Apex 460 -> "C12" fixed grid bias schemes

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monkeyxx

Well-known member
Joined
May 23, 2010
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67
I've gotten pretty far with my apex 460 mods, the mics are sounding great.  I thought I was done until I encountered a post from Open Plan Recording where they figured out how to modify the Apex psu to provide the fixed -1.1V to the grid, like the original C12.  In the sound clips below you can hear the difference compared to a diode biasing method, that I got from Fox Audio research, that I'm currently using.

https://soundcloud.com/open-planrecording-studio/sets/c12-diode-vs-psu-1-bias

The problem is I don't know where to begin on modifying the power supply.  The old C12 schematics are basically illegible to me, hard to read, some numbers are smudged, pF and mF are written the same,  lines zig zagging everywhere.  I know JJ Audio has figured this out also for their "Dutch." 

I guess the Apex 460 psu is pretty similar to the C12 in the B+ supply in that there's a lot of series resistors with filter caps between them, but the values and number of parts is different, also there's a single diode(?) in the C12 and four in the 460 in the rectifier.  Also the Apex uses zener diodes to ground and these are not in the C12.  And the cathode in the C12 is connected to ground directly, instead of by resistors and caps as in the original Apex 460 (or two 1N4148 in series in my case).  Do you think I could just rebuild the B+ section of the PSU only following the C12 schematic if I could actually read it?  And the attached grid bias section.

I don't know if anyone hear can offer advice?  Like what is that 300K 1/2 W resistor in the C12 schematic going to pin 4 of the pattern selector to nowhere? 

I suspect this is a valuable mod, for getting closer to the C12 sound.  I do know my tube microphone cables have an extra unused conductor I can employ for carrying this voltage if I can get at how to do it.
 

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  • psu-stellar similar to apex 460.jpg
    psu-stellar similar to apex 460.jpg
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here's the best C12 schematic I could find

 

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  • AKG-c12 ACTUAL GOOD SCHEMATIC WITHOUT LINES ALL OVER IT!!!!.pdf
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Stock Apex 460 is cathode bias:  you'll need to generate a negative bias in the PSU, send it down the cable, and then attach it to the grid via a biasing resistor.  You can look at my C12 clone schematics for a hint on how to do this, but it's not the only way.

In order to be truly "fixed bias", the bias must be constant as the Q/operating point of the tube changes.  Even the original C12 is actually not fixed bias, in that the negative bias level will change as the tube draws more or less current (in a DC sense at least).  So if you really want "fixed bias", you'll need to generate a constant low-impedance negative voltage level in the PSU and then get it to the grid.

Make sense?
 
Thanks a lot I actually just stumbled across your schematic in your Official C12 Clone thread and yes that might be just about good enough for me!  I wonder if the diodes to ground method is more or less of a truly fixed bias than the standard C12 bias scheme.  I'm sure there's an electrical difference, at least, since there's an audible one.  I am not on the EE level yet, more of an aspiring "tech" with some aspirations and a little hard math here and there, but mostly just practical experience not quite yet matched by theory.  I am a musician!  At this point, things like the C12 grid bias schematic, and Neve amplifier schematics, are slightly boggling to me.  They are certainly more complex than typical guitar effects, seeming to "stack" higher when drawn, more than a straight signal path with + voltage coming from the top and grounds to the bottom.  A big muff for example seems very "linear" when drawn.
 
I have to link the schematic it's beautiful, so easy to read!
 

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  • c12_schematics_v3.pdf
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Just to be clear:  my schematic is similar to the original C12 (exact for stuff inside the microphone), however my PSU is fairly different:

1) I use a full-wave rectifier for the B+ supply:  original C12 uses a half-wave rectifier
2) I use a linear regulator for the heater supply:  original C12 uses a Zener reference supply

However the B+ is of the spirit of the original, with cascaded RC stages.  Mine is missing a Zener reference like is used on the Apex 460.

I like the linear regulated heater supply much better than the original:  it tracks better with current and with PSU case temperature, and is even extremely stable across different tube types, and can be adjusted by fiddling with a single pot.
 
Matadors schematic is perfect for figuring it out!!Great work Matador!!

Cut D1-D2 from ground and apply the negative Bias part circuit to the Apex psu just make sure you re-ground anything that is un-grounded when you cut C1&c2 from ground and discharge those caps before you start!!
Also I used Pin 4 to get it into to the mic.
 
monkeyxx said:
...I encountered a post from Open Plan Recording where they figured out how to modify the Apex psu to provide the fixed -1.1V to the grid, like the original C12.  In the sound clips below you can hear the difference compared to a diode biasing method, that I got from Fox Audio research, that I'm currently using.

https://soundcloud.com/open-planrecording-studio/sets/c12-diode-vs-psu-1-bias

I can't find info about what the PSU-1 bias means? Also, is there info somewhere about how they made the shootout? Sounds and looks very much like a same take with two mics (don't know how to download from soundcloud to confirm) which also means a difference in mic positioning, different capsules etc. which again would make the shootout quite useless.
 
The shootout was done with two mics at the same time with the capsules as close together as possible and the guitar about half a meter away . The mics are identical in every way, the only difference  was the biasing method.
The difference is  easy to hear and repeatable try it out  for you self :)
 
This is another test with the same mics Diode Vs Psu acoustic guitar chords and drum room..Listen for the extra bass bump on the psu biased mic!!

https://soundcloud.com/open-planrecording-studio/sets/opr-c12-deluxe-diode-vs-psu

 
OPR said:
The shootout was done with two mics at the same time with the capsules as close together as possible and the guitar about half a meter away . The mics are identical in every way, the only difference  was the biasing method.
The difference is  easy to hear and repeatable try it out  for you self :)

I don't have truly identical sounding pair to test. So much variation between the capsules  Have  you tested that the mics really sound identical if biased with the same method? Is there a thread about this I've missed?
 
Both mics sounded very similar before the changes no where near the differences I'm hearing so i'm satisfied that there is a difference between the two biasing methods why wouldn't there be?
You can do the test yourself with one mic, you just need a repeatable source and record position,try your speakers and the same mic position.
 
OPR said:
Both mics sounded very similar before the changes no where near the differences I'm hearing so i'm satisfied that there is a difference between the two biasing methods why wouldn't there be?

I hear the difference, just don't understand the electronics behind it  :) (EDIT: Diode bias is more "fixed" than the C12 way? EDIT 2: Too eager to post, now I see it; C12 method also eliminates the cathode bypass cap!)

OPR said:
You can do the test yourself with one mic, you just need a repeatable source and record position,try your speakers and the same mic position.

I'll try that. Would be also interesting to compare to cathode bias.
 
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