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Old 4th September 2005, 04:41 PM   #11
00940 is online now 00940  Belgium
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Eli Duttman : I'm sorry, I've been out of town for a little while. Thanks a lot for your comments.

Would this schematic match your description ?
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Old 12th September 2005, 07:00 PM   #12
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Default Re: Question : pentode ccs schematic ?

Quote:
Originally posted by 00940
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."
Sorry about the delay (took me a while to find an envelope to draw it on). Here it is:
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Old 12th September 2005, 10:32 PM   #13
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I would do it the other way. Pentode as the signal input tube. Triode's cathode to negative rail through the current setting R. Triode's grid to the negative rail.

The triode will be a fine load for the pentode's cathode, and the pentode's plate will be more tolerant of B+ ripple. Pentode's anode to B+, g2 to one end of a 100k resistor, and B+ to the other. 0.5 uF to 1 uF cap between g2 and the pentode's cathode.
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Old 12th September 2005, 10:49 PM   #14
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That's not as advantageous as it might seem. The ripple on a triode plate will be attenuated in the CF output by a factor of mu (20 in this case). Whatever ripple is present on the screen will be passed unattenuated. Now, you do have an RC rolloff, but with the values you give, the attenuation isn't any better than mu anyway. Sure, you could go up another factor of 10 on the cap, but that increases size and cost.

It's pretty easy (see my preamp article) to get the ripple down to ridiculously low levels, so why not put the big cap toward that end? That way, one can take advantage of the much superior CCS characteristics of a pentode plate.
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Old 12th September 2005, 11:15 PM   #15
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ALright, I pulled the sheet on this beastie. It isn't a very good tube for the use you all propose. Two EF184 will cost $5USD and they have 15 mA/V transconductance. That amounts to ~50R output Z for a cathode follower. They have 90kOhm plate Z so they make fine sink-CCS too.

If I were going to the trouble of cutting a bunch of holes and assembling a proper power supply, that's the way I'd do it. Or one could take Sy's route and rig the input valve as a triode...
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Old 13th September 2005, 08:13 AM   #16
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Quote:
Originally posted by Bandersnatch
Two EF184 will cost $5USD and they have 15 mA/V transconductance. That amounts to ~50R output Z for a cathode follower. They have 90kOhm plate Z so they make fine sink-CCS too.
Agreed, EF184 is a very nice valve indeed and if you're going to gp to the trouble of nice metalwork specifically for a cathode follower line stage, then it's a much better choice, However, if you need a triode cathode follower with pentode CCS in an existing piece of equipment and have room to bash a single hole with not much HT, then the PCF80 could work very nicely.

I take the point about ripple rejection and g2. My back of the envelope sketch used the minimum voltage possible and the curves were for g2 = 170V, so I just took g2 to the HT. A better idea would be to do some tests with a lower g2, allowing RC smoothing to g2.
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Old 13th September 2005, 10:20 AM   #17
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and the sad thing is those ECF80 come at 7 per Tektronix 502, so I have a small sack of them. All European Amperex. The pentode section is really a 3-grid pentode, and the triode section is like twice the gm of a 6SN7.

I would suggest an even more complex arrangement, involving some ~100V voltage references and stack the pentode on top of the triode, cascode style. This still leaves one with the trouble of a cathode sink...
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Old 14th September 2005, 09:57 AM   #18
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Default 00940 mail

Sorry this is off topic.....

00940
Dear Sir - I could not email You - this is why
I am sending You this email..... Do You know where to find RKV headphone schematic?
Best regards
sunny
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Old 22nd January 2007, 12:30 PM   #19
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Quick in - quick out ....

I've had Pete Millett's original article in AudioXpress (low mu triode CSS preamp) for breakfast (well litterature to my tea and cereals) for severeal days, and though the design is flawless I still wanted to make some changes.
The 6AS7-EL34-duo is a bit too strong and consumes a little too much current for the heaters so I started thinking of other duos that could do the trick but with less heat. What I came up with was the 12B4A-triode and an EL84, or possible an 6AQ5 as CSS. Or using the Russian 6N6P (6H30P, 5687, E182CC) as triode and the 6AQ5/6BQ5 for pentode.
Would that be reasonable if you still want some current capability?
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