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Old 30th January 2011, 04:50 PM   #1
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Default Pentron tube microphone mixer/preamp

I found this Pentron MM4 microphone mixer and would like to try using it for recording. It has 4 channels each going through one section of a 12ax7. I thought I would try the circuit as is and then simply wire 2 channels with cascading gain each through 2 sections of 12ax7 with 2 outputs instead of 1.

I dont have much experience with this but understand some basics. I have not tested it yet. I will definitely replace the power cord with a 3 prong and most likely the filter and cathode bias caps. should the selenium rectifier be replaced? How do you do that without changing the B+ voltage?

Any ideas or suggestions are welcome thanks for your help

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Old 30th January 2011, 05:35 PM   #2
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Selenium rectifiers are problem areas. I would at least see if it's okay but know that you'll eventually have to replace it. While it's still working, measure the B+. Replace it with a silicon diode (1N4007) and series resistor, which you adjust to restore the proper B+. In fact, set it for about 5% low because of the slow increase of power line voltage over the years.
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Old 30th January 2011, 06:06 PM   #3
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This mixer uses high impedance mixing and should therefore only be fed into something with a high input impedance - 500K or more to be safe.

Replace the selenium rectifier. The increase in HT voltage will not materially affect the circuit operation in this case.


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Old 30th January 2011, 07:48 PM   #4
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The diode gets matched up with striped side same side where + would be on the selenium rectifier?

I'll be plugging the output of the mixer into my Presonus firebox which has 2 xlr balanced (1300ohm) preamp ins, 2 instrument (1M) preamp ins and 2 balanced line ins (10k). So I'll use the instrument ins, correct?

If say I wanted to make a 2 channel preamp out of this, one side being XLR microphone and the other being a tube DI maybe I would want to put a transformer on the XLR side so I could plug a SM57 into it or even add phantom power...what would be a cheap but decent transformer to add? I don't want to go too crazy with this project but a 1 channel mic pre and tube DI for bass would be really cool and useful for home recording projects...
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Old 30th January 2011, 10:56 PM   #5
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100K pots loaded on 300K resistors, it is fine for them. If you load 330K on zero ohm you can get theoretically best summing: less dependence between channels, trade off: less gain.
The Devil is not so terrible as his math model is!
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Old 30th January 2011, 11:20 PM   #6
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Transformers, well cheap and decent don't generally coexist.

It's common to use a high input impedance regardless of source. However, low source impedance means small voltages and consequent low output with more noise. A cheaper way is to make an amplifier rather than use a transformer.

And yes that's the correct direction for the diode.

I think the output of your new preamplifier will be too much for the instrument input of the power amplifier. Best to use one of the balanced line inputs; you can safely ground one side of the balanced input.
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Old 31st January 2011, 04:21 AM   #7
Jim W is offline Jim W  United States
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I'd agree with replacing the caps and the rectifier, but would probably use a better diode than a 1N4007. A BYV26E fast recovery diode will set you back a whole nineteen cents at Newark.

Don't electrocute yourself.
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Old 31st January 2011, 05:33 AM   #8
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If you load 330K on zero ohm you can get theoretically best summing: less dependence between channels
Should work in practice too, just by adding an anodefollower with another 12AX7 at the output. Try Rfb 200-300kohm. Cga could maybe be a problem though.
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Old 1st February 2011, 07:33 PM   #9
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If you make this for stereo I would split the cathode connections to one 4.8k resistor for each section bypassed with a 25 mfd cap. That is 1200 ohms times 4 , for 4 tubes to maintain the same bias and reduce crosstalk between channels.
Replace the selenium rectifier too.
Hope this helps
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Old 1st February 2011, 07:39 PM   #10
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Thanks that seems like a good idea. Why is that a good idea exactly? does it just help to keep each section properly biased?

If I cascade the gain stages can I just take from right after the 330k resistor following gain control right into the grid of the second stage?
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