[Amps] Big Bang in the Thermionic Cosmos - TSPA

[John T. M. Lyles]

This morning on NPR, heard that scientists have gotten enough data 
from a new satellite studying kozmic debris to say that the universe 
is 13.7 billion years old, +/- 1%.

Now, I wonder if we hams will ever come to agreement on something 
here, which has used plenty of bandwidth here for the years I have 
been reading this. I was almost going to post this last week, then 
decided "no, I have plenty of work to do and besides, it hasn't 
mattered how many times the references have been quoted, or 
experience has been expounded here". The same bent filament helix and 
parasite theory keeps getting shoved out as the  only possible way 
that certain triodes have been dying. So one can assume that this 
will perpetuate forever here, until the principals fade away into 
senility or SK. However, as Ian said, it is important to restate some 
facts for newcomers and for the record, as so many have come to 
believe only a certain science is possible, based on the repeated 
assertions of a few on the web.

Thanks for repeating the outgassing phenominum Ian. I wholeheartedly 
agree with it, as it is based on 70+ years of work in the field of 
high power tubes. It happens quite often in our work with larger 
tubes here and thank goodness it is explainable. Otherwise our 
technicians witnessing this would have to be taught that there is 
much more magic in radio frequency engineering  that cannot be easily 
explained. We understand parasites, can measure the presence of them, 
and can find the sources in the circuits if we are careful. What we 
don't prescribe to is that VHF parasites are so prevalent to bend the 
helix of filaments regularly, or sputter gold in a planar triode, or 
other topics most suited for nightime AM talk radio. Some or most of 
the failures can be explained by various theories, and MOST can be 
explained by flash arcs inside tubes.

Below I have quoted paragraphs and Rich's replies to them - which are 
quite predictable (he has a vested interest in his parasite 
cure-alls for his business as well as his technical integrity - those 
oft' quoted 1980s QST articles and the subsequent 'eimac employee 
Foote' letter). I don't disagree that nichrome wire makes a good 
suppressor, having built-in resistance that gives the desired result. 
In the 1980s I used it for some chokes and suppressors in some high 
powered amplifiers and oscillators at two companies that I worked 
for. One supressor design was patented, for a particular VHF cavity 
amplifier using Eimac tetrodes of up to 20 kW plate dissipation 
rating.

I have Richs anode suppressors in my SB220 at home, and they seem to 
work well, in place of the stock Heathkit suppressors.  But I cannot 
fall to being a " blind-faith believer" from the continually 
insistance that so many of the failed amplifier tubes are from the 
obvious VHF parasitic mode. The autopsied carcasses do not 
demonstrate this.

We have all been through this fifty times now. It is sad that Bill 
Orr is deceased (he used to read here), that he is being dragged in 
as having not understood bypassing grids on triodes, etc. Bill Orr is 
probably one of the most respected ham/tube engineers of the 20th 
century, as I have learned much from his many handbooks and articles, 
and chatting with him. I can not write off his designs. I have 
learned a lot as a ham from ARRL HB, but the W6SAI  Radio HB is 
required reading as well, for complete handling of RF amplifiers.


>  >But more gas continues to slowly evolve into the "vacuum" space. This is
>>a perfectly normal process, even in a tube with perfect vacuum seals
>>(leakage is a totally separate problem). To maintain the quality of the
>>vacuum throughout the life of the tube, the manufacturer creates a
>>specially activated metal surface inside, called a "getter". The getter
>>will react chemically with any gas atoms that strike it, and will keep
>>them trapped on the surface. It's a kind of passive, maintenance-free
>>vacuum pump.
>
>**  Concurrent with a big bang, the circuit breakers typically trip. 
>Does the gettering process take place when the tube is cold?   If it
>does, can the gettering process take place in the time it takes to remove
>the tube from the amplifier and test it for gas with a high-pot?

The gas which causes bang is probably not going to show up on the 
high pot afterwards. For one thing, if the energy is limited properly 
by a good power supply design, it helps to clean up and condition the 
tube for further operation. On our big triodes and tetrodes with 
built in ion pumps, we see the results of the bang, after the power 
supply crowbars, as a brief spike in the ion current meter. Often it 
is very minimal, and can only be noted if you happen to be watching 
the current. We usually watch this meter as we are raising the power 
or duty factor of big tubes as we are working them at higher levels 
for the first time.


