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

[Ian White GM3SEK]

Joe Subich, W4TV wrote:
>
>Rich writes:
>
>> > On the other hand, if most damage to the grid occurs very
>> > quickly, then one would probably want fast-acting, electronic
>> protection.
>>
>>
>> ... provided that the transistor used to interrupt grid-current
>> was up to the task.
>
>There you do again ... there is no need to directly interrupt the
>grid current with a transistor.

I don't know of any electronic protection circuit that actually works 
that way. Despite requests in the past, Rich has never shown an example 
of what he claims.

>As long as the anode has sufficient
>current limiting (arc protection) which is normally provided by the
>"glitch resistor," it is sufficient to bias the tube to cutoff.
>That can be done by the normal T/R circuitry, particularly if fast
>(electronic) bas switching is used.  when coupled with an electronic
>disconnect for the HV (e.g., shut off the primary to the HV) any
>overcurrent pulse is not likely to last long enough to cause
>significant damage.  By the way, these are the techniques used
>in the WD7S and G3SEK/GM3SEK control boards.
>

Contrary to earlier assertions, many hams have a strong wish for 
amplifiers that can handle a wide range of faults in their stride, 
recover quickly and keep going if at all possible.

That's certainly what I personally want from an amplifier. I would never 
use any method of fault protection that routinely requires a 
screwdriver, a soldering iron and a big hole in the operating time - not 
when there are other highly effective methods that can often get you 
back on the air within moments.

My Triode Board follows the same two-level fault logic as many other 
control systems. A grid over-current fault (occurring on its own) is 
often due to excessive drive or incorrect loading. This fault is 
detected inside a few milliseconds and will simply switch the amp 
off-line, but will not shut the HV down. The fault is reset simply and 
naturally, by releasing the PTT, and then the operator can fix the 
problem and try again. A small control adjustment is probably all it 
needs, and the amp is back on-line with minimal interruption.

A more serious fault involving a sudden surge in anode current will 
switch the amp off-line and also shut off the HV (if the board is 
interfaced to the HV supply as recommended). Typically this fault may be 
due to an arc from anode to ground, but a glitch resistor as recommended 
in Eimac Bulletin 17 should be enough to protect the tube. Such faults 
can sometimes clear themselves, and if so, a simple press of a Reset 
button brings you back on the air.

The downloadable User Manual on my website explains all this in more 
detail, and includes full schematics.



-- 
73 from Ian GM3SEK         'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek