Thanks Steve. I always have this gnawing doubt creep into my mind when it comes to this sort of thing; dunno why but I do!
Remember These?
#301 Re: Remember These?
Sorry, I couldn't resist!
- IslandPink
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#302 Re: Remember These?
It wasn't obvious to me at first, so you're not different !
I could try and explain it so : if you have the feedback connected, it's not like the higher gain is there, then the feedback reduces it - there' no gap between the two things - the feedback is already applied in nanoseconds, so the higher swing is not 'seen' at the 13E1 grid.
Also... the gain of the 13E1 from grid to cathode is not any different with & without the feedback. Hence you only need the grid bias that is consistent with the output power from the 13E1.
I could try and explain it so : if you have the feedback connected, it's not like the higher gain is there, then the feedback reduces it - there' no gap between the two things - the feedback is already applied in nanoseconds, so the higher swing is not 'seen' at the 13E1 grid.
Also... the gain of the 13E1 from grid to cathode is not any different with & without the feedback. Hence you only need the grid bias that is consistent with the output power from the 13E1.
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#303 Re: Remember These?
My poor brain can't compute this.
If I make a single ended amplifier that has a gain of 100 from end to end then apply feedback. I have to shove a lot more voltage in at the input to get the same volume I would have got without feedback. Gain has gone down, so it must have gone somewhere.
If in a negative feedack system you have to shove more up the input, compared to the open loop situation, then whether the output valve sees that signal or not, you still have to provide it don't you? I provided the high drive; are we saying that, that drive provision is unnecessary, I think it is very necessary, otherwise it just makes a nonsense of everything I learned. With negative feedback, you have to put more in to get the same out. Why is there a feedback equation if not?
If I make a single ended amplifier that has a gain of 100 from end to end then apply feedback. I have to shove a lot more voltage in at the input to get the same volume I would have got without feedback. Gain has gone down, so it must have gone somewhere.
If in a negative feedack system you have to shove more up the input, compared to the open loop situation, then whether the output valve sees that signal or not, you still have to provide it don't you? I provided the high drive; are we saying that, that drive provision is unnecessary, I think it is very necessary, otherwise it just makes a nonsense of everything I learned. With negative feedback, you have to put more in to get the same out. Why is there a feedback equation if not?
Sgt. Baker started talkin’ with a Bullhorn in his hand.
#304 Re: Remember These?
Yes, the key as Mark said is open loop gain and the feedback factor. Most of it is here
As Mark said, the output valve doesn’t know about feedback, all it knows are its grid and cathode, so the voltage swing on the grid is the voltage swing that needs to produce the required output into the load. If you ran it open with no feedback and supplied the grid with the same signal you would get the same output as if you had feedback. The feedback allows the system to apply the correct signal to the grid of the output valve such that its cathode produces the required result.
In Steves example of the single ended amp with a open loop gain of 100 (40dB), then if you apply 30dB of feedback, the overall gain of the system will be 10dB, the 30dB fain of the feedback will consume 30dB of the system gain.
This is where it gets problematic with parasitics and group delay. If HF is not part of the feedback because of losses in the loop then at HF the system as the original gain of 40dB at HF as there is nothing to consume the gain. If there is a delay in the system (as there will be), then at some frequency the feedback will change from being negative, to being less negative, and then become positive.
Nothing is ever as simple as we might hope.
But back to this amp, the output valve load lime is a 8 ohm cathode follower loadline, and that will show you what signal on the grid will be needed to produce the signal on the cathode. Of course headroom is good, but how much is the important factor. What rearly interests me (and is why I would like to attach a scope to the thing) is will the grid be driven positive into A2 and with the grid circuit you have what happens then...
As Mark said, the output valve doesn’t know about feedback, all it knows are its grid and cathode, so the voltage swing on the grid is the voltage swing that needs to produce the required output into the load. If you ran it open with no feedback and supplied the grid with the same signal you would get the same output as if you had feedback. The feedback allows the system to apply the correct signal to the grid of the output valve such that its cathode produces the required result.
In Steves example of the single ended amp with a open loop gain of 100 (40dB), then if you apply 30dB of feedback, the overall gain of the system will be 10dB, the 30dB fain of the feedback will consume 30dB of the system gain.
