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Old 2nd October 2003, 07:15 PM   #21
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Tim, I assume you mean this?

Click the image to open in full size.

If so, here is the current regulation (green=driven, red=strapped):

Click the image to open in full size.

The GK strapped configuration shows about 26uA. Much worse than the 0.5uA when the grid is driven.
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Old 2nd October 2003, 07:30 PM   #22
SY is offline SY  United States
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Perhaps they are, but are we straining out gnats while swallowing camels?
We're doing that when we use tubes in the first place.
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Old 2nd October 2003, 08:07 PM   #23
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Quote:
Originally posted by runeight
Tim, I assume you mean this?
Yes. Except, you can get rid of the 10k between stages and adjust the screen supply resistor for operating point.

Quote:
The GK strapped configuration shows about 26uA. Much worse than the 0.5uA when the grid is driven.
Not suprising, it's running purely on plate resistance (high though it may be), rather than amplifying the error signal. Likewise, it'll also trash out pretty good when driving anything.

Oh - how about putting a CCS in place of the "current sense" resistor? I bet that would perfect it.

Thanks for satisfying my curiosity,

Tim
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Old 2nd October 2003, 10:31 PM   #24
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Quote:
Oh - how about putting a CCS in place of the "current sense" resistor? I bet that would perfect it.
Good idea. Kind of an infinitely recursive design. Each CCS with another CCS inside of it. I like it.

One last thing on the pentode option. I thought I would try a 6BM8 because it is a triode/pentode package and because I have the model. The pentode has half the S of the 6JC6. I wondered if it would work. Also, you get your triode and pentode in the same bottle. Only two sockets for a stereo line stage.

Here's the diagram.

Click the image to open in full size.

To get both devices into good operating region (as I judge it) I raised the B+ to 400V. Idle current is about 4.6mA.

Here is the current draw with a .05v 1KHz input signal. Again the fourier transform:

Click the image to open in full size.

Still below 1ua and so, from the perspective of simulation vs. reality, no different from the 6JC6.

The gain is almost twice the previous configuration. Here is the 2nd harmonic for about the same output voltage:

Click the image to open in full size.

About 0.25%.

And, finally, the response curve:

Click the image to open in full size.

My conclusion is that if you really want to use a pentode as the CCS device, except for the PS change, this is a pretty good option.

Frank, I wonder if a 6GV8 would be a better choice?

However, all of this work still tells me that a triode pair will be just as good. You can get them two to a package too, while keeping a reasonable supply voltage.
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Old 2nd October 2003, 10:59 PM   #25
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Hi,

Quote:
Frank, I wonder if a 6GV8 would be a better choice?
Not necessarilly, that's a fine reg. for low current/ low voltage demands though....around 200 to 250VDC @ 5 mA at most.

One of my favourites is the ECL82/6BM8 for lowish voltages.

Cheers,
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Old 3rd October 2003, 08:02 AM   #26
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Good work that you are doing.
Here is a link that you may wish to read and then try in your sim.

http://www.dddac.de/at08.htm
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Old 3rd October 2003, 09:58 PM   #27
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Hi,

And what if I tell you that some penthodes can be used to replace a choke? ( Tim loves me already)

For, all things considered, this is what we're doing here except we try to keep current constant.

Moreover, I know of at least one tetrode that can mimmick a nice choke... and it ain't all that small a H value either.

There's a lot of clever things you can do with tubes...there really is no end to it.

Cheers,
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Old 3rd October 2003, 10:58 PM   #28
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> Kimmel says that a triode cannot be considered a true CCS since "because it hasn't enough gain...

It Depends.

The usual trick is about a 470Ω cathode resistor, with the grid tied to the bottom of that. The CCS effective impedance is then essentially Mu*(1/Gm) which is Rp which is about the same as a simple plate resistor.

Runeight is using a 20K cathode resistor. That forces 3 extra parts to keep DC bias. But then the CCS impedance is Mu*(20K+(1/Gm)). For the 6DJ8, that comes out to about 600KΩ, a very high value indeed. You get 90% of the benefit of a "perfect" CCS.

Another point: 1/Gm is 200 to 2K for most tubes, and 200Ω happens only with "best" (most fragile) tubes at high current. But with the 20K resistor, they all work about the same: you can use a rugged 12AU7 or 12AT7 instead of the 6DJ8 or other too-hot tube. The impedance then depends mostly on Mu, not Gm or the ratio of Gm to cathode bias resistance.

When you get to Pentodes: the effective Mu is very high, but you must drive the Screen to track the Cathode. The obvious way is a capacitor from Cathode to Screen. You also need a path to supply Screen current, usually a resistor. When you do all that, the Pentode's very-high Mu is swamped by loading effects. I put far less trust in the available Pentode models than the triode models, but I think Runeight is right when he shows only modest or even no improvement from a ballasted triode. You could bootstrap the screen with yet another tube, but where are we going?

As Frank says "There's a lot of clever things you can do with tubes...there really is no end to it." Recall the words of the preacher in the Bible: "Of the making of many things there is no end... there is no new thing under the sun."

I think the simulation should have a load: say the typical grid resistance and capacitance, or the IHF standard Line Out load. If you cascode and bootstrap and CCS a tube to death, you make an ideal infinite object that can't interface with any real world.

For anybody who really wants to dig: just after WWII the MIT Radiation Labs (RADAR) released a 10-volume summary of their research. One volume is general amplifiers, and talks about such things. In general, as you reduce current while keeping supply voltage high, gain rises (but bandwidth decreases). They did try near-CCS topologies. In real life, the improvement from using another tube as a CCS is not as good as using an extra tube some other way, such as another amplifier stage or a buffer. You never see such frills on commercial gear from the heyday of tubes: they knew better.

There is no remembrance of former things... Ecclesiastes 1:11

(because runeight and I smashed them with hammers....)
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Old 6th October 2003, 12:06 AM   #29
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And what if I tell you that some penthodes can be used to replace a choke?
Frank, you knew I would have to ask how? So I could simulate it.

PRR, thanks for the additional illumination. As always you managed to do in a few paragraphs what took me a couple of pages and several thousand drawings.

Quote:
(because runeight and I smashed them with hammers....)
Et tu, Brute?

. . . for the former things are passed away. . . . Behold I make all things new. . . . Revelation 21:4,5

I trust we can be forgiven.
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Old 6th October 2003, 09:56 AM   #30
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Hi,

Quote:
Frank, you knew I would have to ask how? So I could simulate it.
Sure....

Vb = 400V
V out = 335V
Va = 60V
Vg2 = 100V
Vg1 = -1.0V
Ik = 110mA
Rg2 = 60K
Rg1 = 470K
Rk = 50R
C in = 8.0F
C out = 20F
Cg2 = 8.0F
Cg1 = 50F

The tube used here is a 6216 as made by CBS-Hytron.
They claim this circuit emulates a 12H choke at 150mA..
Other than this application note, I fail to see anything extraordinary about this tube tube so I think it's safe to assume other penthodes can be made to work in a similar way.

Cheers,
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