> 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.
I certainly have.
> 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.
....and you know it is age and moisture, and not an arc?
I've got boxes of 50 year old micas here of the same style
that aren't split from moisture.
> 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 you somehow know the arc takes out the tube, and not
that the bad tube makes an arc?
How would you know that?
The FACT is the HV in the AL82 is 3600 volts or so, while
other amps are down around 3000 or less. This is why very
rarely in AL80's do you see damage.
When the AL82 was designed 3-500Z's almost never arced. The
Eimac's were great. This isn't the case today with the
foreign tubes.
Truthfully if the AL82 was being done today it would have a
20 ohm or so limiting resistor. Why 20 ohms? Because the ESR
of the filter caps and choke combination is already ten
ohms.That limits surge to 360 amps or less, since the arc
path itself has some impedance. Adding ten ohms would make
it 180 amps or less, but adding 20 would get it down to 120
amps or less.
As for the idea a grid choke limits current, let's go back
to basic Ohm's law. When the anode faults to the grid from
gas, a plasma forms in the tube. See:
http://www.w8ji.com/fault_protection.htm
There are only two ways the plasma is extinguished and the
fault current stops:
1.) The HV is lowered until the current flow stops.
2.) The grid and cathode rise to the anode voltage, so the
current stops.
NOTHING else will magically stop the current.
If we assume nothing in the anode lets go, then the only
other option is the grid must rise to anode potential. When
it does that the little 500V rated mica caps have 3000
volts on them, and they arc. Now the grids are grounded
again despite the choke opening. The capacitors actually try
to protect the chokes.
If the grid actually has a successful choke opening and the
little caps magically can hold off six times their rated
voltage, then the grid rises to near anode voltage. Now the
plasma simply passes current through the grid to the cathode
system.
It's almost silly to consider that some sort of fusing
system.
The only proper place to add a fuse or series resistance is
the anode.
>And that is supposed to be designed to perfection.
No, that would be the National. :-) I don't know why they
still aren't around since a perfect product would certainly
have a longer sales life than the marginally designed AL-12
series, which only have run 25 years now.
On the other hand the AL82 and virtually all other amps on
the market today or in the past were designed to give the
best value for dollar of cost. Not a single penny was spent
that wouldn't benefit the customer for the target goal. Most
engineers understand it is very difficult to build something
that is priced to dominate a market but doesn't cause too
many problems.
One of the very best amps on the market was the SB220, it
had everything on the edge of being almost too cheap. When
the power limit was raised to 1500 watts output, the whole
game changed.
73 Tom
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