[AMPS] AL-1500, AL-82, AL-1200, QRO???

[Ian White, G3SEK" <g3sek@ifwtech.com (Ian White, G3SEK)]

Rich wrote:
>
>>
>>Rich wrote:
>>>
>>>>
>>>>Rich wrote
>>>>> The glitch R needs to be in the positive HV lead.
>>>>
>>>>Piece of education needed for me here. The glitch resistor is there to 
>>>>limit fault current in event of a flash-over. The only source of these big 
>>>>currents is from the filter capacitor and the only route for the current 
>>>>back to that cap is via the 25R resistor. So why does the resistor need to 
>>>>be in the +ve line.
>>>>
>>>The typical positive filter cap ckt is insulated for several kV.  
>>>The negative filter cap/cathode circuitry/fil. transformer CT is not.  
>>
>>The negative end of the filter cap can easily be insulated to those
>>standards - you just have to remember to do it.
>>
>One does not have to remember anything if one puts the glitch R in the 
>positive HV lead.  
>
Except to insulate *both* ends to full HV. A possible advantage of
placing the glitch resistor in the negative return is that one end is
always close to chassis potential.

>>The cathode circuitry and filament transformer (assuming we're talking
>>about GG triodes) would not see a spike caused by a direct short or arc
>>from B+ to grid or chassis - they are protected by the glitch diodes. 
>
>Limiting the V-drop across the glitch R to 1.5v will hardly allow it to 
>limit current. 
>
Stop and think again. The cathode circuitry, meters and transformer are
protected by the diodes, but the glitch resistor must NOT be shunted by
the diodes - it sees the full HV. 

>> If an arc inside the tube penetrates through the grid to the cathode, 
>
>I have autopsied dozens of tubes.  I have never found a penetrated grid.  
>  
Maybe it depends on the types of tubes... or maybe you've led a
sheltered life :) 

I have seen it on an autopsied GS35b tube, which has a flat oxide
cathode and a domed mesh grid. The grid had about six arc marks on it.
Some marks were on the grid alone, with no mark on the cathode
underneath. One (probably the one that ended the tube's career) had
burned a hole right through the grid and then burned the cathode
underneath. 

A couple were especially interesting - there were marks on the cathode
directly below the marks on the grid, but they had done that *without*
burning through the grid. My speculation about these would be that when
the arc hit the grid, it pulled that local area of the grid very
positive for a few nanoseconds (which could happen because the grid mesh
has a finite self-inductance) and that stripped the oxide coating
directly underneath. 

(Note to GS35b users: that tube had been operated at close-on 5kV.
Normal users don't need to worry.)

>
>>those
>>components are not protected any differently by a glitch resistor in the
>>B+ or the B-minus. As Andy said, it's the same surge current flowing
>>around the loop,   ...
>
> It can not be the same current if the Glitch R drop is limited to 1.5v 
>by the glitch diode.  

You're imagining the diode in the wrong place, shunting the glitch
resistor.

>---------
>In "The Almost Perfect Amplifier"  (*QST*/January, 1994), I recommended 
>using a rheostat to adjust filament V.  During the grate parasitics 
>debate, at least five members of the Rauchian camp argued long and hard 
>that a rheostat can not be used to control filament V.  
>
Forgive me, but I'm not seeing the relevance of that statement to the
present discussion.


73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                          'In Practice' columnist for RadCom (RSGB)
                           http://www.ifwtech.com/g3sek

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