This is where it gets problematic with parasitics and group delay. If HF is not part of the feedback because of losses in the loop then at HF the system as the original gain of 40dB at HF as there is nothing to consume the gain. If there is a delay in the system (as there will be), then at some frequency the feedback will change from being negative, to being less negative, and then become positive.
Nothing is ever as simple as we might hope.
But back to this amp, the output valve load lime is a 8 ohm cathode follower loadline, and that will show you what signal on the grid will be needed to produce the signal on the cathode. Of course headroom is good, but how much is the important factor. What rearly interests me (and is why I would like to attach a scope to the thing) is will the grid be driven positive into A2 and with the grid circuit you have what happens then...
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
#305 Re: Remember These?
Yes, but the summing point is not the grid of the output valve, thats inside the loop.you have to put more in to get the same out. Why is there a feedback equation if not?
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
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#306 Re: Remember These?
Gotcha now
There's no need for the driver to produce wall to wall swing, only what's needed by the output stage. Every day's a schoolday
I now get why Bruce uses a simple cascade with a 6N something P twin triode, on the Mini-Beast. Both sections are identically configured.
I've just clip leaded out one of the 220R resistors on the driver, so its now identical to the input stage. Works perfectly well.
Last edited by Cressy Snr on Fri Jan 04, 2019 11:09 pm, edited 1 time in total.
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#307 Re: Remember These?
I'll try & translate
You feed a signal into the input valve grid ; as you do so, the cathode (due to feedback) is going up and down with it - quite a bit - as the feedback is applied. So, you don't get the gain you expect from the first valve.
After that, gain-wise, everything operates as expected if there wasn't feedback.
Sorry, I missed your extra comments - but you are obviously catching up now. Good job !
You feed a signal into the input valve grid ; as you do so, the cathode (due to feedback) is going up and down with it - quite a bit - as the feedback is applied. So, you don't get the gain you expect from the first valve.
After that, gain-wise, everything operates as expected if there wasn't feedback.
Sorry, I missed your extra comments - but you are obviously catching up now. Good job !
"Once you find out ... the Circumstances ; then you can go out"
- Cressy Snr
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#308 Re: Remember These?
You know what this means now: another blasted schematic.IslandPink wrote: ↑Fri Jan 04, 2019 11:09 pm I'll try & translate
You feed a signal into the input valve grid ; as you do so, the cathode (due to feedback) is going up and down with it - quite a bit - as the feedback is applied. So, you don't get the gain you expect from the first valve.
After that, gain-wise, everything operates as expected if there wasn't feedback.
Sorry, I missed your extra comments - but you are obviously catching up now. Good job !
Thank God it's on the computer this time.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
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#309 Re: Remember These?
Yowsa !
We love your schematics !
We love your schematics !
"Once you find out ... the Circumstances ; then you can go out"
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#310 Re: Remember These?
I would suspect you don't want to be trying to drive the grid of the 13E1 positive, with cap coupling from the driver stage Another argument for sufficient drive and no more.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
- Cressy Snr
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#311 Re: Remember These?
I could bring it to your place sometime this month, before I put it in a box.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
#312 Re: Remember These?
It'll be interesting to see what Nick makes of it...Cressy Snr wrote: ↑Fri Jan 04, 2019 11:32 pmI could bring it to your place sometime this month, before I put it in a box.
Sorry, I couldn't resist!
#313 Re: Remember These?
That would be good.Cressy Snr wrote: ↑Fri Jan 04, 2019 11:32 pmI could bring it to your place sometime this month, before I put it in a box.
--
Nick
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
#314 Re: Remember These?
That was what I was thinking. The CCS will help keep it under control to some extent.Cressy Snr wrote: ↑Fri Jan 04, 2019 11:18 pm I would suspect you don't want to be trying to drive the grid of the 13E1 positive, with cap coupling from the driver stage Another argument for sufficient drive and no more.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
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#315 Re: Remember These?
OK, here are some boring graphs of the amp driving an 8 Ohm dummy load.
There's barrel distortion on my iPad camera, but it should give the general idea,
1KHz sine wave
There's barrel distortion on my iPad camera, but it should give the general idea,
1KHz sine wave
Last edited by Cressy Snr on Sat Jan 05, 2019 2:18 pm, edited 1 time in total.
Sgt. Baker started talkin’ with a Bullhorn in his hand.