[Amps] Alpha 8100/GU74B Tetrode question

[Leigh Turner]

Hi Gary,

The GU74B tube has a rated maximum screen dissipation of 15W, and a typical
operating screen current of circa 20 to 30 mA @ 350 Volts, equating to less
than 10 Watts.

Under normal amplifier working conditions up to 60 mA (two tubes) of screen
current flows through your 15 Ohm series stopper resistor, equating to a
modest 54 milliwatts of resistor dissipation. 

The only way the screen current could rise to levels that would stress the
resistor would be under abnormal fault conditions such as the plate voltage
being removed, or the tank loading grossly maladjusted. However inn such
circumstances the amplifier's screen protection circuit should kick-in to
safely remove the screen voltage under such abnormally high screen current
conditions. 

I presume merely replacing the open-circuit screen resistor restored the
amplifier back to full working condition and there are no other associated
peripheral issues?

I imagine the resistor is a 1W carbon composition type and acted as a
sacrificial fuse during some short-term menacing current surge event, i.e.
it performed the obligatory function it was intended to do.

73

Leigh
VK5KLT


-----Original Message-----
From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
Behalf Of Gary K9GS
Sent: Monday, 14 May 2012 4:40 AM
To: AMPS Mailing List
Subject: [Amps] Alpha 8100/GU74B Tetrode question

Hello all,

I have an Alpha 8100 with a pair of GU74B/4CX-800 Tetrodes.  I recently 
had a problem occur with the amp going into a fault condition as soon as 
any drive was applied.

Through some help from RF Concepts, I isolated the problem to an open 
series resistor that supplies screen voltage to the tubes.  It's a 15 
ohm /1 W resistor.

My question.....what would be the most likely cause for this resistor to 
fail open?

-- 

73,

Gary K9GS

Greater Milwaukee DX Association: http://www.gmdxa.org
Society of Midwest Contesters: http://www.w9smc.com
CW Ops #1032   http://www.cwops.org

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