Looking for pointers on Deflection Amplifier Output Stage Design

[LukeSimon]

Can somebody point me towards books, websites, or published journal articles that fully explain deflection output stage amplifier design?

I have been reading Grob's "Basic Television" textbook. Bernard Grob was a head engineer and instructor at RCA during their golden era, when RCA was pretty much inventing every technological innovation in television.

The third and fourth editions of "Basic Television" are great as they cover black and white, color, tube, and solid-state TVs. Grob explains in sufficient detail everything about a TV needed to build a custom circuit from scratch... everything except the precise output voltage and current parameters for the output stage of a deflection amplifier.

Grob gets VERY close, describing the details of how to design sync separation, phase detection, oscillator, sawtooth waveform, and he covers the high level design of the power amplifier output stage... but when it comes to the exact formulas to use for the deflection output stage amplifier and impedance matching transformer, specifically the output voltage and current needed to deflect the raster by X-degrees horizontally and Y-degrees vertically... he just doesn't provide the necessary details.

I am looking for enough detail to DIY a custom tube TV circuit from scratch. Similar to the amount of detail available in the DIY tube audio community, where it is clear how much voltage and current the audio amplifier's output stage needs to provide to properly drive a loud speaker with a given specification. So if you have any pointers, even a retired TV engineer that I should try to contact, please help! I want to try to spark a DIY TV community by building a custom TV circuit and publishing the designs open source, similar to the DIY tube audio community.

[maxhifi]

Unlike tube audio, not many people have walked this path in the internet era. I would suggest primary sources. A lot of great info in the periodicals of the day. Start by digesting everything tv related you can find ih RCA Review, Electronics, Radio and TV News, etc. You can find much info at www.worldradiohistory.com, but it will be basic research - no spoon feeding here. The info is there but finding it is another question! If this project bears fruit I'm quite interested to learn about it!

At least these days it is online and free! This kind of info used to live only in corners of technical libraries.

Also keep in mind that no "tv cookbook" exists, because tvs were too complex to DIY, and commercial products contained trade secrets. Most educational materials were aimed at service men not designers
Speaking of which Google's patent search may also be useful.

[jhalphen]

Hi to all,
Hi LukeSimon,

Here's a similar endeavour by two French guys, Frederic A. & Pierre G. on their web site.

A Miniature 2" CRT Monitor using a Scope tube (Green phosphor).
- All tube design
- Every important section (Scanning, Video Amplication, Sync, Power supply) is studied in fine detail with an analysis of the Pro's & Con's of each design. The study is divided into 19 chapters for each sub-assembly.
- Loads of Scope photos, construction snapshots, voltage/current measurements.

http://www.cfp-radio.com/

scroll down to "Television"
then "Etude autour d'une TV-moniteur miniature"

Link via Google translate Fr to English :

https://www-cfp--radio-com.translate..._x_tr_sch=http

Sample off-screen photo of the finished monitor (see below)

Best Regards
jhalphen

[LukeSimon]

Originally Posted by jhalphen

Here's a similar endeavour by two French guys, Frederic A. & Pierre G. on their web site.

A Miniature 2" CRT Monitor using a Scope tube (Green phosphor).
- All tube design
- Every important section (Scanning, Video Amplication, Sync, Power supply) is studied in fine detail with an analysis of the Pro's & Con's of each design. The study is divided into 19 chapters for each sub-assembly.
- Loads of Scope photos, construction snapshots, voltage/current measurements.

http://www.cfp-radio.com/

This is amazing and very close to what I am looking for! Thanks!

My goal is to design something that will work with late 1990s era 90-degree deflection fine dot pitch color CRTs around 13-inch in size, such as the M34AFA13X02. There are plenty NOS CRTs from that era out there, especially from places such as https://www.relltubes.com/ My understanding is that the yoke designs starting in around the 1990s for "bubble curved" screen, 90-degree deflection CRTs (like the M34AFA13X02 ) are wound in a way that automatically corrects for pincushion distortion. So that will make the circuit design more simplistic. However, these yokes use parallel windings and a much lower series inductance than the yokes described in the 1950s thru 1970s television theory textbooks that I have access too. Grob mentions parallel yoke windings in his 1970s edition of his textbook, but it sounded like a new technology at the time and so he didn't go into any detail other than mentioning it.

