*NEW* VBNIGC schematic

[Franz Gysi]

Because the pictures in other threads are lost:

An externally hosted image should be here but it was not working when we last tested it.

An externally hosted image should be here but it was not working when we last tested it.

It was starting here, Digi and I where working on a print.

Franz

 

[Franz Gysi]

... another picture

An externally hosted image should be here but it was not working when we last tested it.

 

[teemuk]

A tube buffer for an inverting output stage indeed makes some sense (as much as any buffer) since the gain of an ordinary inverting OpAmp is also defined by its source impedance. You don’t want the source impedance to vary and buffering the input forces the source impedance seen by the OpAmp to remain rather constant. Yet, I personally see no point in the inclusion of a tube (to do this buffering), as this decision typically involves addition of high voltage rail and a filament supply, which tend to complicate things and add cost. In a cathode follower configuration the tube doesn’t clip very “pleasingly” either (which I guess is pretty much the only point in using a tube anyway). Even to get some clipping the tube should be driven with a very hot signal and in that point the power chip would be abruptly overdriven long before the tube. As I see it, the buffer is pretty much a transparent element as is (it's not providing "tube sound") and therefore you could just replace it with an ordinary OpAmp or FET-based circuit.

Addition of a buffer to a non-inverting OpAmp makes no sense since the source impedance is a small concern. Anyway, I guess the point of this design was not to be “rational” in the first place. At least tubes look nice.

 

[planet10]

Yet, I personally see no point in the inclusion of a tube (to do this buffering), as this decision typically involves addition of high voltage rail and a filament supply, which tend to complicate things and add cost. In a cathode follower configuration the tube doesn’t clip very “pleasingly” either (which I guess is pretty much the only point in using a tube anyway).

The point of using a tube (in particular a triode) is that they are still the most linear device yet invented.

dave

 

[teemuk]

Gimme a break! A tube may seem very linear on paper but you forget that in reality we are comparing circuits, not devices. In practical applications you can achieve far better linearity with transistor-based circuits that employ a lot of negative feedback for error correction. I doubt that the circuit in question would beat an ordinary OpAmp buffer when it comes to comparison of linearity.

I say the tube is used in the circuit because it is
a) simple
b) a tube

 

[leadbelly]

I say the tube is used in the circuit because it is
a) simple
b) a tube

Exactly my point! Using a tube as a voltage follower is actually a bit silly, as a lot of work the top guys are doing in the tube world, including this forum, is using solid state followers in their tube amps because they are so better suited to the job of providing good current drive, needed for A2 and AB2 amps, etc. Just look at SY's, tubelab's, and smoking-amp's work (please don't be slighted if you're off the list).

EDIT: although you do have to give credit to Dave's point about linearity. However, since the tube is being used as a cathode follower the better linearity is moot.

 

[planet10]

employ a lot of negative feedback for error correction.

Their are a lot of people that would like to avoid NF (a CF isn't a good example of that thou)

dave

 

[teemuk]

EDIT: although you do have to give credit to Dave's point about linearity. However, since the tube is being used as a cathode follower the better linearity is moot.

I did not say that I would disagree about triode being a linear device. I disagree about the circuit it's used in being linear. Point is, to exploit the triode’s inherent linearity fully you have to have a top-notch design that often employs stuff like constant current source loading, using regulated supply etc. A simple tube buffer circuit like that shown in the first post of this thread would likely beat a similar (basic emitter follower) transistor circuit (both BJT and FET) in linearity but typically you do not use transistors in circuits like that if you want them to perform decently. Good transistor circuits excel in comparison to simple tube circuits such as this one.

What comes to issue of avoiding the use of negative feedback, I must say that it’s not my shame if people choose not to use that technique. I know that it has it's side effects but if linearity is the main concern I see no reason why not to use NFB. And like you mentioned, a cathode follower already introduces 100% degenerative negative feedback (although its local). That’s why it has no voltage gain and also clips less softly / pleasantly than a typical common-cathode stage.