Hello Leigh,
I did the same calculation that you did and reached the same
conclusions. I don't have the amp up and running yet as I didn't have a
replacement part. I'm hoping I have something suitable at the office,
otherwise I'll have to locate one. Dayton is also coming up this week
so that's another possibility.
By the way, the resistor is a metal film type of construction.
On 5/13/2012 8:53 PM, Leigh Turner wrote:
> 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@contesting.com [mailto:amps-bounces@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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