Lou Wrote:
>>> Don't expect miracles by grounding the grids. Lou
Carl Wrote:
> Lou, it will be a miracle if a tube survives a flashover
> without the grid
> "fuse" or a healthy surge resistor.
Let me inject a little factual history of WHY Heath and
others used that asinine idea of floating the grids with a
choke.
Collins came up with a very good idea in the 30S1. That amp
used a cathode driven tetrode that could NEVER be allowed to
have any grid current. The tube had a rated grid dissipation
of zero watts.
Collins cure was to float the control grid through a low
value mica capacitor, and to have the grid return through a
high impedance. The small capacitance from grid to ground
and the fact the tube never drew grid current meant a high
impedance capacitive divider would reduce the power applied
to the control grid. Since the cathode to grid impedance was
always very high (the path was never in conduction) the
divider worked perfectly. The GK capacitance formed one
branch and the small C to ground the other. There was no low
impedance shunting the capacitive divider in proper
operation, because the tube never drew grid current in
normal operation. It was a good stable linear divider that
reduced power applied to the grid.
If the tube ever started to draw grid current the impedance
from cathode to grid would drop like a rock, and the grid
would effectively be tied to the cathode. This would reduce
grid drive and protect the grid.
This scheme really only works in an AB1 tetrode, because the
screen shields the input from the output and the control
grid never draws current.
That very good idea got all screwed up when someone thought
"gosh, that works with the 30S1 so well we might do that in
a 3-500Z". What they forgot is the grid to cathode impedance
is not high, the grid path conducts. It is AB2, not AB1. Now
the upper section of the divider is a variable resistance in
parallel with a capacitance, and the divider is not so
stable.
This is why, if you do a two-tone test on an amp like the
SB220 or L4B, you will see the IM products vary from low
bands to high bands and are not as good as possible. If you
simply remove the worthless grid chokes and capacitors and
ground the grids the amp is more stable at VHF, and has
better HF IM performance. The gain is also more frequency
stable between ten meters and 80 meters.
Now one might wonder why people used that method. The answer
is Bill Orr.
When I did an amp for Heath without the floated grids, Bill
was right on the phone. He pushed hard to use his "super
cathode driven" idea to give negative feedback. So I tried
his idea, and found it made both stability and IM
performance worse. Bill went over my head, and so I showed
someone else at Heath the test results. We took an SB220 and
pulled the grid chokes and bypass caps and grounded the
grids, and both gain flatness with frequency change and IM
performance improved. We could remove turns from the
parasitic suppressor and the amp would remain stable, but
when the mica's and chokes were added the amp would take
right off.
Now Carl seems to think, as some people do, that the grid is
a good place for a fault protection fuse. That's actually a
horrible idea, as just a little thought will show.
Say a tube has an anode to grid fault from gas, and the grid
is momentarily tied to the anode with a low impedance. Now
the choke opens.
What happens now is the grid, after it explodes the mic
caps, rises to B+. (If it does not rise to B+, it cannot
limit current in that path.) So we have a grid biased 3000
volts in an environment of plasma (that's what started the
arc) and the filament is only a very tiny fraction of an
inch away. Now the arc that formerly went to the grid goes
through the grid to the filament, and the filament and
everything in that circuit takes the full impact of the arc.
It's really a very dumb place to put a "fuse", because it
simply moves the problem into more areas.
If you want to have fault protection for the tube, put a
current limiting resistor in the anode. That resistor should
be at least a few times higher than the supply ESR to be
effective, or to make a useful change. Leave the grid at
ground to at least afford some protection for the cathode
system. The grid will have the arc anyway, you can't stop it
by lifting the grid after the fact. Even if you do, a 3000
volt positive grid will simply transfer the problem through
the grid to the cathode, so you really don't fix a thing.
Unless there is some compelling reason to do so, floating a
grid in a GG amplifier using a triode (especially a AB2
triode) is not the best idea. It certainly does not protect
anything, and it certainly does not make the amp work
better.
It really all got started because one guy made it his
campaign to make sure manufacturers used his "super cathode
driven" circuit.
73 Tom
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|