Fusing and Floating Grids
Fusing or Floating Grids
High power tubes
should never be
fused on the control
grids. The reason is
quite simple, any
leakage current will
offset contact
potential bias.
A San Carlos, CA Eimac applications engineer
pushed manufacturers to us ea circuit he called super cathode drive. He wrongly
claimed the circuit would add negative feedback. This was not an Eimac
suggestion and virtually all of the staff disagreed with him. This was something
he personally promoted.
When I worked for Heathkit and Ameritron, I received
several calls from him trying to get me to float the grids off chassis. I made
IMD and stability measurements and did circuit analysis. There was never an
advantage to floating grids, and there was generally a few disadvantages.
We were scheduled to remove the grid chokes and directly ground the grids in a
revision of the SB220 series (tentatively called the Warrior II). Unfortunately
Heathkit moved out of the amateur market before that amplifier reached
production.
There was one Ameritron amplifier where I added
grid resistors in an attempt to better balance tubes. These resistors were
not intended to be fuses!
The resistors were to balance grid current better between tubes as the tubes
aged and lost emission. This was the AL811H amplifier.
Unfortunately, despite my best efforts, these resistors would fail in hard tube
anode flashovers or if the tube “leaked”. These grid resistors offered a very
small balance improvement, but they put the exciter at increased risk of damage
from an amplifier tube failure. When I evaluated the positive results against
the potential cost, adding the resistors was clearly a mistake. Consequentially,
these resistors should be immediately removed from early AL811H amplifier and
the grids should be directly grounded.
I also suggest the grids in
amplifiers like the Kenwood TL-922, SB-220 series including the SB 221 and HL
series, and other amplifiers be directly grounded with the shortest widest
ground lead to chassis.
Self Biasing Tubes
Some articles claim the grid resistor or chokes act as
fuses or, even worse, were installed as fuses. They claim when the resistors or
chokes blow open from a tube fault, they act as fuses and allow the tube to self
bias off.
Such claims are ridiculous. Let’s explore what
contact bias or self bias is, and why it does not work in high power or
defective tubes.
There are two forms
of self bias on a
control grid:
-
Contact potential
bias - Grid leak bias
Note: Grid
leak bias is often
confused with
contact potential
bias, but the
operation and
applications behind
them are totally
different.
Contact Potential
Bias
Contact potential
bias is formed by
space charge near
the hot cathode of a
tube. When electron
emitters are heated
electrons boil off.
They collect in a
cloud around the
cathode. This cloud
supplies all the
charges available
for normal operation
of the tube.
Because the control
grid is in the
cathode cloud, a few
electrons will land
on the grid. If the
grid has high enough
resistance to ground
it can develop a
small negative
potential (excess
electron charge) of
a few volts.
Suitable resistances
are in the order of
several megohms.
This is a very
unreliable source of
bias. Contact
potential bias is
useful only in very
small low voltage
tubes, and it is
even unreliable in
them if the tube is
less than perfect.
Grid Leak Bias
Grid leak bias
is developed when a series
resistance, with a
suitable shunting energy
storage capacitance,
is placed between a
control grid and the dc
return path
to the cathode. When the grid is
driven positive
current flows to the
cathode. This
current causes a
voltage drop across
a modest value
resistance,
and the resulting
voltage drop charges a
leak capacitor. The
capacitor charges with a
negative potential
towards the grid and
a positive potential
towards the signal
source. After many
RF cycles of
rectified grid
current, the
grid leak storage capacitor will
charge to a
reasonably stable
voltage. Of course
if grid
drive is changed,
the capacitor moves
to a new voltage
level. Bias
voltage is
proportional to grid
current and the
value of series leak
resistance.
Grid leak bias is
commonly used in
class C amplifiers
or self-excited
oscillators. It is
generally just a
portion of total
bias. It is
undesirable in
linear amplifiers
because it
encourages gain
compression,
although very small
levels of grid leak
bias can be used to
equalize grid
current in parallel
tubes.
Fusing the Grid
A
few people claim grid leak
bias or
contact bias can be
used to place a tube
into
anode current cutoff
during a fault
condition.
The thought is, by
placing a fuse in
series with the
control grid to the
chassis, the open fuse
will act like an
infinite resistance
leak or contact
potential bias
source and safely cut
anode current off.
(also see
vacuum tube failures
and
Fault Protection)
Destructive fault
current comes
from two sources:
-
Excessive grid
current from
abnormal drive
levels or loss of
load. This “soft fault” is a
grid current fault
that is
generally at a
fraction of normal
anode current levels
in typical medium
power tubes. -
Excessive leakage or
flashover from the
anode-to-grid. The
normal cause is a
high vacuum arc from
gas, mechanical
failure, or debris
in the tube. This is
a “hard fault” that
exceeds anode
current by a very
large margin.
Grids, regardless
of fault type,
should never be
floated. This is
because:
-
Contact bias only
works with small
tubes, and even
then never exceeds
a few volts. -
Grid leak bias
only works with
normal alternating
drive current, it
requires an energy
storage capacitor,
and it develops
over time as the
capacitor charges
from alternating
cycles on the
grid.
Note:
This is true for
screen grids also.
It does not just
apply to control
grids. Screen grids,
just like control
grids, must have a
reasonably low dc
impedance to ground
to prevent
destructive runaway
from a positive
grid.
One participant in
the Amps Reflector
is convinced grid
leak and contact
potential bias are
the same. He uses
that as
justification that a
grid fuse is OK,
since grid leak bias
is OK.
Here is the unedited
text from two peer-reviewed engineering
textbooks that deal
with vacuum tube
amplifier system designs:
The
text above goes on
to caution beginners
to NOT use or
rely on contact
potential bias in
higher power tubes,
since contact
potential bias is
generally unreliable
and only works in
special cases.
Clearly the peer
reviewed engineering
textbook above (used
in my EE classes at
Toledo University in
the late 60’s)
states contact bias
and grid leak bias
are different.
Grid leak bias comes
from intentional
grid current caused
by grid drive from a
signal source.
Contact potential
bias comes from
random electrons
collecting on the
grid.
Text
below comes from
Giacoletto’s Electronics
Designers Handbook.
This engineering
book was a standard
reference used in
sophisticated vacuum
tube system design,
and was considered
as one of the very
best available by
vacuum tube
equipment engineers.
In the following
text, Giacoletto separates
bias into four
distinct classes:
Notice Giocoletto,
as in the earlier
text by Shrader,
makes a clear
distinction between
contact potential
bias and grid leak
bias. Dr.
Giocoletto, like
Shrader, cautions
against high power
use of high
resistance grid
paths. As a matter
of fact Giocoletto
specifically
cautions about free
ions, and he clearly
states high
grid resistance values
are useful in
receiving tubes
only! You can find
that statement near
the end of Giocoletto’s text.
It
has also be
suggested that
Terman somehow
supported the idea
of open grids (high
or infinite grid
resistance). Terman
actually claims the
opposite. Here is
what Terman
actually says:
Hopefully this page
will dissuade people
from following
the very bad advice
to ever use grid fuses.
We should always ground
the control grids, employ
fast electronic
current limiters
for soft faults, and
use HV anode
path fault
limiting to protect
against hard faults.
We should never use
protection systems
intended to float a
grid, either screen
or control grid, in a
transmitting tube!