>  >If the arc is extinguished quickly and not too much energy is dumped
>>into the tube, the tube can recover completely. There may not be much
>>visible evidence that the arc ever occurred (depending on the tube
>>construction). If the tube was hot, the getter can collect the gas
>>within a few seconds.
>
>**  reference please

See below, also experience with over one hundred tubes in my job.

>  >So it's all a matter of time-scales. An arc can happen faster than the
>>getter can handle the gas release - but the getter can do its job faster
>>than anyone can possibly pull the tube out of the amplifier to test it.
>>
>**  please explain the reason for a bent filament helix after a big bang.

Here is what I was planning to post a week ago, and then decided to 
let it rest. Here are some references, for the record, the archives 
and the newcomers  here. Please do not request reprints of these from 
me, as I have not scanned them, and am not being paid to be another 
RF consultant for  amps at contesting.com. One should be able to find 
them at a resonable technical library in a corporation or university. 
Maybe when I retire someday I will start a website reprinting old RF 
and tube papers. I have put a few of them up on the AM Window 
tutorial pages, those related  to AM transmitter designs.

I had looked these up on the subject of flash arcs and their damage 
in big tubes. I encounter occasion burps in large tubes in my work, 
usually they trigger a crowbar and no lasting damage occurs.  Most if 
not all of them are what I would term outgassing related or a 
breakdown of materials inside the tube as the average power is raised 
above previous operating points. Most high power transmitter and 
industrial RF users are accustomed to a sometimes lengthy 
conditioning phase of 'breaking in' a new tube in the rig.

Rich (AG6K) often refers to Flyer's IRE paper from the early 1930s on 
the WLW 500 kW RCA rig, in which much early work on suppressing 
parasitics was published. This paper does much to demonstrate ways to 
eliminate parasitics by balancing amplifiers, shortening leads, 
deQing stray resonances, using good bypassing, decoupling power 
supplies and so on. Fyler discusses trigger parasites, which pop up 
only during part of the cycle of an RF or AF amplifier. Also negative 
resistance effects of grid current, making dynatron oscillation. 
Choke coil parasites, lack of neutralization parasities, relaxation 
oscillator parasites, TGTP modes which can cause parasites and 
others.  The importance of this paper is that they succeeded in 
building a 500 kW MW transmitter at that time-1935. They had many 
parasites to conquer. 

Fyler also discusses a surge phenomena in transmitter tubes called 
'gas flash' or 'Rocky Point effect'. It is stated as another type of 
surge which can stimulate trigger parasites (which cause high 
voltages to pop up at unlikely places in the circuitry). To quote 
"The random occurance of the flash arc in vacuum tubes, frequently 
not concurrent with modulation peak potentials, indicate spasmodic 
gas-pressure variations. The  intensity of the visual flash arc 
varies from a barely noticeable incipient breakdown in the tube 
producing a momentary audible ping, to an intense power arc resulting 
in a virtual short circuit between tube anode and cathode or grid 
accompanied by a loud metallic sound from the anode. It is possible 
to burn out the filament and grid wires with a sustained arc.


Two of the original studies on flash arcs - which were originally 
called Rocky Point effect - were published in the 1930s.

"The Flash-Arc in High power Valves" by Gossling, GEC Ltd, Wembley, 
in Jour. IEE, V. 71, page 460-487, 1932. Its a long treatise on the 
subject, and parasitics are discussed as well as other causes. Most 
of the problems were found in certain valves made in a batch for one 
particular transmitter. A lot of this paper discusses evidence of 
arcs or  bursts inside tubes. The main points are that paralleling a 
lot of tubes is dangerous (a common practice then to get higher 
power) as the energy deposition can be quite high in one valve if it 
arcs internally. Also that the tubes seem to clean up from the flash 
arcs if the energy is limited.

"A Discharge Phenomenon in Large Transmitter Valves" by Heyboer, 
Philips Technical Review, V. 6, #7, page 208-214, July 1941.
This one does discuss the forces which can bend the primative 
filament wires oftransmitting valves. Heyboer calculates the 
mechanical force on a filament wire. It is one of the first 
references that recommends a series resistor in the plate supply lead 
to prevent damage. The actual oscillatory events are recorded and 
predicted in experiments with a filament in an evacuated bottle. They 
are damped oscillations from the inductance and capacitance in the 
power supply and interconnections, not VHF parasitics. On the order 
of a few KHz, and only for 20 cycles or so. The evidence is 
thoroughly presented in this paper and the fix is obvious: limit the 
deposited energy.