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Old 30th Dec 2019, 10:01 pm   #1
bigbri5050
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Default Help with 5840/EF732 mic schematic please.

I have a schematic for the 5840/EF732 pentode being used at 12V in a Telefunken microphone. It has a 12V rail, 100k anode resistor, 1 Mohm used on each of the grid terminals. I imagine that it should produce a high quality sound. However, I want to use it to 'warm' up the sound with varying amounts of second harmonic distortion. Can you advise how I might vary the circuit in order to acheive this. There is a cathode resistor of 1k with a 100uF electrolytic in parallel. Should I vary this or reduce the control grid resistor, (1M)?

I would appreciate any help as the low rail voltage means there is not a lot of expertise available on line.
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Old 31st Dec 2019, 5:02 am   #2
Radio Wrangler
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Default Re: Help with 5840/EF732 mic schematic please.

Those things would likely greatly reduce the sensitivity and/or drastically curtail the frequency response before producing much effect.

With only a 12v supply, it's not likely to be terribly linear as it is.

classic capacitor mikes with valve amplifiers in them tand to be quite valuable, if left in original condition, so you may be better leaving the microphone alone and consider something like a guitar effects pedal after it.

David
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Old 31st Dec 2019, 8:53 am   #3
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Default Re: Help with 5840/EF732 mic schematic please.

Welcome to the forum fellow Lincolnite. As David says mucking about with the circuit could effect frequency response however not doing anything is no fun and you learn nowt.

In a nutshell reducing the anode resistor and or voltage usually results in a less linear OP, IE more distortion, but you lose gain. There's a few things you could try like tying the screen grid to the anode, running it as a triode and taking off the cathode cap, all this will reduce gain though.

With only 12v you haven't got much wiggle room, but AFAIK you should have 48v available from phantom power. However if you use a dual opamp as buffer and gain stage with the valve in the middle you might get somewhere. You really need to draw some load lines and breadboard any ideas. From what you say you haven't actually got the mic, so no worries.

There's some great info here on drawing load lines and other very useful info - http://www.valvewizard.co.uk/index.html start by downloading and reading the triode gain stage PDF. Also there's a forum dedicated to DIY studio gear here - https://groupdiy.com/index.php as well as of coarse the diyAudio forum.

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Old 18th Jan 2020, 8:43 pm   #4
bigbri5050
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Default Re: Help with 5840/EF732 mic schematic please.

Thank you for your help with this. I should have said that the input will be coming from a guitar and not a microphone. I can easily get a lot of input voltage.
What I wanted was to get some very mild distortion, using the overdriven pentode to give variable amounts of second harmonic and even tones above that.
Like you say. I will experiment. I have a job lot of valves. I really like the idea of using the phantom supply. Nice lateral thinking.

Thanks again.
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