PCF80 -Designing a Pentode Constant Current Sink - help needed. - diyAudio
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Old 25th July 2008, 10:40 AM   #1
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Default PCF80 -Designing a Pentode Constant Current Sink - help needed.

Hi all,
I am designing a pentode constant current sink based around the PCF80. I have a few options for operating conditions. I ideally want the current to be about 10mA. I will use Zeners to set g2 as I haven't got space for rc filtering the screen.

If I set the g2 voltage to 100V and have the plate at 100V I only have a g1 of -0.5V which will make for a not very stiff CCS.

Alternatively I can choose to set the g2 at 170V and have the plate at 100V which puts g1 at -2V. This should be a bit stiffer.

However if I set the g2 to higher than the plate then screen current is likely to be excessive. There are no figures for this so I would like to ask if anyone can give an estimate of g2 current if running the plate at 10mA and screen at 170V. I guestimated that it might be as much as 4mA which puts the g2 at 0.68W diss which is very close to its limit of 0.75W.

Has anyone out there built a CCSink with the PCF80, or has experience of screen current with a similar pentode.
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Old 25th July 2008, 10:53 AM   #2
Merlinb is offline Merlinb  United Kingdom
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My calculations suggest about 3mA screen current, making 510mW at 170V. Of course, if you want to be more cautious then just run the screen a bit lower, say 150V.
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Old 25th July 2008, 11:09 AM   #3
Yvesm is offline Yvesm  France
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Old 25th July 2008, 11:11 AM   #4
Yvesm is offline Yvesm  France
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Old 25th July 2008, 11:11 AM   #5
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That sounds usable. How did you derive that figure.
The standard tables suggest that with the a plate at 170V, g2 at 170V and g1 at -2V the screen current would be at about 3mA. Since the scenario I am proposing is worse than that your estimate of 3mA seems to be a bit conservative.

Shoog
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Old 25th July 2008, 11:13 AM   #6
Yvesm is offline Yvesm  France
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Old 25th July 2008, 11:14 AM   #7
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Hi Shoog

On page 108 of Valve amplifiers, 3rd edition, there is a schematic of a ECF80 based CCS. Mr. Jones uses 200V on the screen and 23V (through a voltage divider) on the grid, and finally a 2k cathode resistor. The resulting plate current should be 10mA, while the screen current should be 2,6mA*.

Erik

Shoog, have you solved your overshoot problems with the tabor clone?

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Old 25th July 2008, 11:14 AM   #8
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Old 25th July 2008, 11:27 AM   #9
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Yves - Thanks for that. The graphs seem to suggest that I should be expecting a grid current of about 4mA which is in line with my guestimate. It certainly look workable.

Erik - that looks very promising, could you scan and send me the page. Theres no point in reinventing the wheel now.
The tabor clone is up and running and working amazingly well. Detailed punchy and sweet sounding. Best I have built so far. Still have a few issues with transformer saturation with 20hz signals, but I can live with that. The overshoot problem was mainly a problem with the CCS in the cathodes. They kept failing and overloading the powersupply. This was causing sever ringing between the output transformers and the power supply transformer. Once I got a robust enough CCS the ringing went away. I had to use TV deflection transistors in the end with a Darlington in a ring of two CCS. It surprising how easily you can stray outside the safe operating area on high voltage transistors in switch-on conditions. In those situations transistors become silicon fuses.

Shoog
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Old 25th July 2008, 11:39 AM   #10
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Erik - google books has that reference so no need to scan anything.

Thanks

Shoog
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