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In Reply to: Why do some people dislike UL? posted by Ray Moth on March 17, 2007 at 14:12:56:
UL makes use of NFB. We knew that. Ok, conventional FB theory takes a sample of the output and feeds it back to the input. It's out of phase, of course, and the out-of-phasedness subtracts out whatever was right in the fedback signal leaving an error signal to perform corrections. (If we could get just the error signal, then there would be no reduction in gain with NFB.)Here's the thought: the input used in UL is not the same input used for the music signal. More to the point, the screen as an input does not present the same nonlinearities (distortion) as the control grid. The result is that the error signal is not the right one for the input being used. According to FB theory, we ought to be able to reduce distortion to an arbitrarily low level provided we have enough open loop gain (and ignoring the realities of phase shift, etc...) But it seems to me that that is not the case with UL given the fact that the FB is applied to an input with different distortion characteristics than that used to apply the music.
To look at it another way, applying a distorted signal to the input of an amplifier can't be good. We can (in theory) get away with it with ordinary FB loops because the distortion is (in theory) exactly opposite that produced by the amplifier itself. We don't have that feature with UL.
Follow Ups:
Dave,I don't know if your thinking holds up. An error correction signal is applied to g2. IF it is appropriate, things will work well.
In any case, loop NFB (hopefully only a few dB.) is needed to further lower O/P impedance and correct for "iron" flaws.
See my post down the page replying to sser2. The need to keep instantaneous plate potential above g2 potential never goes away.
Eli D.
"The need to keep instantaneous plate potential above g2 potential never goes away."That is an impossible need to meet, in either UL or pentode mode. At full signal, the plate voltage will swing from close to 0v up to twice B+. How can the screen voltage always be lower than that?
Obviously, g2 potential must remain positive. However, when the "idle" state potential difference between g2 and plate is significant, multi-grid power O/P tubes are most linear. Also, quite a bit of the distortion generated is of the HIGHLY objectionable IM variety.
Eli D.
Hi.Here what I have done to my ST-70 triode strapped EL-34.
Instead of the common practice of 100R bridging the plate & g2 I used before, I have them 32V apart by replacing it with a string of 5x6.2V zener, a SS diode & a 10R resistors.
"Most linear" or "highly objectional IM variety" generated or not, I am happy I have done so as it sounds better to me now.
I don't think its primary effect is applying NFB. It is more like modifying the pentode characteristics a bit. It makes the pentode a lot less senditive to the applied load and g2 voltage.If one takes the U-L as applying NFB to a pentode, the arguement that a triode is a pentode with NFB becomes a whole lot more believable. The triode being the case where the g2 is attached to the plate or 100% U-L taps.
The distaste for U-L can probably be traced to the amps that first used it. Their designs left a bit to be desired from the POV of an inspired builder.
"If one takes the U-L as applying NFB to a pentode, the arguement that a triode is a pentode with NFB becomes a whole lot more believable."Good point!
What's going on with the usual NFB idea is the output is fed back to *THE* input. In the pentode, the output is fed back to *AN* input. In the triode/pentode case the pentode( taken as a beam tube with beam plates attached to the cathode for simplification ) degenerates from a 4 element device to a 3 element device when the g2 reaches the anode.
Not sure I fully understand your point of "the" vs "an". In a typical global feedback scheme, the signal is returned to the input stage but not the actual input "port" e.g. The input signal is applied to g1 of input amplifier but FB signal is applied to the cathode of same amplifier. Really not conceptually unlike UL is it? The only common topology that comes immediately to my mind where "THE" output is fed back to "THE" input would be a "plate follower".
The cathode or grid input has the same effect. The same resistor that is being used in the cathode circuit to develop the voltage could be moved to the grid circuit. I was considerng it to be the same...perhaps a bit of a simplification on my part.
scratch some of that. writing w/o morning stimulant dose.but still, moving the cathode is just the same as moving the grid. In the circuit it is a whole lot easier to move the cathode( to effect grid-cathode ) than to try changing the grid-ground relationship.
Changing the voltage at the cathode not only changes Vgk, but also Vak. It might not mean much, especially in the case of a tube with high plate resistance, but something with very low ra and mu? I dunno, the cathode (even ignoring input impedance) is not *exactly* the same input as the grid and won't have *exactly* the same distortion characteristics. The difference might be genuinely negligible compared with all the other realities involved, but...
You have it right. I do think it's a small thing. As implemented with a pentode, Va-k change is trivial. When it's done with a small resistor underneath( between ground and the cathode bias and bypass ), I think I can ignore the cathode's input Z. For a triode at the front of the amp, the FB signal might amount to a few % of Va-k. I agree with your assesment that this is also likely a trivial effect. It is a good thing to realize when you're 'neglecting air resistance', yes?
"The distaste for U-L can probably be traced to the amps that first used it. Their designs left a bit to be desired from the POV of an inspired builder."That may be what I'm trying to find out. What kind of defects were in those deigns?
Let's see, marginal power supply, high impedance driver's, full loop NFB, AB1 bias for maximum power, OPT load for maximum power with distortion corrected as mentioned earlier. As a general example, look at the Dynaco St.70 modification list. To be more general, consumerist style electronics of the same sort available these days.Build an amp of any style with the care and attention that Josh at E-Luv applies and I think you'll be able to get away with a whole bunch of Forbidden methods.
Or realize that the wrong methods have come to defined 'conventional wisdom'?Just which EL amps have you heard?
actually I've only smelled one. Nice smoke though.I'm not sure how I'd define conventional wisdom. I was probably thinking of something more like conventional dogma when I wrote it.
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