pentode screens

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Mike H
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Re: pentode screens

Post by Mike H » Tue Dec 10, 2019 4:02 pm

Audible effects might be due to there being a capacitor across the filament, and not actually the HF – e.g. the Coleman reg doesn't do that (put a cap across the o/p). And probably also, what kind of cap. Hence my idea for using a series resistor as an attempt at isolation. A small value choke may not hurt either, bearing in mind an AC heater winding on a mains transformer is also a low value choke.
 
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IslandPink
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Re: pentode screens

Post by IslandPink » Tue Dec 10, 2019 6:25 pm

Maybe ther eare other explanations Mike, but there's plenty of scope for the (eg) 60kHz ( and it's typical 50mV ) to intermodulate with the audio signal in the 5-10kHz area, and produce side-bands. It may be worth tinkering with some calcs to use a small choke in series and a cap across the filament to get a low-pass filter that cuts the 50 to 60KHz down by eg a factor of 5 or more. What you do want to avoid ( from the Lehane fil work ) is having the output impedance of the supply ramping up or down significantly anywhere within the audio band. That just kills the tone for an octave or two. wherever it occurs.
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Mike H
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Re: pentode screens

Post by Mike H » Tue Dec 10, 2019 7:13 pm

Yes understood
 
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izzy wizzy
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Re: pentode screens

Post by izzy wizzy » Tue Dec 10, 2019 9:05 pm

I was looking at some application notes from Murata https://www.murata.com/~/media/webrenew ... 6to30.ashx to get a handle on this stuff. I thought the CM choke helped, about 8mH @ 4A rated from memory for GM70. Was going to try 3 terminal capacitor as well and then found combination units at Farnell of CM choke and 3 term cap all in one module (didn't buy it). I had some resistors in there before all that to drop voltage before the CM choke and thought it was headed in the right direction. IIRC, this stuff is designed to work in the range of frequencies we're interested in. I still may fiddle with some of this before the Lehane regs at some point to see what happens.

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