[Amps] Re: 4-400 Question - It's Grounded Grid!

[bearlabs at netzero.net]

Message: 8
Date: Wed, 26 May 2004 15:27:43 -0400
From: Will Matney <craxd1 at ezwv.com>
Subject: [Amps] Bear, 4-400 question
To: "amps at contesting.com" <amps at contesting.com>
Message-ID: <40B4EFAF.1090809 at ezwv.com>
Content-Type: text/plain; charset=us-ascii; format=flowed

Bear,

"Q1: 4-400 No fil voltage applied, B+ applied, screen grounded, 
gridopen, floating. Tube fries??"

Without the heater running to heat the cathode, you shouldn't get but a 
minute bit of current draw through the tube due to capacitance. The only 
thing I can think of would be the anode arcing to the screen grid. Some 
tubes have the screen setting close to the anode. I'm not sure how close 
it is on a 4-400.

... Hmmm, there's a possibility.

"Q2.5 ...either case *with* fil voltage??"

Correct, with the cathode heated by the filament, it will produce a copious amount of electrons to be drawn towards the plate.



Q3: Manual states: "...Should there be excessive grid current and no plate current, suspect a discontinuity in the HV circuit. Probably
no HV is being applied..." How is this possible?? (are they assuming
drive is being supplied by the exciter?)

If screen voltage is applied with no plate voltage, and if the control grid voltage is low enough, the screen will draw current (acting like an anode), especially if the tube is driven either by the control grid or the cathode. But the screen will still draw current if not cut off by the control grid. This is another reason for the saying, "never apply screen voltage before the plate voltage".


Sorry - this stupid webmail thingie doesn't "do quotes"...

 NO SCREEN VOLTAGE! Grounded Grid Triode operation of the 4-400.s :- )
(Henry 2KD-3)


   _-_-bear

Q42: did I ask this one? The amp uses 2 10ohm 1/2w carbon resistors
bypassed by some .001 ceramics to ground the screen/grid pins. Is
this preferred practice these days?? Seems like a failure of these
resistors is a guarantee of smoking a relatively expensive tube?

Will Matney
US Amp





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