[Amps] Grid fuses (was: Life and gain of 3-500Z)

[Roger Glover]

----- Original Message ----- 
From: "Tom W8JI" <w8ji at w8ji.com>
To: "Roger Glover" <r.glover at btinternet.com>; <amps at contesting.com>
Sent: Friday, July 21, 2006 8:36 PM
Subject: Re: [Amps] Grid fuses (was: Life and gain of 3-500Z)


>> This, surely, will the make the grid rise (positive), fully saturating 
>> the tube, valve, bottle thereby shunting HV to ground via the cathode, 
>> blowing the primary protection.
>
> That's a slow boat to China! First it has to dump all the energy of the 
> supply through the cathode,
> then it has to ramp primary current up to fuse meleting levels while all 
> that energy is duming in the cathode circuit and perhaps even back through 
> the input to the rig.

How does tying the grid hard to ground help, apart from the profits of the 
tube retailers?

>
>> What damage to the grid (the most delicate electrode) occurs during this 
>> 'event' <might> be less with the above devices fitted  (select your 
>> preferred order) rather than tying the grid to ground with a solid strap.
>
> Why would the grid be less subject to damge? It is the first thing in 
> line.

The fact that the grid "is the first thing in line" is not in dispute, 
HV-grid fault current and its duration is.

It seems that you are inferring that a frangible component in the grid 
circuit, when subjected serious overload provides no more protection than a 
solid strap.

>
> The only thing a grid resitor does is allow the fault to shift to the 
> cathode for a new wave of destruction.

A cathode is surely more suited to dump 'event' current than a grid?

>
>> I use R's, I've got 1985 full o/p 3-500's, once in a while there's 'pop', 
>> I swear, fit new R's, a fuse and am back on the air in 15 minutes with 
>> the same bottles.
>
> Too bad you didn't just ground the grids and install a fault limiting 
> resistor in the supply positive lead. A high voltage surge rated 20 ohm 
> resistor would hardly allow a pop to be heard, and you wouldn't even have 
> to take the cover off to repair it. Just reset a breaker and away you go. 
> The real advantage is you don't risk dumping all the nasty stuff back into 
> the cathode circuit.

I  wrote, you snipped:-
"
For 3-500 factory or kit built amps, whether it be a fuse, resistor or the
original inductor at the grid that takes a HV-grid event (via a glitch
resistor) it will blow open.
"

IME, heavy grid straps, even with a 25 ohm HV glitch resistor, a plate-grid 
'event', is bad news for the 3-500 tube grid.

Roj





>
> 73 Tom
>



Roj