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Old 27th August 2009, 03:48 AM   #21
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I guess I don't know how to figure Cm in this case. Do I use the plain pentode gain or the gain that I will get with feedback to multiply by the grid-to-plate capacitance to figure Cm? If I take a pessimistic approach, Cm could be up to 30pF in a KT88?

If I want a corner frequency at 200kHz, that would give me a feedback resistor of 26.5k. This seems low and likely to load the driver down. I don't know what this will do to the source follower. I have 5mA idle current through it right now with a 50k load resistor. It would be easy enough to bump that up.

On the other hand, a 100k feedback resistor would give me a corner frequency of 53kHz which means I may not be as flat as I would like at 20kHz.

Then I will have a capacitor in series with the feedback resistor to block DC. I'm thinking I should shoot for a 2Hz cutoff frequency there, does that sound right?
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Old 27th August 2009, 03:58 AM   #22
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Of course, you should calculate for the real gain, with feedback applied!
Speaking of 2 Hz corner, it depends on other LF poles in the global feedback loop; also power filter time constants must be considered, otherwise you may easily get LF instability through B+ chain, since OPT on such a frequency is almost short.
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Old 27th August 2009, 06:29 AM   #23
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Quote:
Originally Posted by SpreadSpectrum View Post
What I don't understand is why do ultralinear circuits exist?
That's an easy one: two words: cost cutting. UL is a lot cheaper than an active screen regulator. Since this is not a concern of mine, the two designs I did using pentode finals include active screen regulation. When using 807s, I found (as suggested by Schade) that parallel feedback off the plates worked as advertised. The other project, using 6BQ6GTBs, didn't need any local NFB at all, since the 6BQ6GTBs produced way less of the nasty high order harmonics. All these needed was a bit of gNFB to take the edge off.

UL basically does the same thing as parallel NFB, but it avoids the necessity for screen regulation, and allows the use of types whose screen and plate operate at the same voltage.

There is just one commercial design that used active screen regulation, and that was a theatre amp, not consumer electronics.

Quote:
Originally Posted by SpreadSpectrum View Post
After my curve drawing exercise, it is apparent that I can have my cake and eat it too. (Well, I do have to provide a screen supply, I guess.) Why would we give up the benefits of the accelerating grid(at a constant voltage) if we don't have to?
We don't have to since we don't answer to the pencil-pushers in the Accounting Dept.

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Originally Posted by SpreadSpectrum View Post
To save a few components in a feedback network and screen supply? Is there a gotcha that I don't know about yet? As I said, I'm still young and haven't had a chance to build a plate to grid feedback project yet.
Yuppers, you got it.
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Old 27th August 2009, 06:40 AM   #24
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Originally Posted by gingertube View Post
With Anode to Grid feedback the feedback resistor is effectively in parallel with the Miller Capacitance. This puts a definite upper limit on the size of resistor you can use and still maintain reasonable high frequency response.
No, that anode-to-grid resistor is in parallel with the reverse transfer capacitance. Cmiller occurs at the input. Simple fix for that: don't return the far end of the resistor to the grid. Take it back to the driver stage, and skip over Crt. That's how I dunnit.
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Old 28th August 2009, 04:06 AM   #25
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Quote:
Originally Posted by Miles Prower View Post
No, that anode-to-grid resistor is in parallel with the reverse transfer capacitance. Cmiller occurs at the input. Simple fix for that: don't return the far end of the resistor to the grid. Take it back to the driver stage, and skip over Crt. That's how I dunnit.
Right. I knew there was something funny about that. We don't care about the
Miller effect, which is a multiplication of the feedback capacitance by the plate
swing relative to the control grid, rather we are worried about where the
feedback reactance starts to become significant in parallel with the feedback
resistor. It's independent of the plate voltage swing.

PS
The MOSFET between the pentode driver plate and the control grid isolates the
feedback capacitance when the feedback goes to the driver plate
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Old 28th August 2009, 11:10 AM   #26
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Quote:
Originally Posted by Michael Koster View Post
Specifically, what's the advantage of E-linear over simple plate-grid feedback.
As I surmise, e-linear is essentially ultralinear plus G2-G1 feedback. In what
way would this be superior to plate-grid feedback?

