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Electrolytic Swap?
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xqtion
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#1
28th February 2012
Old 28th February 2012
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Electrolytic Swap?

Hi,

I have swapped 3 electrolytic caps (C20, C21, C22 all 1uF/450V) of my tube mic to polypropylene.

My question is, do I have to get rid of the bypass Wima film caps that follows?

Thanks!!

ps: I think C22 is actually 1uF/450V since there were actually three 1u/450V electrolytics unlike the schem.
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Electrolytic Swap?-prodipelsp_schems1.jpg  
#2
1st March 2012
Old 1st March 2012
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No, you don't need to get rid of the bypass caps. They're fine.
And I think you may be right about C22. It looks like pin 1 of the connector is the high voltage plate supply, in which case C22 would need to be the highest-voltage cap in there.

Is this a cheap modern tube mike? I think it's silly that they've got a 12AX7 dual triode and they have half of it (one triode) sitting there doing nothing. I'm also curious what's going on with the transistor in the plate circuit. Doe the power supply have an extra knob or button for something?
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xqtion
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#3
1st March 2012
Old 1st March 2012
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Yeah it's a Prodipe Lamp Studio Pro tube mic (around $300). I thought it had a great potential stock thus deciding to improve it But I think the designer made a slight variation on the actual circuit verses the schematics I attached. There are slight differences from looking at the pcb compare to the schematics.

Anyways the PSU only has lo-cut, -10 pad, and a pattern selector.
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1st March 2012
Old 1st March 2012
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Where does pin 6 of the cable go in the PSU box? It's an odd circuit. Whatever's on the other end of pin 6 turns transistor Q7 on and off. If it's on, R41 is grounded and you have a pretty basic cathode-biased triode circuit. If Q7 is off, your cathode resistor is suddenly 153K (R41 + R11) which pretty much shuts off the tube, but it would also raise the cathode voltage high enough to destroy C31, which it says is rated for 16V. Does that match your microphone? Where did the schematic come from?
xqtion
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#5
2nd March 2012
Old 2nd March 2012
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PSU Schems

Here's the schems for the PSU. I got the schems from Prodipe tech support.

Maybe Q7 acts as a delayed start/stop switch to protect the tube for longer life span ?
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Electrolytic Swap?-prodipe-psu-schematics.jpg  
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2nd March 2012
Old 2nd March 2012
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Nope, it looks like Q7 is for the -10dB pad switch. I guess by cutting down the cathode current they're reducing the gain of the amplifier.
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2nd March 2012
Old 2nd March 2012
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Hmm... Interesting~
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2nd March 2012
Old 2nd March 2012
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Cold, or cool biasing reduces gain..?

Not a problem for the 16volt rating of C31.

http://www.freewebs.com/valvewizard1...Gain_Stage.pdf

This mic seems to be well designed, and should not have been modded severely.
Except for C21, and maybe C29.

Why all the different post for this mic mod.
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3rd March 2012
Old 3rd March 2012
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Could someone explain what happens when you extreme cold bias a triode.
I used formula IV on page 36, but formula III with cathode bypass cap is relevant here.
Also, Lm317 seems to be a constant current source, but drawn as a constant voltage source.
Should the connection between R4 and R5 be there?
Leo..
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