A fuse is cheap and easy means of protection. I've always used them. I agree
that other protective methods/circuitry offer a quicker response time to a
potential catastrophic glitch. BUT!... A fused B- has saved my @$$ many
times. Having had no less than 5 glitches in home grown amps running HV PS
of 4000 and 5000 volts I've not lost a tube yet ( Crossed fingers and knock
on wood ). The amps included 2 X 4-400, 4 X 4-400, 3cx3000 and 3cpx5000.
All 4-400 amps have been grounded grid and screens. Had I been using
tetrodes or pentodes with screen voltage I might not have been so lucky.
Regards,
Larry N5BIP
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of Tom W8JI
Sent: Thursday, July 20, 2006 5:09 AM
To: Gudguyham@aol.com; noway@nohow.com
Cc: amps@contesting.com
Subject: Re: [Amps] Grid fuses (was: Life and gain of 3-500Z)
> In a message dated 7/19/2006 3:17:48 PM Eastern Daylight
> Time,
> dezrat@copper.net writes:
>
> That's news to me.
>
> In my experience, a fuse is one of the most reliable of
> all electronic
> components, especially when compared to the active devices
> in an
> electronic protection circuit.
>
> To each his own.
>
> Bill, W6WRT
Bill,
What is the typical reliable opening current and opening
TIME of a fuse compared to the safe holding current? Have
you ever looked at it?
What is the reliable opening point (where it goes
near-infinite resistance) of a 1/4 watt resistor compared to
safe working current? Have you ever looked at that?
How many designs do you see that use resistors as fuses to
protect components?
Under what conditions would the resistor open?
Under what condition would an electronic circuit open? How
long would it take?
If we had a fault from anode to grid, which is the ONLY case
where a resistor would reliably open, would we want to
remove anode voltage from the tube or would we want to let
the grids float up to full high voltage, making the grid
several thousand volts positive with respect to the cathode?
Where would that several thousand voltage go, if not to
ground, when the grid is closely spaced to the cathode?
I think if you stand back and look at the claims fuses or
(much worse yet) resistors are a good idea in the grid by
looking at the entire SYSTEM and what you actually want to
protect, you will see the answer. Of course the decision of
what we want to believe can be based on measurable and
documented fact or on blind faith in someone's word or
guess. I prefer fact, others may not.
73 Tom
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