AKG C12 A / B Version Build

[Spencerleehorton]

Hi,

Looking into building this next week now as have drawn up the schematic and going to produce a few pcbs.
Just have a few questions regarding a few bits:
9 position switch - is it 8 x 56k resistors on the switch?
What switch would be good to use?
I have got 15uf 63v rather than 75v as only 54v at This point.
I'm going to use styrene 5000pf 630v should this be ok rather than ceramic?

Regards

Spence.

 

[Spencerleehorton]

I was thinking the switch was going to be in the microphone, but just realised that it's in the Psu!
Have tried with lorlin and works fine, just wanted to verify that it's 56k on every point.


Regards

Spence.

 

[Spencerleehorton]

Also looking at this mod for reducing plate voltage, what's the advantage?

Regards

Spence.

 

[Matt Nolan]

I suspect that "mod" is just something done by someone concerned by their over-high local mains voltage. They've just upped the PSU bleeder current a bit to pull the B+ down a little.

 

[Spencerleehorton]

Have sat down and worked out B+ transformer arrangement and I'm getting 188v AC so 263v DC after bridge so R4=15k, R5=15k, R6=3.3k to give me 120v.
Just need to sort out H+ arrangement, I have 24v AC so this will be 33.6v DC so will need to adjust first resistor a little to 120ohm.
Am I looking like I'm in the ball park?

Regards

Spence.

 

[Matt Nolan]

Quick calculations suggest that you could change the zener from a 9V to a 12V and, yes, a 120 Ohm or maybe a little larger would be fine for that trimmer pot. You'll be setting it somewhere around 90 Ohms I think to get the 122mA regulated heater current.

You may find you get fewer volts from your transformer under load, or more, depending on its VA rating. So be prepared to maybe change some component values. You could use a 1W or larger 40 Ohm resistor as a dummy heater while you experiment with setting up the power supply, to get you in the ball park, before connecting your precious Nuvistor up to it.

For the B+, I think 15k, 15k, 6k8 should get you in the ball-park. Again, it will depend on the transformer voltage under actual load. Tweak one of those 3 smoothing resistors and / or the 330k bleeder to get your voltage to 120 with the mic connected and fully warmed up. Likewise the heater adjustment. Do final tweaks to it after the mic has been on for a good long time. Measure the voltage at the mic, not in the PSU, and adjust the trimmer pot.

 

[Spencerleehorton]

The dilemma I have about it all is that I want to supply 2 mics from within this one psu.

I have a 2.5 a 24v txt for the H+ and a 188v 0.410ma for the B+.
Hopefully this can supply both mics?
I am having trouble tracking down a 15w 100R or 120R is there any other wattage pots I could use?
I just found a 100R 10 watt pot, I'm hoping this will be ok?
I am also still unclear how this arrangement comes down to 5v at 122ma.

Regards

Spence.

 

[Matt Nolan]

The dilemma I have about it all is that I want to supply 2 mics from within this one psu.

I have a 2.5 a 24v txt for the H+ and a 188v 0.410ma for the B+.
Hopefully this can supply both mics?

Assuming you mean 410mA and not 0.410mA, then yes these should be fine.

I am having trouble tracking down a 15w 100R or 120R is there any other wattage pots I could use?
I just found a 100R 10 watt pot, I'm hoping this will be ok?

Well, let's see, it looks like the original pot may be over-specified, perhaps due to limited selection of components. There's about 125mA going through it. Power = Current Squared x Resistance = 0.125 x 0.125 x 100 = 1.56W but it will be inside a case with poor ventilation and other things getting hot like the PNP transistor and the trafo cores, so we should go bigger and 10W will do it happily.

I am also still unclear how this arrangement comes down to 5v at 122ma.

Figure out the approx trimmer potentiometer setting in advance using Ohm's Law. R = (Vzener - 0.6) / 125mA

The PNP / Zener circuit is a constant current supply. It works because so long as we are in the normal operating region of the PNP, it will develop approx 0.6V across the base-emitter junction. So long as the Zener is in a good point on its operating curve also (they're running the original at about 20mA, I think) then it will have a pretty stable voltage: its specified voltage. You might want to look at the I/V curve of the Zener you pick, and adjust the 1k R2 if need be, to put it in a nice place on its curve.

The base of the PNP and the low end of the Zener are at the same voltage. The top end of the zener is 9V (or 12V) above the base. The emitter of the PNP is 0.6V above the base. So, the pot has to have the difference between these two voltages across it - i.e. 8.4V (or 11.4V).

So, if we have a fixed voltage across the pot then the current through it will be that voltage divided by its resistance. Turning that around, you can set the current by varying the resistance. That current splits to go to the heater (which wants 122mA) and through the 2k bleeder resistor (which will take 2.5mA if the voltage is 5V). 124.5mA in total. You could practically ignore the bleeder resistor in your calculations.

