>
>
>> Now, to Rich's point (I believe)...the fuse may in fact blow faster in
>> this arrangement than in the case of the PA-77 where a sampled
>> switching transistor operates a relay to force a standby condition.
>> The relay uses no special acceleration circuit. So, it would be an
>> interesting test to measure the "break" time of the fuse versus a
>> transistor/relay combination.
>
>Fuses are notoriously unreliable, if you doubt that test some...or
>look at references. Unreliability is the whole problem with fuses.
>
// Any port in a storm. In the Alpha 70-V, a fast fuse successfully
protected the grid. In my Fluke DMM, a fast acting fuse protects the
instrument.
>A fuse takes many minutes...or can even require hours....to fail at
>125-200% of rating.
// Surely, if it is a slow-blow fuse. Fast acting fuses are not
slow-acting.
>For light overloads, a fuse is both slow and
>may not be reliable at all for opening and protecting the system.
>Even with large overloads, some fuses can be very slow.
>
>For example, the Littlefuse 3AG Fast Acting 250mA fuse typically
>requires 1 ampere for 20mS to open, 1/2 ampere for .2 seconds, or
>0.4 amperes for 2 seconds. They only guarantee, with a 200%
>overload, that it will open in 5 seconds!
>
// The grid of an 8877 is rated to carry over 400mA.
>I find it hard to believe people actually think a fuse is faster than an
>electronic circuit, or better in the case of grid protection.
// Why didn't the AL-1500's electronic grid protection circuit protect
the 8877 against the gold-sputtering damage shown in the autopsy photo at
Figure 24 on my Web site? Why did Eimac's customer service rep, Mr.
Reid Brandon (a.k.a. Mr. XXX), tell QST that Eimac's Mr. W. B. Foote was
not authorized to inform me about gold-sputtering?
>The grid certainly has much more thermal lag than 20mS, but not enough
>thermal lag to handle 5 seconds!
// Are you aware of the temperature needed to boil gold?
> The thing that eats the grid up is
>either manufacturing or material problems in the tube, or lengthy
>periods of improper operation.
// The thing that gets damaged is not the molybdneum grid, it's the
gold-plating on the grid. / I re-read your Grate Parasitics Debate
dissertation/pontification on electrons evaporating gold. It was quite
delightsome.
>
>While I'm not a big fan of the single transistor threshold detectors,
>the general advantage of electronic overload is it has a very
>predictable threshold, rather than one of time vs amount of
>overload. Electronic systems can be designed to prevent people
>from "slightly hammering" the grid for extended periods of time,
>since that "slight hammering" has the effect of slowly displacing
>the plating from the grid over time.
>
// chortle. So why does the AL-1500 have a track record of 8877
failure? Hmmmm - it's Eimac's heat dam problem?
>Now the real laugh in all of this is a certain left-coast person
>suggests using resistors as grid fuses. He actually advocates, or
>has advocated in the past, removing (not augmenting) electronic
>protection systems and replacing them with resistors!
>
// An amplifier whose "protection" circuitry has already allowed failure
of one 8877 should be entrusted with a new, $700, 8877?
cheers, Mr. Rauch
- R. L. Measures, 805.386.3734,AG6K, www.vcnet.com/measures.
end
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