Pentode tube help (7905)

Cmos cmos at SHOL.COM
Fri Aug 14 18:57:49 EDT 1998

Hello, I am working on a mobile transmiter.
I can not get any amplification out of the
one stage that uses a 7095 pentode tube.
the filiments for all the tube are conected in series
across the dc power and some of the biasing voltages
are comeing off the filoment circuit.

I think that the power going to the filomint circuit may be
backwoards, I got 2 different schematics, each one has it
a different way.
This tube(7905) is a directly heated cathod(the cathod is
part the heater. I looked it up in my RCA tube book and
the curves look as if some are positive voltages for the
control grid voltages. I thought that when the control grid
is 0 volts it is in maxumum condution (ie. a rectifier) and
that the control grid is to always be less then 0 volts.
Can some one explane this ?

Ok, now for back to my circuit.
With the filoments conected the way they curently are
i have the folowing voltages with respect to the center of the
filoment, pins 4 and 5:

Control grid(#1):   -3.1 volts
screen grid(#2):    + 165 volts
supression grid(#3): -9 volts
plate:                   290 volts

The thing that seem so to me is that the supression
 grid is -9 volts not the same potentional at as the cathode,
can any one explane this ?

Ok, here is that the voltages would be if I switched the
filoment power the other way, I did not do this yet, but
this is what I would expect, not figuring in the efects of
grid-leak biasing though.

Control grid(#1): +3.1 volts
screen grid(#2):  +165 volts
supression grid(#3): -3.1volts (or the same potential as if it was
                                                   connected to pin 1)

Again all the voltages are with respect to pins 4 and 5 (ie. the
meters negative lead in on pins 4 and 5)
Can some one tell me if the second case mite work better,
even with the control grid at +3.1 volts ?

Thanks for any help.

Jim Martz,

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