Screen current through cathode resistors

[bigwill]

In amplifiers with pentodes as the output devices, we all know there is often a small cathode resistor so that the bias current can be sampled. If it's small enough, say 10 ohms, you can forego a bypass capacitor.

The problem with this is, the screen current also goes through this resistor, which is highly distorted compared to the input or output waveforms. Even with a 10 ohm resistor there ought to be at least a little bit of degeneration so surely this can cause a bit of added distortion...

Is this actually a problem in practise? Should we in fact tie the cathode directly to earth and measure bias from a small ANODE resistor?

[DF96]

Screen current roughly follows anode current, except when the anode voltage dips below the screen voltage, so it is only on peaks where this could be a problem. Provided the cathode resistor is significantly smaller than 1/gm the effect should be small.

[HollowState]

Originally Posted by bigwill

Even with a 10 ohm resistor there ought to be at least a little bit of degeneration so surely this can cause a bit of added distortion...

Actually degeneration from an un-bypassed cathode resistor, which is negative feedback, reduces distortion.

[kevinkr]

Originally Posted by HollowState

Actually degeneration from an un-bypassed cathode resistor, which is negative feedback, reduces distortion.

In some cases it has been quite clearly documented that this form of local feedback may increase distortion rather than reduce it.. (I think SY might have some insight on this issue, I don't remember the references, I do remember encountering this to some degree in some of my designs. It certainly raises rp which is generally not desirable..) Where the plate and screen currents sum in that resistor I suspect if the screen current is highly distorted it might degrade the linearity to some degree, as DF96 points out if this sampling resistor is small compared to 1/GM it shouldn't be an issue. A 1 ohm current sampling resistor would certainly eliminate this as a concern and of course then no minor feat of math would be required for the conversion.. (I've used 1 - 33 ohm resistors in the past so you can see what I am getting at - now use 1 or 10 ohms only.. )

[Alastair E]

If you're only checking the BIAS, then the cathode-current will be the Screen Plus the Anode....

Distortion doesnt really apply to Bias Measurements, as you're just sampling the volts across the cathode resistor...
--Bias is usually checked with no-signal anyway...

[DF96]

The resistor may be present for checking the bias, but it is still there when signals come along. In most cases a small unbypassed cathode resistor will add a little 3rd order distortion, arising from the re-mixing of device 2nd-order distortion. Not to worry, provided the resistor is small enough.

[Miles Prower]

Originally Posted by bigwill

The problem with this is, the screen current also goes through this resistor, which is highly distorted compared to the input or output waveforms. Even with a 10 ohm resistor there ought to be at least a little bit of degeneration so surely this can cause a bit of added distortion...

Is this actually a problem in practise?

No. A 10R current sampling resistor is tiny in comparison to the actual rk (~1/Gm) so it makes no difference. (Different story with SS, where 10R could be excessive by an order of magnitude at least.)

Should we in fact tie the cathode directly to earth and measure bias from a small ANODE resistor?

No. That presents a possible safety issue. Even if you have a plate current meter permanently installed, a metal case could possibly become "hot". Better to keep the current sampling resistors on the cathode. Besides, cathode current sampling resistors can do double duty as a last ditch "fuse" to possibly prevent OPT burn-outs.

[ChrisA]

Originally Posted by bigwill

Even with a 10 ohm resistor there ought to be at least a little bit of degeneration so surely this can cause a bit of added distortion...

10 ohms seems high. I typically use a 1 ohm resistor. I think the amount of added distortion is so low that it does not matter. The only way to know for sure is a Spice simulation as I'm sure the effect to to small to measure on the bench.

[HollowState]

Originally Posted by kevinkr

In some cases it has been quite clearly documented that this form of local feedback may increase distortion rather than reduce it..

Ah, so degeneration (negative feedback) now increases distortion instead if reducing it. I think Reich, Tremaine and several others just rolled over in their graves hearing that bit of modern day wisdom. I'd be interested in knowing the exact and precise account of that.

[kevinkr]

Originally Posted by HollowState

Ah, so degeneration (negative feedback) now increases distortion instead if reducing it. I think Reich, Tremaine and several others just rolled over in their graves hearing that bit of modern day wisdom. I'd be interested in knowing the exact and precise account of that.

I know it sounds odd, and I will need to dig for references - did not make sense to me either, and I used lots of local feedback in my early designs, so this caught me somewhat off guard as well. I think SY may have done some actual measurements illustrating the effect or I might be completely off base here..

Edit:

Referring to Pullen here: http://www.pmillett.com/tubebooks/Bo...onductance.pdf p.32 which I quote below, your original understanding and mine were correct. I'm not sure who told me it wasn't true:

TRIODE DEGENERATIVE AMPLIFIERS
The equation for the gain of this amplifier was derived in Chapter 2,
and is:
K = -gm RL / [ 1 + ( gm + gp ) Rk1 + gp RL ] (8)
where Rk, is the portion of the cathode bias resistor which is not bypassed.
The equation for the load line for this amplifier is slightly
modified from that of the ordinary triode amplifier:
eb = Ebb - ib ( Rk1 + RL ) (23)
In other respects, the design technique is unchanged.
Example 13. To illustrate the effect of degeneration clearly, the design
of Example 4, Case 1, may be modified by assuming Rk1 = 400 ohms.
Find the change of amplification and distortion.
As Rk1, is negligible compared to RL, the same data may be used, giving:
.
ec 0 -2 -4 -6 -8 volts
K -14.1 -13.0 -11.8 -10.7 -9.4 .
DISTORTION
The distortion generated by the degenerative amplifier may be calculated
using either Equation 15 or the Fourier technique. Using Equation
15 with a peak-to-peak signal voltage es of 8 volts, the amplifier
of Example 13 will have a distortion of 5%. In a similar manner, a
peak signal of 4 volts yields a distortion of 2.4% ( Ec1 = -4 volts ). As
can be seen from page 13, the distortions without degeneration are 6.0
and 3.0%, respectively.

DISTORTION
The second-harmonic distortion of an input signal as generated in the
amplifier may be determined by using the small-signal amplifications
in the following equation:
D = 25 ( Kp - Kn ) / ( Kp + Kn ) (15)

Should put that contention to rest.. I think the real crux of the matter is that it just raises rp significantly and that may affect the AC load line adversely depending on what it is driving..