Valve line output stage operation

[Mike Phelan]

I have now updated a thread like this that I put on here a while ago, so I'll delete that (or Jon will) during the archiving.
I thought this might be useful for our members who have just started with valve sets. I will archive the thread after it has run for a while - any boo-boos and suggestions welcome.


Valve line output stages.
It is worthwhile giving this subject a bit of an airing, as there are probably a few misconceptions about its operation, and the basic principle is the same on both colour and monochrome sets from the post WW2 era to the demise of the valve.

As tubes got bigger and flatter, more EHT and therefore scan current was needed. Running at 10125 and eventually 15625Hz, the amount of energy wasted by turning the current off during flyback and that needed for the scan became unwieldy.

So, the ingenious reclaim or efficiency diode was devised; instead of the energy being wasted, most of it is put back, and as a bonus we have a high-voltage supply for the line output stage, CRT first anode and field oscillator. The unsung hero of the piece!

A few assumptions and conventions first:
·Voltages have all been rounded
·Zero forward voltage on PY is assumed.
·CStray is a combination of one or more actual capacitors and the capacitance of the transformer, scan coils and other things, and appears in series and parallel to the transformer – easier to call it one capacitor in parallel. There is usually a real capacitor to tune CStray to an exact harmonic of the line frequency.
·Tx is shown as a single winding; in practice it will be several, and includes the scan coils.

Ok – here we go. We’ll say that it is already running, Cb is charged, the spot is about ¾ way to the right and PL is turned hard on by its grid drive. The anode voltage is very low – say 50v – the valve is really a switch. We are at t0
Tx current is increasing linearly (L and C are very large as C is really the entire HT line) as the scan progresses and moves the spot to the right. The current comes from Cb and the HT via PY. As the scan progresses and Cb charge is used up, all the voltages on Tx fall slightly, so more comes from PY as its cathode approaches 300v. Energy is being stored in the form of the magnetic field of the inductors - LOPT and scan coils.

At t1, flyback is initiated by the PL grid being driven negative by the drive waveform. Now the fun starts. The valve is switched off; as Tx had a heavy current, this current cannot just be stopped. Tx and CStray are now a tuned circuit with (1) connected to Cb and sitting on 600v.
Tx is now a generator, not a source; the falling current gives the first part of the flyback and charges CStray to a very high voltage, because of the inductance x rate of change of current.
We are at t2.
Aside: Without any intervention, Tx and CStray would just go into oscillation until losses made it decay to nothing.

CStray immediately discharges into Tx, so the voltage falls and Tx current increases in the opposite direction, sending the spot to the left. As the voltage falls, point 2 drops below HT, and PY conducts.

This is t3 and the end of the flyback.
PY and Cb across the winding stop the oscillation by damping the tuned circuit, leaving the core magnetised and the current at maximum but in the opposite direction. Most of the energy from the scan has been recovered.

As PY is now conducting heavily points 1 and 2 cause current to flow into Cb, charging it to a slightly higher voltage.
Tx is now a generator, not a source, so the current is in the opposite direction to t0 – t2, but the voltage is in the same direction.
The current decays almost linearly towards t4, and the spot moves to the right. Eventually the current would decay to zero exponentially, but just before t4, when the spot is just left of centre, the positive grid drive from the line oscillator slams the PL hard on – the current through Tx now starts to increase linearly because of its inductance.
The time-constant is long and the voltage is about 700. The two overlaps on the current curves cancel each other out to give a fairly linear scan.
We have reclaimed enough of the flyback energy to provide about 40% of each scan, and as well, give us a 700v supply for other things.

The PL is switched off from the flyback to just before the centre of the scan, the PY is switched off during flyback and switched on hard at the first half of the forward scan, then only a small replenishment current flows when the PL turns on and "uses up" some of the boost voltage.

[FERNSEH]

I believe that before WW2 A.D.Blumlein proposed a energy recovery system in the line output stage. The recovered energy from the flyback contributes the first 40% of the horizontal scan. As far as I know no pre-war UK made TV sets employed the idea although some Baird TV sets did use a damper diode which is not the same thing.

DFWB.

[ppppenguin]

Originally Posted by Mike Phelan

Alan Dower Blumlein is something of a forgotten genius, as was E L C White.
I think the damper diode was just to conduct to stop the "ring" rather than making use of the energy. A bit like sticking a diode across a relay coil.

ELC White was very much the circuit design genius at EMI research. He tends to be overshadowed by Blumlein. At least Blumlein now has a decent biography (by RW Burns, published by the IEE).

