Why do you suppose that Collins put those little 47 ohm resistors in each
grid on the 30L1 amplifier?
73
Gary K4FMX
> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
> Behalf Of Tom W8JI
> Sent: Monday, July 23, 2007 9:58 PM
> To: amps@contesting.com
> Subject: [Amps] Grounding the grids
>
>
> 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
>
>
>
>
>
>
>
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