[Amps] warm up time for TX tubes

[Leigh Turner]

Sounds fine John.

The principal requirement here for Tx tube longevity with indirectly-heated
cathode is not to draw any plate current during the 3 minute heater warm-up
and emission stabilization phase; your proposed arrangement here meets that
criteria.

My amp using the robust GU74B tube is now in its 14th year of flawless
service still with its original factory shipped tube. It uses the warm up
approach you outline.

However, the elegant amplifier design uses a novel soft-start arrangement
based on a phase-controlled Triac that slowly ramps up the AC mains over
several seconds so the filaments/heater and the HT supplies all come up
gently without cold inrush current or electrolytic switch-on surge.

Leigh
VK5KLT

-----Original Message-----
From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of John Flood
Sent: Monday, 11 April 2016 3:45 AM
To: amps at contesting.com
Subject: [Amps] warm up time for TX tubes

New to this reflector.   

Hope someone can help me understand tube warm up requirements.

Specifically, what are the necessary conditions for voltages at plate,
screen and control grid during Warm up for indirectly heated tetrode tubes.

I have seen directions that say no plate voltage during warm up, but some
amps (MLA2500) apply plate voltage and limit cathode current to a few mA by
self-bias during warm up.

What I have in mind for my amp with gu74b (4cx800) is to apply fil voltage
for 3 minutes while screen is at 0v, plate is full on and G1 is biased to
cutoff (approx. -75v).  

Is my approach workable?

And

What are the necessary conditions condition for the tube during warm up?..

Thanks in advance for your help

John  K4dlx  

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