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Old 15th August 2002, 10:08 PM   #1
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Default help on power supply issues and more!

Hey everyone. I am building a preamp for recording to my computer. The main idea is this:

cap input power supply with filter choke after first cap. 330 VDC after complete filter stage.

mic input (transformer) w/pad
line in (guitar/instrument) w/pad

-> EF86 gain stage as pentode
-> 6DJ8 mu stage

Also, I want to take the output, amplify it, rectify it and feed it back to the EF86 screen grid for a basic compression circuit.

Now, my main question now is, after the choke I have a bleeder resistor to get the critical inductance down to what I have (9 Henrys) which means (30 mA * 330 VDC). I got that formula from an old ham radio book my dad gave me. Does that make sense? So, that means a 10k bleeder resistor right off of the 330 VDC. That thing gets damn hot. Is this OK?

Now, I'm wondering if I can get any opinions at all about my main preamp idea. As far as tube choice or whatever. I was thinking an all tube preamp ('cept for the diodes) but I'm probably going to rectify and regulate the heater supply. And what about this, a current sourced EF86 (small signal pentode)? Has anyone current sourced a pentode? I've looked through the internet for this but nothing.

Anyway, please let me know what you think and any help would be greatly appreciated! Thanks!!!!

- Matt Mitchell
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Old 16th August 2002, 02:30 AM   #2
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Old 16th August 2002, 02:35 AM   #3
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Location: Dallas,Tx
For an explanation on how the EF86 works in this circuit
Look for the info on the Mullard 3.3 kit
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