Hello Amp Fans:
I have experimented with "grid fuse", while investigating grid equalization
and trying to eliminate the need for using matched pairs of triodes.
I believe what I tried at the time was about a 27 ohm 1/4 watt resistor.
What I found was that the resistor was very sensitive to grid current peaks.
The value that I selected for use with a pair of 3-500ZG triodes, in an SB
220 amplifier, seemed to be very sensitive to slight over drive, or so it
seemed.
The resistor would open circuit perhaps once in 3 months, while I tested
this system on the air in daily use. I can only assume that high current
peaks caused it to open, I never heard any arcing or other anomaly from the
amp.
There was no other damage during the testing. Nothing else failed except
the resistor itself. I did not experiment with other resistance values,
clearly this current level which cause the resistor to open was very close
to the maximum expected grid drive level.
I believe the grid equalization concept is worthwhile, and I increased the
resistor to a 1/2 watt value. This eliminated the open resistors, and the
amp performs flawlessly still today.
I can not prove or disprove the value of this protection technique, but I
can state that it did no damage to the amplifier, and did indeed prove to be
quite sensitive to driving current.
"One test is worth a 1000 expert opinions" as they always say. This was my
personal experience.
73 and Happy DXing,
Mike
W2AJI
----- Original Message -----
From: "Gary Schafer" <garyschafer@comcast.net>
To: "'Tom W8JI'" <w8ji@w8ji.com>; "'Paul Christensen'" <w9ac@arrl.net>;
<amps@contesting.com>
Sent: Thursday, July 20, 2006 9:32 AM
Subject: Re: [Amps] Grid fuses (was: Life and gain of 3-500Z)
>
> >
> > Even in wire circuits, electronic devices are faster and
> > more reliable. That's why the move has been away from fuses.
> > If resistors made good fuses, we would have a resistor panel
> > in our houses and not a breaker box. We would use GFI
> > resistors, and not GFI breakers, to protect people from
> > equipment faults.
> >
> > 73 Tom
>
> We don't see wide spread use of electronic devices in place of circuit
> breakers in our home distribution panels even though they may be far
> superior to circuit breakers.
> We have to sometimes select the device that is the most economical and yet
> provides an acceptable tolerance of protection.
> As an example, circuit breakers used for AC mains interruption are very
> unreliable compared to a fuse. A breaker can have trip errors as much as
> 600% or more over what they are speced at. Yet they are used extensively
in
> electrical circuits instead of the much more reliable fuse because of
their
> ease of use, convenience for resetting and the assurance that the proper
> size device remains in the circuit and to not have to go find a new
> replacement.
> But if you have a critical circuit a fuse is a much better choice than a
> breaker.
>
> **(I have a friend that worked as an electrical power engineer and tested
> many new breakers and found very wide tolerances in them)
>
> The same can be said about amplifier protection circuits. The ideal
> protection device is usually an electronic type protection circuit. But
how
> many hams will pay for those type devices?
>
> In commercial use amplifiers it would of course be silly to have to remove
> half the amplifier to replace a fuse when the amplifier needed to be
quickly
> returned to service. But a ham can tolerate the time.
>
> It comes down to compromise. What is better, a circuit with no fuse at all
> or a fuse that may not be the ideal device in the particular circuit.
> As for grid circuit fuses, as long as the fuse does not cause further
damage
> to the tube it would seem a good thing to do. As evidence of no further
> damage I understand there have been many amplifiers where the grid choke
has
> opened and not damaged the tubes. Now if there is more evidence that tubes
> are damaged in this situation than not, then the fuse may not be such a
good
> idea.
>
> 73
> Gary K4FMX
>
>
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