Long Tail and Local feedback

[ray_moth]

Now, if the tail resistor is replaced by a current sink, making that a split tail configuration requires an unusual dual current sink, but it does indeed improve DC balance. In fact, using BJT composite sink + mirror, insures DC balance to within fractions of a percent. AC balance can also be independently adjusted by including a plate to plate pot that has the wiper connected to +B via a cap.

I don't undedrstand how dual current sinks would work. AFAIK, the purpose of a current sink in the tail of an LTP is to ensure that the summ of the currents in the two halves of the splitter is constant, thereby giving "perfect" AC balance. Splitting the tail resistor above the current sink helps to improve DC balance, as well as providing negative current feedback. Why go further than that?

[Yvesm]

Originally posted by Tubes4e4


. . .

A beefier high gain alternatives to 12AX7 would be for example EC91 (single system 7 pin miniature, µ=100). Maybe 12AT7, too.

Tom

Agreed, 12AT7 will be happy as LTP PI specially when feed with the 400V on the output tubes anodes.

R13 is superfluous, just return R4 and R6 to ground

Yves.

[ilimzn]

Originally posted by ray_moth
I don't undedrstand how dual current sinks would work.

On their own, they would not. Reread the post, there is mention of a cathode to cathode resistor in a dual current sink system.

AFAIK, the purpose of a current sink in the tail of an LTP is to ensure that the summ of the currents in the two halves of the splitter is constant, thereby giving "perfect" AC balance.

Making the sum of currents constant does not insure AC balance, not unless you count differential output, but the 'balance' does not truly apply. If the two triodes have different characteristics, both DC and AC balance will be compromised in the sense that the signal out of one plate and the other will not be equal amplitude.

Splitting the tail resistor above the current sink helps to improve DC balance, as well as providing negative current feedback. Why go further than that?

Splitting the tail resistor above the current sink will improve DC (and AC) balance but reduce gain due to current feedback. In fact, effectively you are lowering the Gm and making the two triodes 'more equal'. With a current sink, there will not be an appreciable change in DC conditions as the sink still insures that the sum of currents remains constant. Things however get interesting if you need your LTP to maintain near perfect DC balance. See attached picture... it was just meant as an example, not necessairly as something the original poster should use in his design, the idea can be used for other things (like auto biassing of class A PP outputs).

[aletheian]

If you are just starting out with tube LTP, you might want to ditch the current sink idea AT FIRST, and just build it up with a tail resistor. Then when you get it working, you can sub in the current source later on. The tail resistor develops it's own voltage in conjunction with ohms law, so you don't need a negative voltage supply either.

[ilimzn]

Actually, in order to save on coupling caps, you may need to use a negative voltage.
The reason for this is that a LTP behaves as a LTP and not merely as a cathoe coupled amp as long as the tail current is fairly constant. In a LTP the two cathodes connect and form a common mode node, where the voltage WRT to ground is the mean of the + and - input + bias voltage. When used as a phase splitter, with one input grounded, the AC component on this node will be roughly half of the input voltage for a reasonable mu. If this is an appreciable part of the total DC voltage on the tail resistor, you cannot assume the tail current is constant. In order to do so, you need to increase the DC voltage drop on the tail resistor to make the AC appear smaller hence the current more constant - this eats away at your B+ and also rises the grid voltage necessary for proper biassing above 0V level, which means you will need coupling caps. Of course, this is a non-issue if they are already there, but the B+ loss can be a problem. This is why a separate negative supply is used, essentially for that voltage to be 'wasted' on the tail resistor in order to better approximate a current source.

[aletheian]

Originally posted by ilimzn
Actually, in order to save on coupling caps, you may need to use a negative voltage.

That's true. I come from the musical instrument amp world, where coupling caps give a nice opportunity to sculpt the midrange tonality by using different compositions to color the sound. I occasionally forget that many hi-fi guys come from the "caps in the signal path are bad" school of thought. heck, I use mylar/paper caps and carbon comp resistors just about everywhere but input stages and power supplies.... want to talk about coloration!

[Bandersnatch]

local NFB and LTP you say? seen this one? I clal the circuit E-Linear, as it is voltage( hence the 'E' ), and Ultra-Linear combined. Works quite well actually. Works SE as well, as published by Pete Millett in AudioXpress earlier this year.
regards,
Douglas

The tail load can be as simple as a resistor and voltage, or a small voltge rail, as in fixed bias and CCS of your choice.

[Gary P]

Here is my take on local feedback...


The Tabor local feedback amplifier

Gary

[ray_moth]

If the two triodes have different characteristics, both DC and AC balance will be compromised in the sense that the signal out of one plate and the other will not be equal amplitude.

I don't see how that can be true. With a good CCS in the tail, any change in plate current in the first half of the LTP must be matched by an equal and opposite change in the second half, regardless of how unmatched the LTP triodes may be. If that were not true, then the CCS would not be doing its job. With a CCS tail, AC balance is assured, assuming the plate load resistors of the LTP are accurately matched.

DC balance, however, still depends upon matching of the triodes making up the LTP. A really well-matched double triode (e.g. 6SU7) should achieve acceptable DC balance. If necessary, DC balance can be assisted by using separate cathode resistors above the CCS. As you say, this introduces negative current FB, with consequent loss of gain and elevated plate resistance.

[ilimzn]

Ray_moth, now that I've re-read you qouting me, I figured out the problem, I must have been more scatterbrained than usual while I was writing that!
Of course, you are right - I was thinking one thing and writing another, namely not AC balance but AC gain per triode in the LTP. This of course, remains different looking at the AC voltage between grid and cathode, but differentially (which is the proper way of looking at that circuit), this does not apply, as the common mode is taken out and the exact gain distribution between the two triodes in the pair becomes largely irrelevant (but of course surfaces as the DC imbalance). Due to the sum of currents being fixed by the CCS, of course, the phase splitting action is always 'perfect' WRT plate current, and if the plate circuit impedances are matched, the AC output is always balanced.
Sorry for the confusion...