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Old 8th April 2010, 05:44 AM   #11
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Quote:
you could use a ccs to drive the screen and zener
Thats the way to go! Use a depletion FET like DN2540 together with a 7-pin gas regulator tube. Then the sand is gone.....

Think you will find it in MJKs latest thread.
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Old 8th April 2010, 04:20 PM   #12
kevinkr is offline kevinkr  United States
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The place where this does make sense is in a VCVS based filter in a line level X-O...
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Old 8th April 2010, 04:24 PM   #13
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Quote:
Originally Posted by revintage View Post
Thats the way to go! Use a depletion FET like DN2540 together with a 7-pin gas regulator tube. Then the sand is gone.....

Think you will find it in MJKs latest thread.
Getting the sag and time constant of the screen cap out of the picture is a side benefit, but my main reason is to eliminate screen current variation from the diffamp tail current in this pentode driver loaded by V/I feedback:

G1=G2/mu Scaled Drive Strawman Design

Michael

PS now this is interesting: "The place where this does make sense is in a VCVS based filter in a line level X-O... "

or maybe using remote cutoff pentodes in a "vari-mu" compressor circuit...

Last edited by Michael Koster; 8th April 2010 at 04:27 PM.
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Old 8th April 2010, 08:43 PM   #14
Sheldon is offline Sheldon  United States
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Originally Posted by SY View Post
Distortion was low, but source impedance was still 1/gm, so not really any better than a triode. The only other performance advantage (in exchange for a higher parts count) was a slightly higher gain. But if you can hear the difference between a gain of 0.95 and one of 0.98, you're a better man than me.
Thanks. May still try it just for something a little different. And I got the parts. But maybe 6JC6 instead of 6AU6.


Quote:
Originally Posted by kevinkr View Post
I've been thinking a lot lately about using a zener tied to the cathode to bias the screen grid in a number of pentode based applications - you could use a ccs to drive the screen and zener - I have no idea how linear this would prove to be. Might be interesting and if it works reasonably well it would save the cost of a large high quality screen cap.. Just a thought.
With a reasonably quiet supply, wouldn't a resistor do almost as well as a ccs, especially with a ccs on the cathode?

Brings up another question that puzzles me. I would think that the impedance of g2 should be paralleled with the dropping resistor, to calculate the cap to cathode. But I'm a bit mystified as to how to calculate, or even estimate that. Data sheets don't show grid curves with other factors held constant. I'm thinking that it would typically be relatively high with a current source on the cathode, as a voltage change at g2 could only change current at g2, in opposition to current at the plate.

Sheldon
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Old 17th April 2010, 05:17 PM   #15
Sheldon is offline Sheldon  United States
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Been thinking about the follower, and came to the conclusion that a mu follower would be the better choice for my project. Here's what I'm planning on using it for, and three different versions of the mu follower. I'm thinking that at the low currents I'm running, the SK170 might be the better option as the controlling device. Or?

BTW, phono amp is from Dmitry Nizhegorodov. Shunt supply a version of Salas's supply.
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Old 19th April 2010, 06:10 PM   #16
Sheldon is offline Sheldon  United States
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Right now, I'm thinking that the gyrator mu follower will be a nice fit. If I use the cascoded version, I'm also thinking it would good to bias up the top device some with a battery. Of course, an enhancement mode device on top would need to be biased. But even with the depletion mode device for the top, some extra voltage across the lower device might be desirable. Without additional bias, the DN2540 would only put about 2V across the SK170.

Sheldon
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Old 20th April 2010, 12:39 AM   #17
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Quote:
Originally Posted by Sheldon View Post
Right now, I'm thinking that the gyrator mu follower will be a nice fit. If I use the cascoded version, I'm also thinking it would good to bias up the top device some with a battery. Of course, an enhancement mode device on top would need to be biased. But even with the depletion mode device for the top, some extra voltage across the lower device might be desirable. Without additional bias, the DN2540 would only put about 2V across the SK170.

Sheldon
An IXYS 01N100 or 10M45 would drop -3V Vgs at low current, which is probably OK for a 2SK170 bottom device in a cascode. Based on the 2SK170 datasheet the Ciss would be about 33pf at 3Vds and would only decrease to 30pF if Vds is increased to 10V.

I'm thinking about a cascode gyrator using a 2N7000 + 01N100 myself, but the 2SK170 is a great choice for lower current operation and the 01N100 will work fine at a couple of mA.

Cheers,

Michael

PS for a single device you might consider the FQP2N60C at about 4pF Crss and 20pF Coss

Last edited by Michael Koster; 20th April 2010 at 12:59 AM.
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Old 20th April 2010, 01:21 AM   #18
Sheldon is offline Sheldon  United States
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Thanks Michael. I was thinking not so much the Ciss as getting on the flatter part of the curve, but 3V looks pretty good there if the data sheet is accurate. Don't know if that matters much here. The 10M45S does look like a better fit than the DN2540, and I have a couple. I'm liking the gyrator idea, as I can set the voltage so that tube drift will be less of an issue. Plus, the voltage from the input supply is just right for the output tube.

Also, since this is the output tube for the preamp, it's a good spot for a slow start circuit for charging the output cap. I've use simple RC circuits to delay power for other projects, and Greg (Geek) mentioned the application for the gyrator. It should flatten any power on thump. Alas, I suspect that I'll still get some thump on power off, as there is little storage in the HV supply.

Sheldon

Last edited by Sheldon; 20th April 2010 at 01:26 AM.
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Old 20th April 2010, 10:29 AM   #19
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hey-Hey!!!,
SY mentions the low gm; consider type EF184. Dropping R to the g2 and bypass it with a cap around 0,5 uF.
cheers,
Douglas
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Old 20th April 2010, 07:34 PM   #20
Sheldon is offline Sheldon  United States
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EF184 looks like a nice tube. I actually got some 6JC6 for cheap. Very similar specs. But I think for this project, the mu follower makes sense. I need one less HV regulator, and the voltages work out nicely. I had thought about using a tube for the mu CCS or gyrator, but don't I don't think I have enough voltage to make that work well(could make the supply a cap input supply). More important, maybe, is that the current is pretty low to feed the output tube plate, in this configuration. That's why the sand based gyrator seems most practical.

Sheldon
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