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Old 24th Apr 2020, 6:31 pm   #8
G6Tanuki
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Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 13,953
Default Re: When to fit valve screening cans?

What are the valves doing?

If the anodes are at significant RF-potential [they're being used as RF/IF amplifiers, frequency-multipliers, VFOs, mixers] then the 'can' isn't really there for heat-dissipation issues, it's there to provide a consistent, predictable and long-term-repeatable capacitance between the anode and ground.

I remember a "distributed line" six-6AK5 RF amp that someone tried to align without the cans. He spent an afternoon on it before I pointed out that the few picofarads of anode-to-can capacitance was part of the design equations and he couldn't possibly get it to be both stable and have low 'ripple' across the necessary 50MHz bandwidth without the cans fitted.
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