Message: 8
Date: Wed, 26 May 2004 15:27:43 -0400
From: Will Matney <craxd1@ezwv.com>
Subject: [Amps] Bear, 4-400 question
To: "amps@contesting.com" <amps@contesting.com>
Message-ID: <40B4EFAF.1090809@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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