Lowering noise in small signal pentodes?

[SY]

Originally Posted by plovati

What explanation had been provided for that claim?

The guy who presented his data believed that the partition noise was eliminated because both plate and screen current went through the plate load resistor. I have not confirmed his data, just putting up his circuit as an interesting alternate approach.

I don't have a copy of his presentation, but if memory serves, the series screen resistor was a stopper, maybe 1k, and the series string was chosen to have one or two orders of magnitude more current than the transistor base.

[Boris_The_Blade]

Time to break out the old HP uV meter

[smoking-amp]

One issue with the screen current bypass/re-combiner scheme is that the resultant tube curves look nothing like what they did before. Forget about it sounding the same.

The rounded knees that pentodes typically have become square, just like a Mosfet. (not the fault of the bypass device though) Normal screen current loss is the cause of the rounded knees. Put it back, and its back to square. More 2nd harmonic shows up then, sharper clipping. I've run the Mosfet/tube-pentode setup on a curve tracer, and the worst looking pentode curves you can find square right up into the cleanest Mosfet like curves. You might just as well use a Mosfet for the whole thing in the first place.

[TheGimp]

If we are discussing small signal amplification, should we be concerned about clipping?

[pointy]

I think the transistor makes a negative supply of the thing that makes the noise (some form of AC).

so the g2 becomes invisible to the noise making problem, which if I remember right is between g3 and the plate.

or something...........................?

[moonbird]

Ahem -- Mr. Gimp -- did you see that link to the cheap eurostrips that I left for you on the thread about breadboarding?

[leadbelly]

I consider the last 3 posts as 3 strikes...please start your own threads.

[TheGimp]

leadbelly, how does questioning about small signal and clipping irrelevant when the opening is:

" Lowering noise in small signal pentodes?
So, I have been searching and reading up on the various sources on how to apply small signal pentodes to input stages, and more specifically, how best to treat g2"

Constitute a strike and attempt to hijack the thread? I am questioning why large signal distortion is an issue when we are talking about using pentodes for small signal amplification.

[leadbelly]

Originally Posted by TheGimp

leadbelly, how does questioning about small signal and clipping irrelevant when the opening is:

" Lowering noise in small signal pentodes?
So, I have been searching and reading up on the various sources on how to apply small signal pentodes to input stages, and more specifically, how best to treat g2"

Constitute a strike and attempt to hijack the thread? I am questioning why large signal distortion is an issue when we are talking about using pentodes for small signal amplification.

1) How exactly does one hijack their own thread?

2) Who said ANYTHING about clipping or large signal distortion? I certainly didn't.

EDIT: Oops, my apologies, you were replying to smoking-amp about clipping...I didn't see that.

[TheGimp]

Apology accepted, no foul.

So back to my question, does clipping mater in this case where we are talking about using pentodes as small signal amplifiers?

Say we are doing a phono input stage, and we are amplifying 4.4mV rms with a gain of 100 (maybe not realistic). We have a supply of 200V and we are biased with the anode at 100V. The largest signal excursion will be 4.4 * 1.414 * *2 * 100 = 1.24Vp-p.

With less than 2v p-p swing we shouldn't have to worry about clipping, and should be able to restrict our operation to a fairly linear region. If partition noise IS addressed by the discussed circuits (and I'm not sure yet), then what are the remaining issues?

Ignoring partition noise are there other noise factors of small signal pentodes that would be of concern?