Blumlein did devise an energy recovery LOP before the war but it was never adopted by real TVs. Possibly the scan power was low enough that designers didn't think it worthwhile. Having said that, all the energy stored in the scan coils had to go somewhere and you certainly didn't want ringing. Hence the damper diode. Another factor may have been lack of good low loss trasnformer cores at 10kHz. Energy recovery only works properly if the iron losses are respectably low. This really had to wait for ferrites which were a post-war invention.

[Duke_Nukem]

I believe that before WW2 A.D.Blumlein proposed a energy recovery system in the line output stage.

He certainly patented a LOP circuit back in 1933. The circuit that was illustrated had a diode-connected triode connected backwards across the output valve (in fact, the circuit looked just like a 70's semiconductor output stage !) and the stored energy was used to perform part of the subsequent scan.

The extra valve would certainly need to have needed good heater-cathode insulation, way above that of usual valves and presumably a special "tv" valve would be required and is probably why the circuit never got used (or at least one of the reasons).

TTFN,
Jon (aka Mr Anorak)

[PJL]

Well, I am your novice target audience..and I'm still confused but that might just be me.

I looked at the circuit for a TV22a and noticed that 1-2 on your diagram is the scan coils and the remainder is the LOPT. I understand the EHT rectifier sits off overwind but it was helpful to see it in the circuit diagram.

Here are my 'dumb' questions perhaps some because I have chosen the wrong circuit:

1. Is the PL turned off just for the period of the flyback or for longer?
2. The PY only seems to capture the stored energy in the scan coils as the stray capacitance is surely just confined to the LOPT?
3. I thought it easier to understand if you consider thet the scan coils are AC coupled and therefore current must be balanced.
4. Does the LOPT (not the scan coils) have the higher inductance and determine the ramp slope?
5. The TV22a width control seems to damp the scan coils presumably to reduce the Cb volts?

[ppppenguin]

Here's an attempt at some of the questions.

Originally Posted by PJL

1. Is the PL turned off just for the period of the flyback or for longer?

In theory the PL is turned on only from the middle of the screen to the right hand edge. In practice it is turned on a bit earlier.

Originally Posted by PJL

2. The PY only seems to capture the stored energy in the scan coils as the stray capacitance is surely just confined to the LOPT?

The PY does not capture the stored energy. It just allows it to flow from the inductors to the boost capacitor. The energy on the forward stroke is stored in all the inductive components. The stray capacitance that governs the flyback is, to a first approximation, the total strays in the entire stage.

Originally Posted by PJL

3. I thought it easier to understand if you consider thet the scan coils are AC coupled and therefore current must be balanced.

Any net DC through the scan coils will shift the picture sideways.

Originally Posted by PJL

4. Does the LOPT (not the scan coils) have the higher inductance and determine the ramp slope?

The ramp slope is determined by the total inductance. I'm actually not 100% sure here. One job of the transformer is to match the impedances of the various parts of the circuit so the inductance seen at the LOP anode is the scan coil impedance modified by the LOPT turns ratio. Am I talking rubbish here?

[Duke_Nukem]

Re item 4, in an ideal world, the LOP would have an infinite inductance (and no leakage inductance). With scan coils connected to the LOP, the LOP will have appear to have and effective inductance of the scan coils inductance multipled by the turns ratio (squared,root,whatever) of the transformer.

Of course in practice you can't have an infinite inductance, but the LOP's own inductance should be large compared to that due to the coils.

TTFN,
Jon

[Mike Phelan]

All
I think Jeffrey and Jon have covered most of the answers, so I'll not add to them.
Al
I did consider that, but decided to draw the line at the point where it is just the basic LOP stage.
PJL
That is what L5 and L6 are, AFAIK. Most LOP stages have them and they are often mounted on the top caps - a few dozen turns.

The scan coils form part of the resonant circuit and I miissed the importance of C20.

Having read the service notes it says that PY is always conducting during the on phase so the ramp slope must be determined by the scan coil inductance. The width control it says is "varying the transfer of energy between the primary of the autotransformer and the deflection coils" and I assume this must mean it reduces the boost voltage by damping - with PY always on I can't see how else you can vary the width.

It isn't always on - as soon as the cathode voltage rises above HT at the start of the flyback between t1 and t2, it is off and remains off until the forward scan starts.

The combined coils resonate with C20 and develop the EHT. On the negative cycle it is clamped by PY but I am not sure what happens then.

C20 and the stray capacitances "tune" the LC circuit during flyback, so the width control lowers the "Q" and therefore the boost voltage and ultimately the supply to the stage. So your answer to (5) is correct, just stated in a different way.