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The troubles with Snivets




 
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Author Topic: The troubles with Snivets  (Read 4579 times)
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John K5PRO
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« on: January 11, 2014, 03:14:03 PM »

On another topic here on "just how bad are transformers driving class B modulators", IN3IEX mentioned a particular 6KM6 tube, and I digressed about "Snivets", mentioned on the datasheet. W3JN quoted this:

"One of the phenomena that has appeared in the image reproduced in the television receiver is known as the snivet. Snivets are thin parallel vertical lines which may appear at one or more positions of the reproduced image. lt has been suggested that the production of such thin vertical lines is due to Barlthausen oscillations developed in the horizontal output tube. Barlrhausen oscillations may be produced in a tube when a positive electrode is arranged between two more negative electrodes. The electrons present in the tube are repelled by the more negative electrodes back toward the positive electrode. The oscillations caused by the alternate attraction and repelling of electrons occurs in a frequency range determined by the dimensions and spacing of the electrodes and the potentials applied thereto. Such oscillations may produce radio frequency radiation in the video carrier frequency range of the television receiver. T is radiation will then be detected and pass through such receiver in substantially the same manner as the video signal. The received radiation is then reproduced on the cathode ray screen in the form of the above-referred to thin vertical lines. "

Interesting stuff indeed. I was looking up beam power tube info and found an RCA treatise on Pete Millets audio website, from one of their internal tube design publications. It seems that receiving tubes got a real boost in development when television came about. Trying to get linear sawtooth deflection fields on the yoke of a CRT wasn't so easy with triodes and tetrodes. Nonlinearity and bad performance would show up easily on a picture, not so obvious with audio and RF circuits for radio. So they really sharpened their pencils at RCA.

Beam power tubes were the big thing out of RCA (and other licensees) and the idea of having sharp electron beams that 'necked' down between screen grid wires to prevent interception and having a space charge region out between screen and plate to repel secondary electrons from the plate were great inventions. Along with this came sharp characteristic curves, the knee on the left side seen in the plate current vs voltage plots for beam power tubes. Turns out that they claimed Snivets were caused by an instability in the virtual cathode region when the plate voltage is swinging low (on the right side of the picture - from horizontal deflection output tube). During these abrupt changes in plate current (at the knee) HF bursts (like barkhausen) would emit from the tube. Early TV didn't have all the shielding, and it get into the tuner. Tube engineers had a paradox, making better beam power tubes had a sharper knee, and high plate current at the knee, so so more snivets. Some of those little embosses and perpendicular flaps that are put on the plates of those tubes were intended to make the plate plane more irregular and try and stabilize the pesky snivets.
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w3jn
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« Reply #1 on: January 11, 2014, 03:44:39 PM »

Thankfully, snivets are no longer a problem in TV sets (although I do dimly recall chasing some down in a solid state RCA chassis some time ago).  But it's still good to be aware of this phenomenon when using beam tubes in a transmitter (or in the audio output of a receiver).
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John K5PRO
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« Reply #2 on: January 11, 2014, 03:47:46 PM »

More receiving tube trivia from television applications came from the RCA Harrison, NJ design facility. Those among us who worked in TV repair are probably more familiar with these.

Sleeping Sickness - development of cathode interface resistance from leaving tubes idling in cutoff, first noticeable in early tube digital computers. Eventually the tube would not conduct, when it was supposed to change states. Fixed
by reducing silicon impurities in the barium coatings for cathodes (that were added for improved emission). Early 6SN7-GTB used as horizontal oscillators in TV would loose sync from the same problem, and the low silicon improved tubes helped.

Blackout - a semi-insulating layer got deposited on the grid wires during manufacture. This would develop a bias similar to what grid-leak bias does when grid was driven positive. It had a negative temperature coefficient, so would manifest as loosing horizontal or vertical sync for a while after a set was turned on, until it finally would stabilize.

Bulb or Mica charge - this would produce a bright vertical line or a jiggy cogwheel pattern on picture, due to too high a voltage gradient in the tube. The little slots cut in the mica insulator plates that held tubes electrodes in place were a solution to this.

Spooks - A strange vertical line or aberration on the left edge of the raster, often near the blanking region or off screen. It was apparently very elusive to the tube engineers and hence the name. Sometimes it would appear on a nearby TV in a service shop. Or would move across the screen randomly. Spooks originated from the rapid plate current risetime (100 nS) in the damper tube in the horizontal compartment. This generated RF noise.  Chokes, shielding all improved this.  

It seems that the barkhausen effect was the best theory as a source of Snivets, as the loadline of the horizontal output tube wasn't a simple straight resistive line but more elliptical. The flyback transformer drove the plate voltage far into the knee region of the beam power tube performance. A particular 25CD6 tube showed a very strange loadline at low plate voltage that had little loops and tics in it. Some of these negative resistance regions were strong sources of oscillation. So perhaps all the cuts and wrinkles in the plates were not so helpful.
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KA0HCP
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« Reply #3 on: January 11, 2014, 04:13:11 PM »

Care to address "Squegging"?
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John K5PRO
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« Reply #4 on: January 11, 2014, 07:00:44 PM »

I didn't see that in the RCA television tube literature but I do remember it in the HP606 signal generator alignment instructions. It was an ugly characteristic, if I remember correctly.
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WU2D
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CW is just a narrower version of AM


« Reply #5 on: January 11, 2014, 08:53:02 PM »

My Blonder Tongue UHF converter fell off the set and scared the cat. Could one of you come over and change out my damper diode? There are already two picture tube brighteners on my Dumont. Can I go for 3?
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These are the good old days of AM
KA2DZT
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« Reply #6 on: January 11, 2014, 09:44:05 PM »

Dumonts,  heaviest TVs ever made if you remember the early 50's TVs,  Solid 3/4" plywood cabinets,  separate audio chassis the size of a small xmtr.  OBTW I have a BT UHF converter (NIB) if you need it.  Sorry, I'm using all the tube brighteners I have.

Fred
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Patrick J. / KD5OEI
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« Reply #7 on: January 12, 2014, 02:23:15 AM »

Back in the day, microwave ovens would cause similar disturbances. Unfortunately the modulation here, clean as it may be, makes all kinds of orderly lines and clear audio in the TV set, and the sound comes on the PC well. I do not know what the neighbors experience. No one has complained and I am sort of afraid to ask, lest I unwillingly be assigned 'ownership' of every messed up reception issue they experience.
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w3jn
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« Reply #8 on: January 12, 2014, 09:29:47 AM »

There's a Youtube video of some buy checking horizontal output toobs on a toob tester, and he found a bunch that self-oscillated in the tester, to the point where they killed the TV in the same room.  Enter "Barkhausen" in the youtube search bar.
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WQ9E
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« Reply #9 on: January 12, 2014, 10:43:13 AM »

There is a nice little article on "spooks" by MB Knight of RCA in the March 1953 issue of Radio and Television News starting on P. 64.

The September 1959 issue of Electronics World (briefly lived successor to Radio and Television News) has a good treatment of multiple forms of spurious oscillation, starts on P. 57.

I remember reading these articles a long time ago.  Unlike one of my colleagues I don't have perfect issue date recall but fortunately that isn't necessary with online search engines Smiley

Both of these issues are available on the American Radio History website.
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Rodger WQ9E
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