[Heathkit] HW100 IF voltages

[KA4INM]

   Dick KF4NS wrote:

/*snip*/
> What I did was decrease the screen resistor back to the original 1K from the 10K mod which was intended
> to reduce the 2nd IF output and increase the BFO level. The assumption was that the IF was too high and
> exceeded the BFO output.

> I made the change to 1K and took measurements at the plate of V4 with my TEK scope. Before the change I
> was reading -8.2V RF WITHOUT the SB610 tap connected and -7.5V WITH it connected. After the return to a
> 1K screen resistor I was reading -7.8V WITHOUT the SB610 tap connected and -6.1V WITH it connected.

> Here is where I get all turned around. Why does the IF signal drop with a lower value screen resistor?
> With an increase in screen voltage due to the smaller screen resistor, why does the output go down? I can
> understand the drop due the extra load from the IF tap for the 610. I also noticed that the signal level
> at the receive output is lower WITH the 1K resistor with a constant level signal into the receiver input.
> I get the feeling that I should not only have seen an increase of IF signal output along with a stronger
> audio output signal.

> I am totally losing my knowledge of theory or I am misinterpreting my test results.

> Can someone get me back on track please ?

   There are several things going on in this case.
First, when we first study tubes we have a tube with a 3 Volt signal on the grid and 300 Volts of
signal on the plate, as power amplifiers work.
  Here we have micro Volts on the grid and the plate.  In this case the screen grid also acts like
an additional control grid.  Some tetrodes and pentodes have a screen grid that is as sensitive as
(or more sensitive) than the control grid, the 6AS6 (I believe) is such a case.
In these cases cathode resistance and the resistor bypass capacitor greatly effects the operation.
These tubes act a lot like two triodes in the "cascode connection," with high gain and low noise.
In the tube there is an electrostatic charge configuration based on the physical construction, where
the electrons travel in sheets of various shapes based on element shapes and position.  These
electrostatic charges effect the electron density in the various "sheets, rods and other shapes" all
in parallel.
I believe you forced the tube (in general) into a negative resistance region.  (The signal is going
to the screen instead of the plate.)
   I would feed a signal generator in at the smallest signal possible and look at the plate of this
amplifier while increasing the screen Voltage from 0 to the normal screen supply while watching the
signal level on the plate with your 'scope to confirm your observations, using a separate screen
variable supply or a 50 or 100 k Ohm pot connected to the normal receiver power.