>> While trying to source a problem in a single 3-500z amplifier, I observed
>> a flash and failure of the tube fillament structure. The amplifier was a
>> homebrew based on the Henry amplifier line and the many Handbook designs.
>What were the conditions when the tube flashed? Was the PA drawing
>grid current withjout drive? Were grid or catode currets excessive?
The amplifier was on, in the "receive/standby" mode drawing a small
amount of unknown grid current and excessive (for standby mode) cathode
The unit was terminated into a proper 50 ohm termination, (cantenna)
>> This amp was plagued by "diode hash" in the standby mode which turned out
>> to be a parasitic just about 50MHz. This was the second tube to "die" in
>> the same amplifier within two months.
Quite interesting to see the cathode current eratic with rotation
of capacitors. Two or three receivers on VHF in the same room were
"swished" with a strong signal. Fortunately, I have the HP spectrum display
on most of the time and quickly set the Bandwith out to 100MHz. I could
easily see the amplifer output go up and down in frequency and amplitude.
Almost a nice oscillator. Also note the bias voltage for the higher anode
voltage was not enough. The installed zenner diode voltage was small for
the anode voltage.
The amplifier was placed back onto the reactive antenna by the
owner with reduced plate voltage (for the installed zenner). The
oscillation problem seemed to dissapear. The owner against my
recomendations proceeded to drive the amplifier with about 35 watts.
While adjusting the the tune and load controls, the flash. The fillaments
were then dark along with some circuit damage.
>To oscillate at 50 MHz, the grids must have had some seriously long
>leads or a tuned circuit near the area of 50 MHz.
There was a serious problem with the "Henry" type of grid current
metering. I dont' bother to measure problems there, I just rip out the
poorly designed circuit and install something that works.
>From my experiences, a 3-500z makes a hell of a powerfull oscillator around
>Even if the tube oscillates, any damage would have to be caused by a
>parasitic that indicated visable current on the elements.
Almost every parasitic I've run accross, causes some type of damage or fuse
blow. I've rarely been able to experience the moment as well as I
did with this amplifier incident.
>The grid current would have to be excessive, or the cathode current
>VERY excessive. Let me explain why....
Isn't a parasitic an undesired or excessive current? Just a question of
the source cause.
>The main cause of failure of a good tube element is rooted in
>thermal problems (ignoring mechanical shocks, like dropping
>the tube). In order to excessively heat an element, excessive power
>must be applied for a finite amount of time. In a tube like a 3-500Z,
>the saturated cathode current is about 100mA per watt of filament
>power, or about 8 amperes.
Both tubes were excellent in mechanical and emission condition. The tube
was tested in an amplifier for over an hour before placement in the
mentioned. I believe the owners original 3-400z was a new tube. It lasted
only minutes. (remember the 3-400z has more gain)
>As a matter of fact pulse duty charts show (operation with elevated
>filament voltage) 2000 volts of positive grid voltage causes the peak
>cathode current to just reach 10 amperes.
>Since that current causes electrons to smash into the grid, kenetic
>heating of the grid is the primary problem with heating. Grid
>dissipation is almost 10 kW during that condition. If the pulse
>duration is excessive, the grid fails much sooner than the cathode.
>In any case, the thermal lag of the filament and grid require some
>time to be expended at high currents (dissipations). The grid
>actually has to become white hot to melt and fail, since it is a
A very loud white flash is good enough for me. A very large rapid
increase of current would cause heating, similar to fillament inrush
problems which take out fillaments.
>The meter's response is much faster than the grid heating, and any
>current large enough to cause a grid failure would surely show
>as greatly excessive current.
The meter and shunt were both destroyed during the flash. There is a
point here I'd like to make in a later post about sizing meter shunts to
act as fuses. Less expensive than a tube. This would not ignore normal
>We aren't talking about a nanosecond pulse failure, this isn't an
>FET. It's a massive vacuum tube.
off topic comment
Repeditive nano second pulses are hard to generate with high voltage. The
common method is to employ a Marx Bank of which I've some experience.
Again, from my experience with rapid electron movement. By the time you
see or hear the flash, it's to late.
>At 50 MHz (the frequency you claim you measured the parasitic at) ,
>the tube can be operated normally at several hundred watts output or
>500-600 mA of total cathode current for DAYS without failure.
An approx visual measurement on the HP 8920A
around 50MHz. Yes, I've tuned large oscillators and amplifiers at VHF
prior to this.
>Common sense tells me if you had a parasitic strong enogh to wipe
>out the filament, it would have loaded the power supply heavily. It
>takes average power to cook the elements, not peak power. Time is
>absolutley a factor.
The power supply was well past overkill/excessive, mentioned in my first
post. This power supply has considerable energy potential.
> The big fix was to replace the garbage grid current metering circuit with
> a "cathode" return lead type. The grid pins of the 3-500z socket were then
> tied directly to the chassis ground through large low XL straps.
>Grids should ALWAYS be as directly grounded as possible in a
>grounded grid triode PA. There are many reasons, and stability is one
I've built and tested circuits with success that use grid pin metering, but
the Henry method is trash (what the owner started with). For an amplifier
with less operating bandwidth it might have merits in special
applications. Not anything I would use.
>One final question Skip, a 4-400 has LESS gain than a 3-500Z in
>grounded grid. So you had a LOWER gain tube fail.
No, I thought I wrote 3-400z. If not, I'll mention it again. The first
tube was a 3-400z.
>Were these tubes all brand new tubes? What was the inrush current?
>Did the HV positive supply line have an appropriate surge limiting
>resistance designed in, or was it a low resistance path?
No, well within guidelines, No, Yes
The first tube was a new 3-400z tested in an amplifier and used at full
output before installation.
The second tube was my personal test tube. Used, full emission and
used in my test amplifier at the rated output before installation in the
All shock and thermal considerations were properly dealt with. No the HV
supply was excessive and not limited in a method I would choose. It is
As mentioned before. The grid circuit was redone, supressors installed,
repairs and metering changes were done.
This amplifier now lives in service and as of my last conversation with it's
owner has full output with no surprises.
The second tube is still on my kitchen counter as of this morning.
I have not spent the amount of time I should to carefully evaluate each
responce given here. But I'm in a hurry to get some other things done.
Any questions or clarifications can be made in the near future.
Please feel free to ask.
Skip May wv6f
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