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The PC97 - tutorial

The Radio Constructor, October, 1961.
    
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The In Your Workshop series was a technical education feature based on conversations between the fictional characters of Smithy the Serviceman and his young assistant Dick. This article is an extract of the section dealing with the new PC97. The UK Betting and Gaming Act 1960 allowed commercial bingo halls to be set up and the initial conversation revolves around an active topic of the time.

Introduction

It was about half an hour later (after Dick had returned his repaired television receiver to the rack and had successfully treated another) that the muttering commenced again.

'One and Three, unlucky for some!'

Smithy paused and listened expectantly.

'Two-Oh, blind twenty!'

There it was again.

'Two and One, key of the door!'

Smithy turned quietly.

'All the Twos, dinkey-doo!'

As had occurred previously, his assistant appeared to be completely absorbed in his work.

'Two and Six, bed and breakfast!'

By looking very carefully, Smithy was just able to make out the movement of his assistants lips.

'Three-Oh, blind thirty!'

'House!' yelled out Smithy.

The effect on Dick was galvanic. He suddenly lost his balance and fell sideways off his stool, the screw-driver he had held in his right hand describing a graceful parabola in the air. At the same time his feet became entangled with his soldering iron cord, causing the iron to trace out an arc of a circle whose radius was the free length of lead from Dicks legs. Smithy avoided the iron nimbly as it flashed past him, noting simultaneously that a crash from the vicinity of the sink indicated that the flying screwdriver had reduced the meagre Workshop stock of cups by one. He was, however, too late to avoid Dicks stool, which. toppled neatly over on to his left foot, the seat edge falling precisely on his instep.

Smithys roar of anguish caused Dick to look up from his prone position on the floor, whereupon he thoughtfully devoted his attention to the spectacle of the dancing Serviceman. After some moments he was distracted by a smell of burning. He got up to retrieve the soldering iron. which was by now almost lost in a cloud of vapourised linoleum. Dick quietly put his stool into its correct position by his bench, and sat down on it.

'What on earth,' he asked eventually, 'possessed you to do that?'

Smithy was, for once, completely at a loss.

'Do what?' he gasped weakly.

'Yell out at me like that. I nearly shot out of my skin!'

'Its your own fault,' retorted Smithy, recovering himself. 'You shouldnt have kept muttering Tombola numbers all the time.'

'Tombola ?'

'Housey.'

'Oh, you mean Bingo!

Grunting in affirmation, the Serviceman aggrievedly took off his left shoe and sock, and very carefully inspected his foot for injury. There was not the slightest evidence of abrasion, of bruising, or of any other damage whatsoever.

'I dont think there are any bones broken,' Smithy remarked eventually, in a relieved tone of voice.

His thoughts returned to his assistant.

'Anyway, what was the idea of saying those numbers all the time?'

A gleam of pride came into Dicks eyes.

'I was practising,' he said, grandly. 'You may not know it, but Im the new official number-caller for the Bingo Club at La Vie Boheme.'

'Where?'

'La Vie Boheme. You know, Joes Caff.'

'I thought it was El Picador since he got the bamboo wallpaper up'

'He put a Formica top on the counter last week,' explained Dick, 'so hes changed it again. He always changes the name when he gets something new in.'

Smithy absorbed this news quietly, and a thought struck him.

'What happened to the previous official number-caller?' he remarked.

Dick looked a little uncomfortable.

'Well, there was a bit of a punch-up,' he remarked, 'You see, he got Number Six upside down and called Doctors Orders during the Snowball House. Nobodys seen him since.'

Smithy chuckled.

'It sounds a dicey occupation,' he grinned, 'and I should watch it, if 1 were you. Anyway, its about time we got back to the grind.'

Tuner Triode

'I suppose so,' replied Dick. 'Incidentally, have you seen this new TV set Ive got here? It doesnt use a cascode in the tuner input stage at all - just a single triode,'

'Ah yes,' said Smithy. 'Thatll be the new Mullard PC97.'

'You dont sound very surprised,' commented Dick aggrievedly.

Im not,' replied Smithy. 'For one thing, triode input stages have been used in the States over at least the last three years.'

'I didnt know that,' said Dick. 'Anyway, how can you get a triode to amplify at VHF? Surely, it would oscillate like the clappers!'

Fig. 2 (a). The basic cascode tuner unit amplifier. The grid of the upper triode is normally held at approximately half the full HT potential. The anode-grid capacitances quoted are those for the PCC89
(b). The anode-grid capacitance of the Mullard PC97 is maintained at the very low level of 0.5pF
(c). Neutralising the PC97 by coupling the remote end of the anode coil to the grid via a neutralising capacitor. The bypass capacitor has a relatively low value, a typical figure being 100pF. The decoupling resistor may be 1 to 5kΩ, and the neutralising capacitor have a range of some 2 to 12pF
(d). The anode circuit of the PC97 can be represented as a pi tuned circuit
(e). The internal structure of the PC97, showing the effect of the screening plates. The special shape of the anode also contributes towards the low anode-grid capacitance
(f). The pin layout of the PC97. Two cathode pins are available.