That French project used a much smaller deflection degree than 90 degrees. It is also not clear from a brief viewing what type of deflection yoke is being used, and it may be deflection plates?

Originally Posted by maxhifi

Start by digesting everything tv related you can find ih RCA Review, Electronics, Radio and TV News, etc. You can find much info at www.worldradiohistory.com, but it will be basic research - no spoon feeding here.

This is what I am currently doing. The guy that runs that website is providing an incredibly valuable service to the world. I hope to help slow the fading of this technology into distant history by reading and digesting as much as I can from that site... and hopefully creating a cookbook DIY TV project to give back.

Next on my list after Grob's "Basic Television" is read Zworykin's "Television" 2nd edition. Zworykin literally invented television, and founded RCA's television division. I've paged through the PDF copy of his book that is available on www.worldradiohistory.com. I see he has an explanation of many formulas for electromagnetic deflection. Zworykin is a once-in-a-hundred-years caliber person, and his 1000 page "bible" is intimidating. Hopefully it has enough details for what I am after.

[Electronic M]

If you are trying to design a tube sweep circuit to drive a solid state era yoke you may want to look into matching transformers....In the 1970s there were test jigs built with a deltagun CRT and tube deflection style yoke that came with matching transformers that allowed them to work with solid state chassis...If you can use a matching transformer to drive a tube yoke with a solid state chassis then you should be able to use a matching transformer to do the opposite.

[LukeSimon]

Originally Posted by Electronic M

If you are trying to design a tube sweep circuit to drive a solid state era yoke you may want to look into matching transformers....In the 1970s there were test jigs built with a deltagun CRT and tube deflection style yoke that came with matching transformers that allowed them to work with solid state chassis...If you can use a matching transformer to drive a tube yoke with a solid state chassis then you should be able to use a matching transformer to do the opposite.

I was thinking that an impedance matching transformer would be needed for both horizontal and vertical deflection output stages. Similar to how tube audio amps typically have an output transformer to step the output voltage down and the output current up. Grob describes a tube amp output stage for vertical deflection and the need for a transformer to couple the beam power pentode plate to the vertical deflection coil. But unlike his chapter on video amplifier design, which is a step by step cookbook, Grob doesn’t specify step by step formulas for determining the impedance matching transformer parameters and beam power amplifier parameters for a given yoke inductance.

Not knocking Grob, by the way. His book is incredible and provides cookbook level step by step details for literally every other aspect of a TV chassis. I think deflection tech is the one thing that changed the most throughout the era of CRT displays. So unlike his other chapters, he kept the explanation at a high level so that it wouldn’t become quickly outdated. Searching scholar.google.com, it appears as though active research was going on in computer driven yoke design up until the early 2000s.

[Electronic M]

If you tracked down a test jig with the impedance matching you could probably reverse engineer it. The 70's RCA jig I have supports everything from the 1955 RCA CTC-5 to stuff from the 90s according to the setup aeapter books the shop that used to own it sucribed to...

[LukeSimon]

Originally Posted by Electronic M

If you tracked down a test jig with the impedance matching you could probably reverse engineer it. The 70's RCA jig I have supports everything from the 1955 RCA CTC-5 to stuff from the 90s according to the setup aeapter books the shop that used to own it sucribed to...

Can you share schematics? Also, can you take impedance and inductance measurements of the transformers you have? We should take inspiration from the DIY tube audio, community and pull together DIY TV schematics as a community. Repair and restoration are only part of what is needed to keep tube television technology alive. Open source DIY schematics that can be simulated in LTspice would be an amazing educational tool for many.

One trend in CRT electromagnetic deflection technology was towards lower impedance, lower inductance yoke windings, as well as from series windings to parallel windings (which helps lower impedance and inductance). I don’t understand why that trend occurred though, but it appears to be aligned with the transition to solid state.

Basically, I want to back port many of the improvements that occurred in the solid state era to a pure tube chassis such as the improved yokes (better geometry), higher CRT G2, focus, and acceleration voltages (better focus and spot size), finer mask dot pitch, and smaller G1 aperture (smaller spot size).

Another improvement that I will try to back port: some of the compactron tubes that were designed for video amplification are small and lightweight enough to plug in to a printed circuit neckboard that has a design similar to a late 1990s solid state neckboard but with compactron amps as opposed to BJT transistors. The advantage would be reduced parasitic capacitance, inductance, and resistance in between the video amplifier output stage and the CRT cathodes. This would allow for higher video bandwidth at the same plate current, or, lower plate current at the same bandwidth. Lower plate current would prolong the tube amp’s lifespan.

