Diego wants 1.29V under his V1a (post#1), Darlington for your V3 would be about that...
NPN might offer better choices than PNP, MJE350 is so just barely getting it in mine...
If you could hold V1a at sufficiently constant current by Triodlington current dumping
method alone, the entire top triode can go away and become a simple resistor. It not
like V1a would see plate resistor and load currents anymore, only plate voltage and Mu.
NPN might offer better choices than PNP, MJE350 is so just barely getting it in mine...
If you could hold V1a at sufficiently constant current by Triodlington current dumping
method alone, the entire top triode can go away and become a simple resistor. It not
like V1a would see plate resistor and load currents anymore, only plate voltage and Mu.
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Sorry for the late response. To be honest you totally lost me as I´m just a noob at this.
The voltage stated in the first schematic are taken from the real amp. I built it following a design without changing anything. So, the bias of the 6sn7 does not need to be 1.29, it could be 1.7 or 2. The plate voltage needs to be so low because it sets the bias of the 6as7 which is now around -23, I can increase that voltage up to 60 and the 6as7 goes to around -19. That´s why I thought the ac coupling could be useful to increase the plate voltage even more. However, I don´t know if the coupling cap will damage the sound more than the gain obtained by running the 6sn7 at higher current.
In any case, I already built the amp so I´m restricted to the same B+ and the 3 tubes and a bit of space restrictions.
The voltage stated in the first schematic are taken from the real amp. I built it following a design without changing anything. So, the bias of the 6sn7 does not need to be 1.29, it could be 1.7 or 2. The plate voltage needs to be so low because it sets the bias of the 6as7 which is now around -23, I can increase that voltage up to 60 and the 6as7 goes to around -19. That´s why I thought the ac coupling could be useful to increase the plate voltage even more. However, I don´t know if the coupling cap will damage the sound more than the gain obtained by running the 6sn7 at higher current.
In any case, I already built the amp so I´m restricted to the same B+ and the 3 tubes and a bit of space restrictions.
If using all same tubes and no sand, go with schematic of post #12.
Yet we are still in process exhausting other options. In Post #1, Diego
threw sand at the problem of CCS for V1a. I propose the same, but in
a totally different way. Taking Wavebourn's idea one possible extreme.
Here: Q1 & Q2 hold V1a cathode at constant current by dumping any
current variance. The Triode experiences only voltage variations, and
operates on nearly flat Mu loadline.
Oddly enough: This is also sort of shunt regulator, slaved to Mu. And
improves power supply rejection observed between plate and ground.
V1a can't tell any different that the current regulation is parallel shunt
rather than series... And we make V1a appear as-if a LOT bigger Triode.
Yet we are still in process exhausting other options. In Post #1, Diego
threw sand at the problem of CCS for V1a. I propose the same, but in
a totally different way. Taking Wavebourn's idea one possible extreme.
Here: Q1 & Q2 hold V1a cathode at constant current by dumping any
current variance. The Triode experiences only voltage variations, and
operates on nearly flat Mu loadline.
Oddly enough: This is also sort of shunt regulator, slaved to Mu. And
improves power supply rejection observed between plate and ground.
V1a can't tell any different that the current regulation is parallel shunt
rather than series... And we make V1a appear as-if a LOT bigger Triode.
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300ohms = 1.3Hz
32ohms = 9.8Hz
8ohms = 38.4Hz
And keep in mind the output impedance is not 1.2K
Invert factor we used to boost parallel current.
It is plate resistance (7K?) divided by about 35.
So whats that now, maybe 200 ohms???
With 1.2K in parallel, maybe 170ohms...
Not even close to as low as I thought it'd be.
I'm almost sure the collector follower impedance is
lower than that. Where did my math go wrong?
Unless boost to quiescent current is different then
boost to Gm? I've went and lost myself again...
32ohms = 9.8Hz
8ohms = 38.4Hz
And keep in mind the output impedance is not 1.2K
Invert factor we used to boost parallel current.
It is plate resistance (7K?) divided by about 35.
So whats that now, maybe 200 ohms???
With 1.2K in parallel, maybe 170ohms...
Not even close to as low as I thought it'd be.
I'm almost sure the collector follower impedance is
lower than that. Where did my math go wrong?
Unless boost to quiescent current is different then
boost to Gm? I've went and lost myself again...
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No cap is perfect, all have other strange things going on besides capacitance.
Sometimes those other effects are significant. Mixing cap types and/or values
helps to flatten out those quirks. Almost always closer to ideal than a single
large cap.
Since you obviously stuck on MKP (and nothing wrong with that) I'd suggest a
pair or more of smaller ones. And none the same value.
--------------
I totally computed output impedance wrongly for the circuit of Post#24.
Quiescent current magnification and Gm magnification are NOT the same.
Plate voltage is set by total quiescent current, resistor drop from B+,
and location on the triode curve that places 1.3V under the cathode.
Triode current is set by 1.3V shunted across 750 ohms. The rest must
come from sand assist. Sand will pull the plate voltage down untill the
rule of 1/(Mu-1) from plate to cathode is satisfied. This voltage rule is
modulated by Mu*whatever is happening at the grid of course...
Triode plate impedance is divided by HFE of the darlington pair.
HFE is highly variable with temperature and manufacturing tolerance.
25x30=750 only the garaunteed minimum for this pair, could be higher.
Fair to say 6300 going by the "typical" 25degree published curves.
What exact impedance does this get us? Dunno... 1 ohm therabouts?
Lets just say it will be plenty low enough to drive headphones.
Sometimes those other effects are significant. Mixing cap types and/or values
helps to flatten out those quirks. Almost always closer to ideal than a single
large cap.
Since you obviously stuck on MKP (and nothing wrong with that) I'd suggest a
pair or more of smaller ones. And none the same value.
--------------
I totally computed output impedance wrongly for the circuit of Post#24.
Quiescent current magnification and Gm magnification are NOT the same.
Plate voltage is set by total quiescent current, resistor drop from B+,
and location on the triode curve that places 1.3V under the cathode.
Triode current is set by 1.3V shunted across 750 ohms. The rest must
come from sand assist. Sand will pull the plate voltage down untill the
rule of 1/(Mu-1) from plate to cathode is satisfied. This voltage rule is
modulated by Mu*whatever is happening at the grid of course...
Triode plate impedance is divided by HFE of the darlington pair.
HFE is highly variable with temperature and manufacturing tolerance.
25x30=750 only the garaunteed minimum for this pair, could be higher.
Fair to say 6300 going by the "typical" 25degree published curves.
What exact impedance does this get us? Dunno... 1 ohm therabouts?
Lets just say it will be plenty low enough to drive headphones.
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Lumping headphone in parallel with 1.2K plate resistor (if we regard B+
another virtual ground) is about 240ohms load. But triode gets at least
750x Gm assist from the sand. So sees loadline slope as-if it were 180K.
And this is worst case, bottom of the barrel, almost reject transistor.
Consider Gm boost with typical HFE is more likely 6300x. Loadline slope
could be 1.5M or higher! Begins to look like a CCS. The triode does not
know or care that CCS effect comes from shunt rather than series.
another virtual ground) is about 240ohms load. But triode gets at least
750x Gm assist from the sand. So sees loadline slope as-if it were 180K.
And this is worst case, bottom of the barrel, almost reject transistor.
Consider Gm boost with typical HFE is more likely 6300x. Loadline slope
could be 1.5M or higher! Begins to look like a CCS. The triode does not
know or care that CCS effect comes from shunt rather than series.
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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.
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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.
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.
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.
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.
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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?
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?
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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.
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.
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.
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.
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