[Amps] Super Cathode Circuit

[Tom Rauch]

>I recall an article in Ham Radio many decades by Bill Orr, 
>with a super cathode circuit.  As I recall, a good portion 
>of the drive was to the cathode and the screen was 
>connected down on the cathode choke.  I have never seen a 
>complete amplifier design, but as I recall there was some 
>degeneration in the circuit.

Colin,

Orr lifted that design from Collins, but like Collins in 
later years Orr made some mistakes in the application of 
negative feedback. The problem is two-fold:

1.) In some systems is the phase changes quite a large 
amount with tuning or over wide frequency excursions.

2.) In some systems the grid to cathode impedance varies 
over wide excursions over the RF cycle, and with drive 
level.

3.) A system cannot depend on the feedback being applied to 
the same element that shields the cathode from the anode. 
This is because the feedback circuit effectively lifts the 
grid from the chassis for RF, so you no longer have a good 
shield. It destabilizes the PA.

Imagine you have a system with tuned circuits or portions of 
tuned circuits in or at the feedback path. As those circuits 
are adjusted the phase of the feedback can change quite a 
large amount. It is very easy to change from 180 degrees to 
something less than 90 or more than 270 degrees as the tuned 
circuits are moved through their range. That's one problem.

Another is the cathode to grid impedance. In a sub-class 2 
amplifier, like AB2, the grid to cathode impedance varies 
greatly with conduction angle. This makes it impossible to 
have stable amounts of negative feedback by lifting the grid 
through a divider unless the divider has many times LESS 
impedance than the lowest grid to cathode impedance. Take 
for example the poor idea of using a 3-500Z with the grid 
floated through a small mica cap ala SB220 and clones. The 
theory is the grid cathode capacitance forms one leg of a 
divider with the grid to chassis cap as the other half of 
the divider.

The problem is the G to K capacitance of the 3-500 is only 
around 15 pF depending on socketing and layout. That 
capacitance at the socket terminals also varies considerably 
with frequency because of series inductance and transmission 
line effects. It is a bit less on lower bands and 
significantly more on higher bands.

The time-varying (due to drive level) grid to cathode 
impedance shunting that capacitor is only a few hundred ohms 
average, and goes from nearly an open circuit during 
negative grid excursions to just a few dozen ohms on the 
peak of positive excursions.

Now the misplaced notion is a capacitor from grid to chassis 
makes a nice capacitive divider in conjunction with the 
grid-cathode internal capacitor as the other leg, but in 
order to divide voltage without shifting phase the FIXED 
capacitances would have to totally  dominate the voltage 
division system. It doesn't come remotely close. The upper 
capacitor in the divider has considerable series inductance, 
and it has a reactance of 3k ohms on 3.5MHz and much less 
than 200 ohms on 28Mhz (because of the series inductance and 
frequency change).

So on 3.5 through 28 the tube has a lower grid divider leg 
of around 38 ohms down to 5 ohms on ten meters. The upper 
capacitor has a reactance of 3000 ohms on 80 down to less 
than 200 ohms on  10 meters. The exact values move around a 
bit from lead reactances.

That 3000 ohm reactance on 80 is in parallel with the 
average grid impedance of a few hundred ohms!!!! This causes 
a HUGE phase error in the divider. By the time ten meters is 
reached the grid to cathode reactance is down to 200 ohms. 
This is in PARALLEL with an average grid to cathode 
resistance of 200 ohms, but that resistance varies over the 
RF cycle from much less than 200 to nearly infinite. This 
not only greatly changes the amount of feedback from band to 
band and over each RF cycle, it changes the feedback phase a 
considerable amount.It causes an increasing phase lead as 
frequency is lowered.

The idea a grounded grid class triode can use a floating 
grid for negative feedback is just silly. We not only can 
calculate the errors, the errors show up in testing. Making 
matters even worse, it is applied to tubes that draw grid 
current!! Harmonics that contribute to IM have uncontrolled 
phase, and the stability is compromised.

This is one reason why the 30L1 demands a long driver cable 
and has a long history of field modifications to improve 
stability, and why amps like the SB220 are improved by 
directly grounding the grids.

The partially floating control grid has a place in a GG AB1 
tetrode where the grid NEVER draws current and the screen 
shields the output from input, but it is very poor 
engineering in a AB2 amplifier or especially a triode!

A resistor in the cathode, if the stray C across the 
resistor or series inductance to the tuned input or cathode 
is low, does not do any of this. It is always a better 
system by far.

But then, it was never exotic enough to be anyone's 
exclusive pet idea, was it???

We get in trouble with these agendas where something that 
works in one specific case is taken outside all boundaries 
and applied to systems where it falls apart, just because it 
is someone's pet idea or cause in life. This applies to 
nichrome as well as that souper cathode drive.

When someone thinks they have the single universal cure all 
for all applications look out!!!

73 Tom