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Old 10th Jun 2018, 9:39 pm   #101
1100 man
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Default Re: DC Restoration questions

Hi David,
I've just been looking at the full circuit of the Alba T866 (red books to the rescue yet again!). That looks to be a very neat & tidy line gated AGC system. From what I've seen of Alba circuits so far, they must have had a very good design team- their circuits have interesting features.

However, I've been experimenting with the preset gain control feature of the T655. This is so simple, just using the negative voltage developed on the grid of the line O/P valve fed through a pot to control the gain of the tuner & 1st IF stage.
As the TV will always be fed from the Aurora, which will be supplied from either a DVD player or VCR, the RF signal level at the aeriel socket should be pretty constant. It won't have to cope with variable off air signals.
The gain of the tuner & IF stage can be preset and any variations compensated for with the existing contrast control. Conveniently in the Pilot, this is in the video amp anode circuit.
So no automatic gain control at all:- just two presets and the existing contrast control!

Cheers
Nick
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Old 10th Jun 2018, 10:10 pm   #102
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Default Re: DC Restoration questions

Hi Hugo,
Many thanks for your detailed explanation. It would seem that the main thing I had overlooked in DC coupling the cathode of the CRT to the anode of the video amp was the heater- cathode voltage of the CRT.

The set under test has been my Pilot PT650. The original circuit with the video AC coupled to the cathode of the CRT and the grid grounded, had the cathode at about 30 to 40V above chassis.
With the DC restorer circuit and the brightness control moved to the grid, the cathode could be operated at about 60 to 70V.
With full DC coupling the cathode is at about 150V above chassis and has the potential, as you say, to rise to HT potential of 200V.

The Alba T655, the design of which I've copied, uses full DC coupling and has 150V on the cathode. However, it uses an AW47-80 CRT which has a fat neck and a chunky electrode assembly.
My Pilot uses a CME2103 CRT which has a thin neck and presumably a smaller gun assembly.

Does anyone know what the maximum heater- cathode voltage is for each of these tubes? I only have basic data for these CRT's. It would be interesting to know.

As it stands, the heater of the CRT is at the bottom of the chain and is at chassis potential. Could the H/K stress be eased by moving it further up the chain? Or does this cause other problems?

Many thanks
Nick
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Old 10th Jun 2018, 10:39 pm   #103
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Default Re: DC Restoration questions

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Hi Hugo,


As it stands, the heater of the CRT is at the bottom of the chain and is at chassis potential. Could the H/K stress be eased by moving it further up the chain? Or does this cause other problems?

Many thanks
Nick
Nick,

I wouldn't move the heater up the chain. You will recall though when I made the mods to my Admiral set (which was once just AC coupled with only moderate CRT cathode voltages) I decided it was better for the CRT to fit a separate small heater transformer, and to make sure the heater and cathode assume the same average voltage, they can then be connected with say a 1meg resistor rather than just leaving the heater floating. CRT's must be given top safety priority as currently a non renewable resource.

Hugo
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Old 10th Jun 2018, 11:30 pm   #104
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Default Re: DC Restoration questions

I managed to mis- type both CRT types in post 102
They should have read AW43-80 and CME2101.

I have since found data, albeit in French, for the AW43-80 which says the cathode can be 200V more positive than the heater or 125V more negative.
So running the cathode at 150V is within spec but maybe not desireable!

I can't find the H-K value for the CME2101, but assuming it is similar, I think, for the safety of the CRT, I will go back to the DC restoration circuit which means the cathode can be run at a much lower voltage.

Hugo, I had noticed your heater transformer in the Admiral. I am interested, though, from a theoretical point of view in the effects of moving the CRT up the heater chain. If the cathode were to be run at 150V, would this not reduce the H-k potential as the heater would be at a higher potential? I am not planning to move the heater: just curious!!

Many thanks
Nick
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Old 10th Jun 2018, 11:57 pm   #105
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Default Re: DC Restoration questions

Hi Nick,
The main reason why the CRT heater is at the bottom of the chain is because in the event of a heater to cathode short in one of the valves in the chain or a S/C heater decoupling capacitor there will be no possibility of damage to the CRT. If the CRT heater is high up in the chain and one valves or decoupling capacitors lower down in the chain went short circuit to chassis the tube heater would over run.
Best to leave the CRT heater at the bottom of the chain.

DFWB.
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Old 11th Jun 2018, 12:00 am   #106
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Originally Posted by 1100 man View Post
I am interested, though, from a theoretical point of view in the effects of moving the CRT up the heater chain. If the cathode were to be run at 150V, would this not reduce the H-k potential as the heater would be at a higher potential? I am not planning to move the heater: just curious!!