I am torn on whether or not to cheat and use a 1990s DST flyback transformer. They have solid state rectifier diodes for the G2, focus, and high voltage anode lines. The majority of the tube audio community is OK with solid state power supplies, and keep the signal paths pure tube. Tube TVs have 2 power supplies: flyback power supply and B+ power supply. At first I will probably cheat and make those solid state, and later design non-solid state replacements. The non-solid state flyback power supply designs are very impressive, especially the ones that use a shunt diode tube for regulating the CRT’s high voltage. Most 1990s solid state TVs lacked high voltage regulation and yet pure tube TVs in the 1960s had it! A great example of newer tech sometimes taking a step backwards.

[old_tv_nut]

Originally Posted by LukeSimon

...
One trend in CRT electromagnetic deflection technology was towards lower impedance, lower inductance yoke windings, as well as from series windings to parallel windings (which helps lower impedance and inductance). I don’t understand why that trend occurred though, but it appears to be aligned with the transition to solid state...

Actually, you said it yourself - it's because of transistors being high current, low voltage devices compared to tubes.

[zeno]

First a correction. Almost all solid state ( SS ) sets do have HV regulation.
Its all done with the main power supply.

Its an interesting project. If you use a modern in line jug I would
try to work with the yoke it has. You may look into Zenith EFL tubes also.
They could light up a room & very reliable.

Another thought was what Panasonic & Sony did in the early 70's. They
had separate hoz sweep & HV stages. Just a cool idea if you want to
be different. It may also make some things easier.

Last note is if you eventually want to build a full blown OTA NTSC set
you can cheat with a Zenith IF strip (SS) & tuner of your choice.
Or if you want tubes look at the Zenith IF's on the sets from abt 1958.
They had 3 6BZ6 or 2 6EH7's & a 6EJ7. Those Zeniths had better
sensitivity & selectivity than ANY TV. They saw things other sets
were blind to.

73 Zeno
LFOD !

[LukeSimon]

Originally Posted by zeno

First a correction. Almost all solid state ( SS ) sets do have HV regulation.
Its all done with the main power supply.

Its an interesting project. If you use a modern in line jug I would
try to work with the yoke it has. You may look into Zenith EFL tubes also.
They could light up a room & very reliable.

Another thought was what Panasonic & Sony did in the early 70's. They
had separate hoz sweep & HV stages. Just a cool idea if you want to
be different. It may also make some things easier.

Last note is if you eventually want to build a full blown OTA NTSC set
you can cheat with a Zenith IF strip (SS) & tuner of your choice.
Or if you want tubes look at the Zenith IF's on the sets from abt 1958.
They had 3 6BZ6 or 2 6EH7's & a 6EJ7. Those Zeniths had better
sensitivity & selectivity than ANY TV. They saw things other sets
were blind to.

73 Zeno
LFOD !

I am hesitant to use a vintage CRT because they are rare and therefore should be reserved for restorations. Restorations are paramount for keeping this tech alive. My “Frankenstein” TV will use vintage vacuum tube diodes, triodes, and pentodes… which are less rare, thanks to the tube audio community hoarding them since the 1970s. I also have 6 NOS M34AFA13X02 CRTs, which is another motivator for taking a Frankenstein approach. I keep using the DIY tube audio community as a sort of guiding analogy. They often use vintage triodes with modern loud speakers, and they even use solid state power supplies. They keep everything non-solid state in the signal path. I am taking the same approach, with a 1990s solid state diode split flyback transformer.

[old_tv_nut]

I presume you are planning to use an existing yoke? That would be a good idea, not only for image geometry and convergence, but because you could measure the yoke currents and drive voltages that you need to produce in an existing set. Then you "only" have the problem of winding transformers that will match a tube circuit to a yoke that was designed for transistor drives without the nearly impossible job of determining yoke design.

Some things to know:

Vertical sweep operates the yoke as mostly resistive, with some inductance. In other words, you need a class A (or close to it) drive, and will have a voltage waveform that consists of a sawtooth plus some amplitude of pulse during retrace.

