[Amps] bending filaments (was al80B)

[Tom Rauch]

> Both apparently can.  Eimac cautions against having more than c, 29A-rms
> of filament inrush current - presumably to avoid distorting the filament
> helices.

Eimac states that as a general rule, and it is very conservative.

The problem arises when the tube has poor materials, or is improperly
manufactured. The AL80 series has a very long history with good tube life.
This filament-grid issue really just started when Amperex tubes were used.
It appears even in amplifiers that have a LONG field history of no failures,
when Amperex tubes are used as a replacement.

For example, I have an AL80A that was used with an Eimac tube since 1983. I
changed that tube last year to an Amperex, and it failed G-K within a week!
A replacement lasted about six months, and failed the same way again. A
change to one more Amperex tube and the amp has run even since.

 I have autopsied a number of grid-fil shorted 3-500Zs that were
> funtioning normally before their grid choke imploded, and/or the grid-I
> meter and/or shunt exoloded, and/or the vhf parasitic suppressor resistor
> more than doubled in resistance without showing external signs of
> heating.  (tubes removed from amplifiers that had c. 60% of 29A of
> inrush).

Carbon resistors age with time and heat. They are notorious for that. But
then I'm sure you know (and choose to ignore) that fact, so I won't rehash
the same old facts of life you choose to reject.

> -  A friend took his SB-220 to work, coupled the anodes to a spectrum
> analyzer, and found there was damped-wave ringing at c. 110MHz at the
> anodes when sending 50wpm dits, even though grid current was normal -
> thus, no vhf oscillation was present.

I doubt it. In order to have damped wave "ringing", there must be a
transient with a response slope much more rapid than the frequency of
ringing,

Even if there were ringing, which I doubt, it goes nowhere towards proving
anything except the amplifier is stable. If it were  not stable, it would
oscillate.

Many or most people don't know how to use spectrum analyzers properly, and
that could 9or could not) be the root of what he thought he saw. If you
collect enough data and discard what you don't like, you will eventually
have something to support your wild theories Rich!

> was a feedback path between the SB-220's anode output and cathode input.
> Unfortunately, there is 0.3pF of feedback C.  At 110MHz, 0.3pf = c.
> 4800-ohms of XC.  This doesn't seem like much until one discovers that
> the length of RG-58/U coax used at the cathode input of the SB-220 is
> resonant c. 110MHz.

First, the .3pF was taken as an equivalent at 30MHz, not 110MHz. The peak in
effective feed through occurs around 200MHz, when grids are grounded
properly.

Second, the length of cables mean little or nothing by themselves. It is a
complex circuit, with stray capacitances and inductances as well as the
input circuit itself part of the system. If you sweep the input of the tube
you will find the VHF impedance at the cathode is low.

The length of cable between by dummy load and amplifier is 1/4 wl at 21 MHz.
That does NOT mean the cable has a high impedance, will ring, or will
support any type of oscillation. Surely even you know that much about
transmission lines!

> --  As you may recall, during our last telephone conversation, you told
> me you had seen many SB-220s that had sustained what appeared to be
> parasitic oscillation damage.

No I don't, but I'm sure you would invent such a conversation if it
supported you!

> >Exactly where does the bending force come from Rich?
>
> Electro-magnetism, Tom.  Get thee to a welding shop and observe arc
> welder cables when an arc is struck.

Let's go into that, and avoid all your diversions. You waste too much time
with spurious emissions!

You claim the force is caused by magnetism. Let's stick to that and see if
it makes sense.

http://www.w8ji.com/vacuum_tubes_and_vaccum_tube_failures.htm

3-500Z's reach emission saturation at about 100mA per watt of filament
power. ~15*5=75watts for a 3-500Z.
~75 times 100mA is 7.5 amperes.

The emission current at full saturation, when the electron cloud is fully
depleted, is HALF of the RMS filament current and 1/3 of the peak AC
filament current during normal operation!

The total force on the filament helice due to that magnetic flux would be 11
grams when distributed over a 7 cm area for a 21 ampere current in a typical
helical structure the dimensions of the filament.

The normal force from filament helice to filament would be much larger just
from normal AC filament current than from full saturated emission in one of
your imaginary parasitic "events".

Which leaves the following question hanging:

If normal peak filament current is 1.4141*15= ~21 amperes, how can an
additional ~7.5 amperes cause a filament to bend? Why doesn't the stronger
force at a lower frequency, especially during inrush, bend the filament? Do
you really expect us to believe a 20-30% rise in peak current in a time
interval too short to measure on a meter pointer would physically bend one
of the hardest metals?

73 Tom