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Old 30th November 2010, 02:18 PM   #31
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Join Date: Nov 2007
Location: Dallas
Originally Posted by Wavebourn View Post
Actually, your Darlington stage has 1k2 output resistance; voltage amplification factor with no load is 1.6. I played with such (well, similar) beasts, they oscillate like crazy, that's why I went with pentodes in my Barracuda project.
That makes no sense whatsover. I am gonna disagree for now.
Darlington collector follower does not have 1K2 output impedance
when you return 1/(Mu-1) voltage feedback to the base.
For exact same well established reason Schaded pentode has
reduced plate resistance with voltage feedback to its grid.

Oscillate? Where is the phase shift? The loop is all local direct
coupled. If some parasitic high frequency thing? Maybe we need
to throw lossy ferrite bead at it, or tiny bypass cap under the
cathode to become the dominant pole. I just don't think any of
that is necessary in a direct coupled local loop.

Overall voltage amplification factor is 1/(1/(Mu-1)))=Mu-1.
The triode loadline is shunt virtual CCS >=180K, not 1.2K.
Going by GE-6SN7GTB curves near this operating point,
I would have to say Mu-1 here looks to be about 20.

I don't think we are operating 6SN7 at the best available point.
This point is still holdout from when it was driving the 6AS7.
We were expecting another 21V rise before reaching the output.
Perhaps 2mA, with 2.5V under the cathode, and 80V on plate?
Obviously would have to pad our input to the Darlington, or do
something else to raise the 1.3V threshold.

Last edited by kenpeter; 30th November 2010 at 02:40 PM.
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Old 1st December 2010, 04:55 AM   #32
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Join Date: Nov 2007
Location: Dallas
Rename BS8.asc.txt to just BS8.asc
All necessary Spice models plugged in.
Ripped from Fairchild and Duncan Amps.
Ready to run without hassle in LTSpice.


Without load, amplification factor and currents exactly as I predicted.
With load, sims neither as good as I said, nor bad as Wavebourn sais.
It definitely struggles more than I expected with 300ohm load.

Or maybe I need to give it longer pre-start time to let DC settle on
those output caps? I need to go re-run with that experiment.
Attached Files
File Type: txt BS8.asc.txt (3.6 KB, 11 views)
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Old 1st December 2010, 05:24 AM   #33
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Location: Dallas
A little better operating point for the triode helps a lot!
Not sure I've picked best operating points for CCS sand?
Could still benefit a stiffer threshold under the cathode.
Cathode current is not quite as constant as I had hoped.

I need to go back and sim Diego's original circuit to see
if this is any improvement, or if I've just made it worse...
Also need to sim all tube circuit of Post#12.
Attached Files
File Type: txt BS9.asc.txt (3.8 KB, 7 views)

Last edited by kenpeter; 1st December 2010 at 05:36 AM.
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Old 1st December 2010, 05:47 AM   #34
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Join Date: Nov 2007
Location: Dallas
Actually, just seems plate resistor wasn't permitting enough quiescent.
Real problem happens when Q2 bottoms out for lack of reserve current.
70mA not enough for this load. Up Q2 to about 150mA and all is golden.

Go back to schematic BS8 and change R6 to 470 ohms, run again.
Much betta! Seems 6SN7 operating point was not the critical issue.


Back to original circuit of Post#1 for a sec:

I do not know now if 70mA was enough for 6AS7 cathode follower?
The rules are a little different for cathode follwer, but not much. I got
a feeling even that circuit wanted more quiescent than it could move.

1V input swing is 20V output swing is 67mA current swing into 300R.
Really pushing it much too close to the limit with only 70mA quiescent.
Assuming +/-1V input, +/-20V output were original intended margins?

Last edited by kenpeter; 1st December 2010 at 06:14 AM.
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Old 1st December 2010, 11:38 PM   #35
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Location: San Josť
I should say that the amp I originally built sounded pretty good with no obvious distortion in spite of running both tubes at suboptimal operating points. The output stage is the better of the two and probably only needs a small increase in current. The drawback of it is the heat generated by the cathode resistors. They are bolted to the chassis and generate 42C at that spot.

The first option I drew, DC coupled with a CCS has the advantage that you can adjust the bias of both tubes with the trimmer in the CCS.
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Old 2nd December 2010, 02:24 AM   #36
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Reasonably faithful LTSpice model of original post#1 amp.
Gain is a lot lower than Mu, +/-1V input won't clip 70mA.

Shunt or series CCS will increase gain from 8x to 21x
Now +/-1V of input can challenge 70mA output quiescent!
Thats what had me worrying CCS loadlined sims earlier...

Adjusted for equal listening levels, 70mA is not a problem.
Attached Images
File Type: gif BS10.gif (76.3 KB, 37 views)
Attached Files
File Type: txt BS10.asc.txt (3.5 KB, 3 views)

Last edited by kenpeter; 2nd December 2010 at 02:36 AM.
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