Thanks,

Michael
hey-Hey!!!,
The circuit is a means of delivering plate to grid FB around the output stage at minimum complexity. No resistor network needed. Single-node PS is also a plus I think; everything gets fed to the OPT's ct( in the PP implementation ).

The output stage's g2 isn't really part of it; it can be riding along in U-L mode, or done pentode, or rigged as triode. It is just plate to grid, at U-L tap ratio of plate signal. One can tap the OPT where you want, or stick with end-of-layer points if y'all want someting other than a specific U-L ratio( anywhere from 20-50% deending on who made the OPT ). It could even be done with a cathode FB topology; fixed g2, cathodes to the tertiary winding and the E-Linear front end attached to taps on the plate winding. While winding the coil it could even be tapped to maintain pentode operation( g2 to tap on the opposite side of the anode CT, but you'd loose the ability to run g2 at other-than-B+ ).
cheers,
Douglas
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Old 28th August 2009, 03:38 PM   #27
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Quote:
Originally Posted by Michael Koster View Post
e-linear is essentially ultralinear plus G2-G1 feedback.
Yes, it is.
My E-linear is derived from Pete Millette's as shown in the
below URL.

http://ja1cty.servehttp.com/E-LINEAR/E-Linear-amp.png

Due to deep NFB of G2 > G1 in the original Pete's, I reduced it
by means of two pcs of 39Kohm which are connected to the
plate of C3g.
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Old 28th August 2009, 03:44 PM   #28
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Quote:
Originally Posted by ja2dhc View Post
Yes, it is.
My E-linear is derived from Pete Millette's as shown in the
below URL.

http://ja1cty.servehttp.com/E-LINEAR/E-Linear-amp.png

Due to deep NFB of G2 > G1 in the original Pete's, I reduced it
by means of two pcs of 39Kohm which are connected to the
plate of C3g.
E-Linear is set-fraction output plate to grid FB. That the output g2 is able to get attached to the same tap( creating U-L ) is not relevant. E-Linear can be done pentode, U-L, or triode output stages.
cheers,
Douglas
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Old 28th August 2009, 04:14 PM   #29
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Quote:
Originally Posted by Bandersnatch View Post
E-Linear is set-fraction output plate to grid FB. That the output g2 is able to get attached to the same tap( creating U-L ) is not relevant. E-Linear can be done pentode, U-L, or triode output stages.
cheers,
Douglas
OK, I think I get it. E-linear is plate-grid feedback using an OPT tap instead of a
divider or shunt network.

What you do with the screen grids is a separate issue.

That was my question, and whether there is any advantage, other than
feedback is feedback, to using g2 modulation in addition to the plate-grid
local feedback.

Thanks, Douglas!

Michael
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Old 28th August 2009, 04:39 PM   #30
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Quote:
Originally Posted by Michael Koster View Post
OK, I think I get it. E-linear is plate-grid feedback using an OPT tap instead of a
divider or shunt network.

What you do with the screen grids is a separate issue.

That was my question, and whether there is any advantage, other than
feedback is feedback, to using g2 modulation in addition to the plate-grid
local feedback.

Thanks, Douglas!

Michael
hey Michael,
I think there is. U-L does not linearize a pentode; its g1 spacing is pretty much like a pentode's IMO. What I do find useful is it reduces its output Z, evidenced by the slant the grid lines now take on. Given the requirements to drive the inductive load( at LF, more or less ), the steeper( v. pentode ) plate curves are useful.

I think that the optimum will be done with a CFB output TX, equipped with taps on its anode winding for the E-Linear connection. Stuff like the Chicago BO-14 comes to mind, but I think I'll have Heyboer wind versions of it with taps at less than original 37% to compliment the 10% cfb tertiary coil.

The CFB method of implementing U-L also offers application to power tubes like TV sweeps with low g2 voltage limits.
cheers,
Douglas
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