The current requirement is set by the heater. The AKG engineers found it best to run it at 5V, at which level it draws 122mA, or thereabouts. We learn this from your schematic. Most likely this is the point of best SNR for the mic.

The current is held constant (within reason) even if the resistance of the heater fluctuates (which it will - lower resistance when it is cold, higher when it is hot), so this is a nice semi-soft-start circuit really - it stops a high current spike going through the cold heater when you first power up. The current is also held constant (within reason) with variations in the supply voltage, due to the constant voltage action of the Zener and the PN junction in the transistor. It's a little feedback loop. The resistance of the transistor emitter to collector path will vary to try to keep the current constant.

I hope that all helps.
Cheers,
Matt.

 

[Matt Nolan]

You might want a little heatsink on that PNP by the way.

You should duplicate all the PSU paths for each mic. You could perhaps share the bridge rectifiers, though it would probably be better to have separate ones, especially in the heater circuits.

 

[tubestation]

Hi,

I have got 15uf 63v rather than 75v as only 54v at This point.

This is the Outputcouplecapitor and the Schematics says Tantal .  Maybee it s more  save to use  a 100 Volt Type If You can t find a 75 V.  Probably a 63 V would be ok but  for my Taste  i would feel better  with a 100 Volt one.  If You have enough Place in the Mic, You could also use a Mundorf MKP  250 Volt ( because it s quiet small  and  sounds good ) for a cleaner  Sound.

I'm going to use styrene 5000pf 630v should this be ok rather than ceramic?

The Schematics says Ceramic  and because  it s the ac Connection from Capsule to Ground  it s a Couplecapitor as well and Part of the Sound. Styrene sound s different .
Because You probably  build with non  original Parts , Chasis , and Headgrill , the Mic will Sound different than the Original , so You  are more free with the  Choice of the Capitors.
Greatings,
Lothar

 

[Spencerleehorton]

Ok, well I've made up some pcbs for the mic and psu, I'll put together and test over weekend, if I get good results I will start making some bodies out of brass Thursday next week!

Regards

Spence.

 

[Spencerleehorton]

This is a very quick mock up, I've just drawn on the circuit then put in ferric chloride, will print out proper ones if it all works!
As you can see some Xmas cello tape holding on the txf, haven't mounted the tube oN the pcb on this one but this will be on pcb on 2nd version.

Regards

Spence.

 

[Spencerleehorton]

Psu.

 

[Spencerleehorton]

Have started to build the head baskets, I have only done fairly rough at the moment but a lot more work will go into these and will be nickel plated. Body I am cutting once I get my hands on some other equipment and will be braised together, then the head baskets will be braised to the body.
The pcb will be housed in a metal box which will be slotted into the body.
The capsule will be attached to the inside metal box.

Update more later.

Regards

Spence.

 

[Spencerleehorton]

Psu taking shape!

 

[Spencerleehorton]

Tested B+ and under 47k load resistor I'm getting 136v so bit high,  will change load resistor to 56k and see how it comes out.

Regards

Spence.

 

[Matt Nolan]

Tested B+ and under 47k load resistor I'm getting 136v so bit high,  will change load resistor to 56k and see how it comes out.

Regards

Spence.

If you mean what I think you mean, it will come out higher with a 56k load. The B+ load from the mic is around 50k according to the schematic. Have you got the bleeder resistor in the PSU and the polar pattern select divider ladder?

Good work so far!

 

[Spencerleehorton]

The resistor ladder I think isn't attached correctly, i have 5 minutes to check it now!!!
Have checked and this is what I get:

CW pos9 - 130.7v
8 = 130.1
7= 129
6=127.3
5=124.2
4=117.8
3=103.3
2=103.3 ?
1=0v

I have 15k, 15k, 3.3k before a 330k bleed resistor with a 47k load resistor
I have attached the lorlin to the B+ to the centre pin and Gnd to pin 1, the other connection is from the 1M resistor to pin 5 on the lorlin.

Does this all sound correct or have I made a mistake here?

Regards

Spence.
Regards

Spence.

 

[Matt Nolan]

Sounds like you have your lorlin switch pins mixed up. The output as you run the switch through the 9 positions should be an even and gradual reduction - eg 120, 105, 90, 75, 60, 45, 30, 15, 0V.

 

[Spencerleehorton]

Yes definetly a problem at the lorlin switch.

Just to go over voltages from diodes I'm getting under load

244v at diode bridge
192.5v after 1st 15k
141.6v after 2nd 15k
131v after 3.3k

Think I should change the 3.3k to 4.7k?
Should bring down to 120v?

Could you explain exactly how to wire lorlin so I'm clear on this please?

Regards

Spence.