Interesting story thats been repeated many times over the years on the
reflector. Sort of a continuation of the Bill Orr obsession with an idea.
However Ive never seen a simultaneous blown choke and grid cap in the
decades Ive worked with the SB-220 series or any other amp using a similar
design.
Yes, a few amps that Ive received had a tube failure as well as an open
choke but they also did not have a HV surge resistor which I have championed
for 44 years. Look at the NCL-2000 and you will see that resistor. The huge
majority had an open choke and a perfect tube.
Early production SB-220 grid caps of the postage stamp variety will split at
the seams from age and moisture ingress but the dipped micas seem to last
forever.
As far as IMD Ive not bothered measuring on HF but on 6M it is always in
the -33 to -35 dB range with the chokes. Stability has never been a problem
with the hundreds Ive converted.
Is the Heathkit design perfect? What is perfection? As far as I am concerned
it works and I see no need to change it now 40 or so years down the road.
OTOH, The Ameritron 2 x 3-500Z AL-82 and the AL-80 with directly grounded
grids have reputations of taking out a tube and plate choke when there is an
arc. And that is supposed to be designed to perfection.
Carl
KM1H
----- Original Message -----
From: "Tom W8JI" <w8ji@w8ji.com>
To: <amps@contesting.com>
Sent: Monday, July 23, 2007 10:58 PM
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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