportedly had a
> big-bang. I have yet to see a barnicle.
3-500's don't have copper cooler. 8877's and such do. Barnacles
are small and fragile, and disappear after an arc. They are common
in vacuum capacitors also.
> >Tube faults like that are intentionally removed by controlled arcing, and
> >uncontrolled flash will do the same thing.
> >
> >If the tube repeatedly arcs, it needs to be replaced.
> >
> >I'm more interested in your claim the grid fails to be a shield in the
> >3-500Z at 80 MHz, and the grid capacitors raise that frequency. How do
> >you determine that?
> >
> A dipmeter. If the grid were resonant where you claim, the amplifier could
> not oscillate at c. 110MHz.
The amplifier does NOT oscillate at "c. 110 MHz, as you claim, when the grid is
directly grounded.
The AL80 series, if the parasitic suppressor is removed and a grid
wire or two lifted, oscillates near 180 MHz. It is a repeatable
oscillation that can easily be measured.
> >If I do a plot of S12 or S21 on a cold tube in a test fixture, would you
> >like to put it on your web page and remove the incorrect data?
>
> If no such dips occur in a SB-220 at the frequencies you claim, the
> ''incorect data'' is not mine.
Your mistake is rooted in your lack of test equipment. A grid dip
meter held near the grid connection to chassis can not determine
the parallel resonant frequency of the grid.
It could be measuring almost anything, because it is the wrong
device for the job.
The only way to measure the grid's isolating or shielding effect
between the anode and cathode is to measure the loss through the
tube from anode to cathode, or vice versa, on a cold tube in the
actual circuit.
I'm sure you think you measured something useful, but the fact is
you have no idea what the grid impedance is using your method.
This is especially true when there are capacitors involved, and
multiple grid pins. You easily could have measured the loop
impedance from chassis to grid support and back to chassis again,
including the path through the chassis.
Even if you isolated a single pin, the measurement would still be
invalid because it does not determine the impedance at the grid.
On the other hand when the signal attenuation is measured from
cathode to anode, the effects of grid shielding can be clearly seen.
73, Tom W8JI
w8ji@contesting.com
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