Many thanks
Nick
If one terminal the heater is grounded, there is a low probability of it assuming any potential much higher than ground, and running off AC it would only be +/- about 9v peak. I guess it could have a poor connection to its socket pin to ground.

But if it is higher up the chain, the valve below it could get unplugged, or fail and further up the chain there are more and more socket-pins & valves below it increasing the risks. If the lower part of the chain opens the voltage on all heaters above in the chain will rise to the supply voltage, peak AC value over 300V, so if the CRT cathode was say 100V DC or less, it could cause failure of the CRT's H-K.

Most indirectly heated valves appear to have a heater cathode voltage max of around 200V. But for a CRT I wouldn't push it anywhere near that just in case.

The grid to cathode voltage that cuts the CRT off is usually no more than 60V or 70v, sometimes less. So with a grounded grid arrangement, that is the max cathode voltage needed, and with a grounded heater too, the max heater cathode voltage and that is generally just fine. But the safest way to avoid any problem, especially if you modify the circuit, is a small transformer.

The other thing that is helpful is a high voltage low capacitance diode between the CRT's grid and cathode, to stop the grid going significantly positive with respect to the cathode in case of circuit modification accidents.

...and I agree with post #105, its also safer, in the event of H-K shorts, or bypass cap shorts, in other valves if the CRT heater is grounded.

Last edited by Argus25; 11th Jun 2018 at 12:07 am.
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Old 11th Jun 2018, 12:40 am   #107
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Default Re: DC Restoration questions

Thank you both for your detailed explanations. That all makes perfect sense! It's very helpful to understand exactly why certain circuit arrangements are preferable to others and why designers did the things they did!

All the best
Nick
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Old 11th Jun 2018, 7:09 am   #108
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Default Re: DC Restoration questions

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I am interested, though, from a theoretical point of view in the effects of moving the CRT up the heater chain.
Nick
The reason Fernseh indicated is an important one.
A Philips book on the World Series receiving tubes for television, 1950, gave a second reason: to minimize hum. Quote:

Some special points of interest relating to series operation of heaters in a television receiver remain to be discussed. The tubes described in this Bulletin have been so designed as to have a very favourable heater-cathode insulation. There is, however, always inevitable capacitance between the heater and the other electrodes, particularly the cathode, and this may result in objectionable hum interference if the position in the heater chain of the particular tube in which the interference might be caused is not judiciously chosen. One end of the heater chain is normally connected to the receiver chassis, to which most of the cathodes are also connected. It is therefore advisable to choose the position in the heater chain of, for example, the frequency changer and the picture tube in such a way that the potential between heater and cathode is a minimum. The heaters of other tubes not likely to cause hum interference, such as the mains rectifier, should then be included in the other end of the heater chain.

Jac
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Old 11th Jun 2018, 8:25 am   #109
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Quote:
Originally Posted by Jac View Post

A Philips book on the World Series receiving tubes for television, 1950, gave a second reason: to minimize hum. Quote:

Jac
The thing about this though it is not the reason why the CRT heater is in the earthy end of the chain. It is the reason why the Tuner, IF & video & audio detector stages are though, because these are low signal level stages, for example at the audio and video detector, the peak to peak signal voltages might only be a volt or two.

The CRT is a very high signal level environment, the peak to peak voltage at its grid or cathode for full contrast is usually in excess of 20 volts, usually 30V to 40V or more, so some 10's of milli-volts of hum due to H-K leakage in the CRT has negligible effects.
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Old 11th Jun 2018, 9:10 am   #110
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Default Re: DC Restoration questions

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Originally Posted by Argus25 View Post
Quote:
Originally Posted by Jac View Post

A Philips book on the World Series receiving tubes for television, 1950, gave a second reason: to minimize hum. Quote:

Jac
The thing about this though it is not the reason why the CRT heater is in the earthy end of the chain. It is the reason why the Tuner, IF & video & audio detector stages are though, because these are low signal level stages, for example at the audio and video detector, the peak to peak signal voltages might only be a volt or two.

The CRT is a very high signal level environment, the peak to peak voltage at its grid or cathode for full contrast is usually in excess of 20 volts, usually 30V to 40V or more, so some 10's of milli-volts of hum due to H-K leakage in the CRT has negligible effects.
Certainly the tuner is also sensitive to hum.

When I look at the circuit diagram of the TX400U, the first 625 line TV in this country, I see that the sequence (from earthy end upwards) is:
1. the line oscillator
2. phase detector valve for the line frequency
3. CRT
4. mixer/osc valve in the tuner
5. RF valve tuner (= first valve)
6. frame oscillator / output valve
7. booster diode
etc.

I suppose that ease of wiring certainly played an important role.

Jac
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