'Not if you neutralise it,' said Smithy. 'Although, even then, you want to design the valve so that the capacitance between grid and anode is as low as possible. With a conventional cascode like the PCC89 the capacitance between anode and grid of the earthed-cathode triode is of the order of 1.9pF whilst that of the earthed-grid triode is approximately 4.1pF. (Fig. 2 (a)). The Mullard PC97 has a much lower capacitance between grid and anode, this being around 0.5pF only.' (Fig. 2 (b))

'I see,' said Dick, musingly. 'Of course, with the cascode the lower, earthed-cathode, triode works into the low cathode impedance of the upper triode. This low impedance should keep voltage gain in the lower triode down, in any case, and thus help in preventing oscillation.'

'Thats pretty near it,' said Smithy, 'although you do in practice neutralise the lower triode of a cascode.'

But Dick was pursuing his ideas in his own way.

'And, of course, theres no risk of feedback in the upper triode,' he continued, 'because the earthed grid comes between cathode and anode.'

'W G Morley,' remarked Smithy encouragingly, 'could not have put it more succinctly.'

'Do you really think so?' exclaimed Dick, forgetting his train of thought.'

'Definitely.'

'Well, I suppose I am a bit of a gen kiddy at times,' said Dick modestly. 'I do know, you know.'

'lm quite certain you do,' replied Smithy soothingly. 'Anyway, lets get back to the PC97. As you have just, in your masterful manner, explained, the basic make-up of the cascode assists towards stability. The very low anode-grid capacitance of the PC97 also assists towards stability, because, amongst other things, it makes neutralising requirements much less critical.'

'How do you neutralise it, Smithy?' 'By getting a neutralising voltage,' replied Smithy, 'from the HT end of its anode coil.' (Fig. 2 (c)).

'But that points decoupled to chassis!' protested Dick.

'Not by all that much it isnt,' said Smithy somewhat inelegantly, 'because you use a decoupling capacitor having a lowish value around 100pF. This lowish value brings you back to the old pi tuned circuit (Fig. 2 (d)) in which the coil is really tuned by the decoupling capacitor and the output capacitance of the triode in series. The RF voltage on the end of the coil remote from anode will be 180° out of phase with that at the anode itself, and so youve got a nice little bit of neutralising voltage all ready for feeding back, via a capacitive trimmer, to the grid. Since the value of the decoupling capacitor is fairly low, enough RF appears across it for neutralising purposes.'

'Its very neat, isnt it?' commented Dick. 'One thing I notice is the extreme simplicity of the circuit around the triode as compared with the usual cascode arrangement.'

'Thats a considerable advantage of the triode,' said Smithy, 'and it makes tuner unit design simpler and cheaper. You will usually find, incidentally, that the bypass capacitor for the anode coil is of the feed-through variety, this being done to keep inductance in this part of the circuit down to a minimum. I should add also that, in practical tuners, adjustment of the neutralising trimmer will not normally be done by the likes of us; it will, instead, be carried out at the factory with special test equipment.'

'How is the capacitance between grid and anode brought down to a value as low as 0.5pF?'

'By putting C-shaped screening plates around the two backbones of the grid,' said Smithy, 'that is, the two thick vertical wires which support the grid wires. (Fig. 2 (e)). Although these plates kill the relatively high capacitance between the backbones and the anode they still leave free access from the working areas of the grid to the anode. The screening plates come out to a separate pin which may then be connected to chassis.'

'This all seems very knobby,' said Dick approvingly. 'Any other points?'

'Oh yes,' said Smithy. 'The PC97 has got a frame grid, with all the advantages that that confers. And the cathode comes out at two pins (Fig. 2 (f)), which reduces cathode lead inductance.

'Wont the gain of a single triode be lower than that of a cascode?'

'It may be a little lower,' said Smithy, 'but that doesnt necessarily matter, because you can pick up the lost gain very easily in the IF strip-anyway. The limiting factor to receiver sensitivity is noise-factor, and the noise factor of a tuner input stage using a PC97 is of the same order as that of a cascode stage.'

'Well, the PC97 certainly seems to be a welcome introduction as far as Im concerned,' commented Dick. 'If only because it may simplify tuner circuits and make servicing easier!'

'Im quite certain youre right there,' commented Smithy, looking at the clock. 'Anyway, its about time we got back to work again.'

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