Hi Peter.
> >The Orr circuit belongs in zero grid current cathode driven tetrodes
> >ONLY.
>
> That circuit (or to be precise, a variant of it) can be used with grid
> current and grounded grid - the super cathode driven approach uses
> something like that although that is with a tetrode. Similarly the semi
> cathode driven amplifier -see Pappenfus.
>
> I believe the '220pf to ground caps' business got used first in the 30L1.
> The simplified schematic I have suggests that the ratio of grid to
> filament RF volts were used for establishing correct tuning and loading,
> which seems a little strange. It may be that approach works with 811As,
> but I agree with Tom that is isn't a general approach.
Let's look at that.
The grid to ground reactance is 70 ohms per tube on 3.5 MHz.
That reactance is fed from the cathode's ~100 volts or so peak
through a series resistance of about 100 ohms average, but that
resistance varies with time from near infinite resistance in parallel
with about 10 pF to a few dozen ohms in parallel with the same
value.
The anode is coupled back to the grid through about 5 pF, sourced
from about 3000 volts peak AC. That's about 9k Xc on 3.5 MHz,
which would allow 1/3 ampere peak current from the anode back to
the grid system.
The cathode to grid resistance in series with the bypass cap
dominates on 3.5 MHz...and produces a modest phase lag in grid
voltage (phase changes with grid current through the RF cycle).
The anode dominates on 30 MHz, because the anodes contribution
to the grid (caused by the 1100 ohm reactance of Cpg) now is 3
amperes peak. So the 8.2 ohm Xc grid capacitance moves around
in voltage mostly from the 3 amperes peak current caused by
anode voltage rather than the grid current or grid to cathode
reactance.
Of course the anode's voltage changes phase with the tank tuning,
causing the grid to shift phase.
Yes Peter!
That looks like a well thought-out system compared to say...a
resistor in the cathode that would always have the proper phase
and an almost constant value of feedback.
Seriously, that has to be one of the poorest systems ever
suggested for obtaining negative feedback! It is a useful system in
a GG tetrode with no grid current, but is silly in a triode with grid
current.
73, Tom W8JI
w8ji@contesting.com
--
FAQ on WWW: http://www.contesting.com/FAQ/amps
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
|