[Amps] grid fuses

[R L Measures]

On Jul 19, 2006, at 4:47 PM, Joe Subich, W4TV wrote:

>
> Lon W. Cottingham writes:
>
>> but why pay $200 ++  for a 3-500Z when a 6 cent, 1/4 watt resistor,
>> can/will save them from certain demise from several very common
>> faults?
>
> If fusing the grid is so "safe" why is it not used in commercial
> transmitters?

If seatbelts are so safe, why weren't they installed in our 1940 Chevy?

> If it protects a tube so well, why has no tube
> manufacturer recommended it (they recommend a surge limiting resistor
> in the anode circuit)?

Tube mfgs finally approved the glitch R in the + lead of HV-PS --  
years after some Amateur
Radio amplifier builders figured out that it was a good idea.
>
> I have worked on many commercial and broadcast transmitters over the
> past 30+ years and not one had any kind of grid fusing.  Every one of
> them had current limiting in the anode circuit and many had electronic
> "trips" that would dump the stored charge in the power supply (known
> as a "crowbar") in the event of a fault.  In fact, a "fuse" in the
> grid circuit is the opposite of what a qualified design engineer would
> use as it would keep the transmitter off the air until a maintenance
> person could physically strip down an amplifier to replace the fuse
> rather than simply reset/recycle when the fault clears.

As I see it, HF transmitting amplifier engineers are/were reluctant  
to admit that VHF and UHFparasitics are not an impossibility, even  
though it was pretty obvious that they were rather blissfully  
ignoring the feedback-C spec from tube mfgs.

>
>> I guess many of us are as blind and closed minded about amplifier
>> theory as we are about politics.
>
> As P. T. Barnum is credited with saying, "There is a fool born every
> minute."  It's those fools that keep snake oil salesmen and e-mail
> scammers in business.

That's one of my favorites, Joe, but I recently found out that the  
statement came from a competitor of P. T. Barnum.
http://www.historybuff.com/library/refbarnum.html
>
> 73,
>
>    ... Joe, W4TV
>
>
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R L MEASURES, AG6K. 805-386-3734
r at somis.org