[Heathkit] HW100 IF voltages

[Dick KF4NS]

Thanks for the comments Ron there are not microvolts on the screen and plate. but I think you would  have 
to look at the schematic of the HW100/101 to see the conditions. I have 300VDC on the plate and B+ on the 
screen through a dropping resistor. What I am reading on the scope is the signal voltage, but you bring 
up a good point, why is the scope reading a negative signal voltage through a 10:1 probe and at such a 
low level??? Hmmm

73, Dick KF4NS

----- Original Message ----- 
From: "Ron Youvan" <ka4inm at tampabay.rr.com>
To: "Dick KF4NS" <kf4nsradio at verizon.net>; <heathkit at mailman.qth.net>
Sent: Tuesday, June 05, 2012 9:01 AM
Subject: Re: [Heathkit] HW100 IF voltages


|   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.
| -- 
|    73 Ron KA4INM - All E-mail sent to this address shall linger in the Google cloud forever!