[Amps] 8877 Grid Trip timing

[Karl-Arne Markström]

Actually the "crowbars" in large transmitters are there to save the "tube innards" (grids, cathode) in case of
internal arcs which are common occurrence. The sense circuit is connected in the plate return, and the ignitron diverts the the major part of the fault current in the order of a few hundred microseconds.

Grid overcurrents due to other causes are usually sensed by more conventional overload relay circuits 
which have time constants of some tens of milliseconds, in order not to trip on modulation peaks.

Standard procedure for checking the crowbar circuit in 100 kW Telefunken transmitters (which, as a whole, were quite tempermental creatures) was to short circuit the 14 kV 15 A rectifier using a 0,3 mm silver wire, which should not burn off or melt. This obviously required some courage by the maintenance personnel.

73/

Karl-Arne
SM0AOM





----- Original Message ----- 
From: "Jim Tonne" <tonne at comcast.net>
To: "Tom Rauch" <w8ji at contesting.com>; "Rick Stealey" <rstealey at hotmail.com>; <amps at contesting.com>
Sent: Tuesday, October 02, 2007 5:33 PM
Subject: Re: [Amps] 8877 Grid Trip timing


> 
> Tom wrote (in part):
> 
> > My homebrew amps trip in less than 2mS of excessive grid 
> > current. They also fault if the reflected power is more than 
> > 20% of forward power.
> 
> Excellent!
> 
> And in the high power shortwave transmitter world those
> transmitters use crowbars and trip even faster than that.  
> Those big tubes are pricey and even a millisecond grid fault 
> can melt it and cost a fortune.
> 
> - JimT   WB6BLD
> 
> 
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