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Old 17th August 2005, 10:02 AM   #1
00940 is offline 00940  Belgium
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Default Question : pentode ccs schematic ?

Long time ago, EC8010 suggested that : "a pair of PCF80 would make a very nice line stage/buffer. Use the triode section as a cathode follower, and sit it on the pentode section used as a constant current sink. Use a 300mA DC constant current regulated heater supply, and you'd have something with performance completely out of proportion to price."

I've a dozen of those tubes, transformers, caps, resistors and so on. Cost would we be near zero

For the cathode follower, ok, I found some articles explaining me how to design them. However, I've some problems to find a simple ccs using a pentode. Could someone link me to one ?

PS: the pcf80 : http://www.r-type.org/exhib/aaa0252.htm
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Old 17th August 2005, 11:51 AM   #2
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You might poke around Gary Pimm's website; he's the ultimate CCS fetishist. There's also some very detailed design stuff in "Valve Amplifiers."
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Old 17th August 2005, 04:39 PM   #3
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Gary Pimms Circuits are probably the best solution.
However a Mu Follower will be a close approximation to a CCS. If you have plenty of voltage to burn then the current sensing resistor under the pentode can be made large which will stiffen the CCS properties. You will have to consider this more as a SEPP stage than a true CCS. I used this approach in building a Parafeed SET amp which has worked really well.

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Old 27th August 2005, 05:49 AM   #4
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pete millett's low mu pre also uses pentodes as a CCS.
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Old 27th August 2005, 08:57 AM   #5
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Let's see if I can give you a good description. At least for now, I do schematics by hand with pencil and paper.

You stack the triode on top of the pentode (triode cathode connected to pentode anode). Connect triode anode to B+. Connect pentode g2 to B+ via 1 Megohm resistor. Connect pentode g2 to pentode cathode via a small cap. Ground pentode g1. Connect pentode cathode to ground via series combo of variable resistor and 10 Ohm fixed part. Use Ohm's Law to set the current by measuring the voltage drop across the 10 Ohm fixed resistance. I like battery bias; so, connect + side of 6 V. Lithium device to triode cathode. Connect - side of battery to 1 Megohm grid leak resistor. Connect grid leak resistor to triode grid. Cap. couple triode grid to wiper of volume control. Cap. couple (4.7 muF.) triode cathode to O/P jack. Wire 1 Megohm resistor across O/P jack.
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Old 27th August 2005, 10:29 AM   #6
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Old 27th August 2005, 10:33 AM   #7
Jaap is offline Jaap  Netherlands
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Default sorry mistake

wrong thread
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Old 27th August 2005, 11:35 AM   #8
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"You stack the triode on top of the pentode (triode cathode connected to pentode anode). Connect triode anode to B+. Connect pentode g2 to B+ via 1 Megohm resistor. Connect pentode g2 to pentode cathode via a small cap. Ground pentode g1. Connect pentode cathode to ground via series combo of variable resistor and 10 Ohm fixed part. Use Ohm's Law to set the current by measuring the voltage drop across the 10 Ohm fixed resistance. I like battery bias; so, connect + side of 6 V. Lithium device to triode cathode. Connect - side of battery to 1 Megohm grid leak resistor. Connect grid leak resistor to triode grid. Cap. couple triode grid to wiper of volume control. Cap. couple (4.7 muF.) triode cathode to O/P jack. Wire 1 Megohm resistor across O/P jack."

You'll have to do that as a schematic before I can follow it.
In my use of a Pentode as a CCS it went in the endode of the triode rather than the cathode.

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Old 27th August 2005, 12:17 PM   #9
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In my use of a Pentode as a CCS it went in the endode of the triode rather than the cathode.
That's correct for a common cathode voltage gain block. The thread's initiator asked for a CCS (sink) loaded cathode follower.

FWIW, I envisage the use of the PCF80 in this manner working WELL. It should be HIGHLY linear and the O/P impedance will be approx. 200 Ohms. Even the NASTY IHF "standard" 10 KOhm load should be handled with ease.
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Old 27th August 2005, 05:16 PM   #10
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"That's correct for a common cathode voltage gain block. The thread's initiator asked for a CCS (sink) loaded cathode follower."

Thanks for correcting me on that one.

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