Horizontal sweep operates the yoke as mainly inductive, with a switch-mode horizontal output that applies an essentially constant voltage to the yoke inductance, producing a sawtooth current (in concert with the damper tube), and a large voltage pulse during retrace. The inductance producing the sawtooth is the parallel of the flyback transformer and the yoke. Impedance matching is obtained by the winding ratio of the taps for the horizontal output device and the yoke. To use a tube HO to drive a low impedance (transistor) yoke, you will need a much greater ratio than was used in the transistor set the yoke came from. This means the tube capabilities will be matched (higher voltage retrace pulse and and lower current during active trace compared to a transistor). In tube circuits the damper tube may need a separate winding ratio of its own, whereas transistor circuits often put the damper on the same tap as the output transistor. Transistor circuits might have HO collector retrace pulse on the order of 1 kv or less, while tube circuits might have multiples of 1 kv. Retrace voltage and retrace time are also affected by the value of the retrace tuning capacitor.

Service info typically says "do not measure" and doesn't tell you the amplitude of the pulse, which takes a special high voltage scope probe to measure, so they didn't want to tempt technicians to blow out their scopes.

By the time sets were in wide commercial production, no designer in their right mind would take the trouble to go back to basic theory to calculate winding counts, inductances and resistances unless forced to by radical changes in design, like going to transistor circuits. Instead, they woud typically take a similar design and modify it slightly to match the new needs (the new yoke impedance is x % lower, so change the winding ratio by that much) and then fine tune the prototypes on the bench.

The flyback transformer is also responsible for generating the high voltage. For least high voltage variation with beam current, you actually want the HV winding to resonate at three times (or even five times) the primary retrace frequency, to present as flat-topped a pulse to the HV rectifier as possible. This is a refinement that I wouldn't expect any DIYer to make, as it depends on the distributed capacitance of the HV winding. Since this was affected greatly by whatever insulating compound was used for the HV winding, it always was trial and error based on previous designs, modifying the number of layers and therefore number of windings per layer. The head sweep designer at Zenith developed a method mixing a silicone liquid bath with the same dielectric coefficient as the potting compound, so a flyback prototype could be run in a beaker of the stuff instead of actually potting it. This way, if the tuning was off, you could modify the prototype instead of starting from scratch.

You can get by without precise tuning of the HV winding, you just may get more HV variation than a carefully engineered production set.

[LukeSimon]

Originally Posted by old_tv_nut

I presume you are planning to use an existing yoke? That would be a good idea, not only for image geometry and convergence, but because you could measure the yoke currents and drive voltages that you need to produce in an existing set. Then you "only" have the problem of winding transformers that will match a tube circuit to a yoke that was designed for transistor drives without the nearly impossible job of determining yoke design.

I am definitely going to use the yoke that was designed for the late 1990s M34 CRT. As I mentioned earlier, this style of CRT has the yoke designed using computer simulations that allow for pincushion correction to come “for free” due to the shaping of the windings. I have taken impedance and inductance measurements of the windings and they are much lower than the values I see in non-solid state TVs.

Originally Posted by old_tv_nut

Some things to know:

Vertical sweep operates the yoke as mostly resistive, with some inductance. In other words, you need a class A (or close to it) drive, and will have a voltage waveform that consists of a sawtooth plus some amplitude of pulse during retrace.

Horizontal sweep operates the yoke as mainly inductive, with a switch-mode horizontal output that applies an essentially constant voltage to the yoke inductance, producing a sawtooth current (in concert with the damper tube), and a large voltage pulse during retrace. The inductance producing the sawtooth is the parallel of the flyback transformer and the yoke. Impedance matching is obtained by the winding ratio of the taps for the horizontal output device and the yoke.

Grob does a good job of describing the variations in the waveforms required for vertical or horizontal deflection, as well as cookbook level details for how to design circuits for producing the waveforms. Where he stops short is the precise formulas for output stage voltage, current, impedance, and inductance matching. I am afraid I will blow up my yoke unless I have a formula for calculating reasonable starting values that I can then tune in a prototype circuit. Remember, I am using a late 1990s yoke in a circuit design that is coming from a 1960s textbook.

Originally Posted by old_tv_nut

To use a tube HO to drive a low impedance (transistor) yoke, you will need a much greater ratio than was used in the transistor set the yoke came from. This means the tube capabilities will be matched (higher voltage retrace pulse and and lower current during active trace compared to a transistor). In tube circuits the damper tube may need a separate winding ratio of its own, whereas transistor circuits often put the damper on the same tap as the output transistor. Transistor circuits might have HO collector retrace pulse on the order of 1 kv or less, while tube circuits might have multiples of 1 kv. Retrace voltage and retrace time are also affected by the value of the retrace tuning capacitor.

