[Big Bad Bob]

On 01/31/16 13:56, John L Stewart so wittily quipped:
Measure Step Size caused by the Screen Bypassing Network
John L Stewart Jan 2016

This paper covers measurements made to determine the frequency response
step size caused by the screen bypass circuit in a voltage amplifier
pentode.

Two similar circuits were used in this trial. Neither is optimized. In
one circuit the screen grid is supplied DC in the ordinary way from the
plate supply. In the alternative circuit a screen DC connexion thru a
suitable resistance back from the following cathode of a DC coupled
split load phase inverter is used. Each has certain advantages &
problems. Refer to the schematics.

The signal generator is direct connected to grid one of the 6U8 pentode
section. There is no capacitor in order that there be no other RC time
constant in circuit that might cause errors during the tests. Notice the
screen bypass capacitor is rather low capacity. The test equipment would
take a very long time while operating at low frequency in order to
acquire the data. Shifting the frequency by using a smaller RC time
constant eliminates that problem. The resulting step remains the same
size.

The Pico Technology Scope/ Spec A has a max input of +/- 20 volts. With
the X10 Differential Probe that becomes +/- 200 volts. So the gain
measurements are all taken off the cathode of the triode section of the
6U8. If taken off the plate connexion of the 6U8 pentode the probe would
need to be set to X100 which would result in less measurement
resolution. The experimenter needs to know as much about what his
equipment will not do as what it will.

For the ordinary circuit- in out Gain Gain db


Bypass cap connected 100 mV 12.54 V 125 42
Bypass cap disconnected 100 3.59 35.9 31
Step is ~ 11 db


Bypass cap disconnected 182 mV 6.14 V 33.7 30.6
Bypass cap connected 182 18.8 102.2 40.2
Step is ~ 10 db almost clipping

For the CF to G2 circuit- in out Gain Gain db

Bypass cap disconnected 99 mV 2.11 V 21.2 26.5
Bypass cap connected 103 7.89 76.6 37.7
Step is ~ 11 db overloaded


Bypass cap connected 50 mV 4.87 V 97.4 39.8
Bypass cap disconnected 48 1.04 21.7 26.7
Step is ~ 13.1 db almost clipping

Terman reports phase shift for screen steps in this range have maximums
of 25-40 degrees.


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I would expect screen resistance to change based on G-K volts, according
to some kind of general curve, and it's probably NON-linear at that,
based on the actual screen volts, plate volts, plate current, blah blah
blah.

But yeah, get it wrong and you howl and screech like a poorly configured
regenerative receiver.