[Amps] 8122 and parasitic excuses TSPA

[John Lyles]

2 subjects in this email:
Burle Industries in Lancaster, PA is still very much in business. I talk with them almost monthly as we are a major customer for their super power tetrodes and triodes. I have seen numerous small tubes such as the 8122-sized Cermelox tetrodes in the office of their chief engineer, so I would say they can and would make it, if they don't now. But it won't be cheap as a Russian tube.


As for parasitics, W8JI (and other commenters) is on target. Without Richard Measures here to jump into his favorite debate, it is a little easier to speak about what many of us believe to be reality. I know this is a very polarized subject on this forum, but in my own experience of 30+ years in power RF, it has been noted that we tend to group anything that seems abnormal, like a flash arc in a tube, a bandswitch failure, high VSWR with incipient arcing of output networks or tuners, and burnt resistors, in the parasitic category. In Roswell, NM, some people also believe that a UFO did crash in 1947 and that the alien bodies were exhumed and studied by the government. I cannot force people to believe what I think on that topic, but parasitics are one excuse that gets overused among radio amateurs, broadcasters, manufacturers and end users of RF power systems. Certainly we have all witnessed their occurance in something, and sometimes the damage is permanent. But too often, ot
 her
causes such as tube gas, VSWR,
overheating, poor design of chokes, etc. bad connections, are the real cause. As engineers and technicians of our craft, we must remember to not sweep everything under the generalization of parasitics, until we can test properly using whatever instrumentation we can get our hands on. 

For example. last spring I witnessed some very strange things happening around one of the 3 MW power amplifiers at 200 MHz at work. They are plate modulated by a pair of 4CW250,000B tetrodes on a floating deck in series with the plate supply. All of the neon lamps on the fuse holders would light up, on that deck, even though the line fuses were good. In addition, a loud buzzing was heard on the PA system speaker in the building. I was called in on a Sunday to help our techs figure out what was going on, as the problem seemed to happen suddenly. Immediately, all of us thought "parasitic" or self oscillation in the modulator. With spectrum analyzer in hand, I proved that wasn't the problem. Shortly later, we also discovered that the bias power supplies for the 4CW250,000B's were running very very hot, and failed. All sorts of strange things were happening. It took us hours to finally understand that we had high level RF leak down the B+ cable from the PA to the modulator. The l
 ow pass
filter network
(L and C's) at the PA had a loose shunt capacitor, so kV of 200 MHz RF was riding up the B+ out of the cavity to the modulator, which is not RF tight like the PA. It wasted at least one 4CW250,000B (shorted) and several power supplies before we found the loose part, which had burning marks where the screw had been loose. All together it cost us over $50K in failed components, and downtime (overtime repairs too). 

After finding the real culprit, we felt better in that we actually found a physical cause to the problem, and it wasn't a mysterous parasitic as everyone had thought. 

73
John 
K5PRO

 

> 
> I think the problem is many people don't really understand 
> what causes problems, and so they look for one Devine answer 
> to all failures. The fashionable answer when someone doesn't 
> really know the answer is "parasitics".
> 
> A tube arcs momentarily, a very common event with high 
> voltage tubes, and it is a parasitic.
> 
> A bandswitch fails, a common event with antenna system 
> faults, poor wiring, poor switch choice,  or poor tuning 
> adjustments.... and it is a parasitic.
> 
> A plate choke burns out, common when there is a series 
> resonance, and it is a parasitic.
> 
> A carbon resistor in a suppressor goes off value, very 
> common for carbon comps operated in high temperature 
> environments, and it is a parasitic.
> 
> What you find is the people who don't really know what the 
> problem was blame a parasitic and say "that's common with 
> xxx amplifier" while the people who do understand the system 
> say "mine never had that problem".
> 
> That's why there is always such a disagreement.
> 
> 73 Tom