This is super helpful for me. I am going to add this to my project notes. Thanks!

Originally Posted by old_tv_nut

Service info typically says "do not measure" and doesn't tell you the amplitude of the pulse, which takes a special high voltage scope probe to measure, so they didn't want to tempt technicians to blow out their scopes.

I have a 3kv probe and a 40kv probe. The horizontal output pentode specs say the plate can handle up to 5kv. Having spent most of my life with solid state TVs and horizontal output transistors, I laughed when I saw the crazy voltages horizontal output tubes deal with. 5kv would immediately destroy a horizontal output transistor 😂. In general, tubes are better at handling high voltages, and transistors are better at handling high current.

Originally Posted by old_tv_nut

By the time sets were in wide commercial production, no designer in their right mind would take the trouble to go back to basic theory to calculate winding counts, inductances and resistances unless forced to by radical changes in design, like going to transistor circuits. Instead, they woud typically take a similar design and modify it slightly to match the new needs (the new yoke impedance is x % lower, so change the winding ratio by that much) and then fine tune the prototypes on the bench.

It is totally understandable that practical engineering would work this way, but also for the current state of things, very worrying for keeping the technology alive today. As these engineers retire and move on, memories fade, and the art of the technology dies forever… unless we do what the DIY tube audio community has done. They have taken their favorite technology to the point where new tubes are being manufactured to this very day such as the Western Electric 300B vacuum tube (https://www.westernelectric.com/300b).

I know I will have to tune values of parts in a prototype circuit. I just wanted a more airtight understanding of the output stage so that my starting point is closer to not blowing up my yoke 😂

[AlanInSitges]

This is such an awesome project! I have often thought it would be cool to do something similar but my level of knowledge is way below what's required to even start. Here's hoping this really takes off.

Speaking of which, and following on with the general theme of deflection and yokes, and not intending to hijack this excellent discussion, I wonder if someone can tell me if all of these huge pulses and inductors, etc., are strictly necessary. I mean, if one were to start over from scratch, in a vacuum, and set out to build a circuit whose requirements are to move an electron beam from A to B over a precise period of time, and given that we are already in possession of a waveform that exactly graphs that movement, would it be possible or feasible to simply drive a yoke winding from a simple power amplifier circuit? Both V and H?

I don't know how much juice is really needed to deflect that beam five inches on a 34CM CRT, but I do remember as a kid back in the 80s building a lissajous project from Radio-Electronics out of a 19" tube B&W set and a second yoke, and powering it from a 6L6 amplifier, and that it could very nearly move the beam off the screen. I've often wondered why, if that's the case, they didn't just put an STK chip or similar in these sets along with a SMPS for the HV.

[old_tv_nut]

Originally Posted by AlanInSitges

This is such an awesome project! I have often thought it would be cool to do something similar...Here's hoping this really takes off.
...

I totally agree!

Regarding output amplifier type:

The horizontal sweep is operating a load that is mostly inductive at horizontal frequency. This is because of the number of windings needed to create a magnetic field to deflect the beam. This dictates that the output circuit apply a constant voltage to the yoke during sweep, so the output ends up just switching on and off. This is convenient because it reduces the power handling requirements (wasted energy) tremendously. The horizontal output device has to conduct for a bit more than the right half of the sweep (to make up for resistive losses) but most of the energy is stored in the magnetic field and transferred to the resonant flyback capacitor and then to the damper diode to make the left half of the sweep.

In both tube and transistor horizontal outputs, you want the switch-off to be as fast as possible, to prevent having both current and rising retrace pulse voltage present simultaneously. In tubes, it's a matter of total power dissipation, and in transistors it's much more critical because the safe operating area (limit of simultaneous voltage and current) is very small. A bad design will kill an output transistor on the first retrace, before you can even figure out what happened.

The vertical output amplifier is much closer to linear, modified somewhat, because at vertical frequency the yoke is mainly resistive but partly inductive. So, it already operates mainly as you proposed.

Electrostatically deflected tubes use linear drive waveforms with incidental power losses in the amplifier but none in the deflection plates.