Universal tube mic psu

[kilmister]

Hello all!

During the years I've been reading this forum and learned to solder something together. I've also managed to build couple of tube mics and some power supply units for them. At the moment I'm in middle of typical Apex460 to C12 modification and needed psu once again.
As a diyer I like etch simple pcb but don't like to make new design everytime application changes. What I wanted was one simple pcb design that I can use for all different kinds of mics just by swapping component values and jumpering some out depending on application. Also I wanted to use components which are available from local milk&bread store.

So here is what I finally managed to put together. It's a very simple design with following options:
-B+ is smoothed by CRCRCRC network and there is place for optional zener diode.
-9 position polarity voltage divider network
-Adjustable bias voltage for fixed bias mics
-Adjustable heater voltage regulated by CRCRCRC and LM317
-LM317 noise smoothed by CRC (for U47 type of bias)

I'm pleased to receive feedback and suggestions.

Happy diying!

 

[muffy1975]

Hey,

this is exactly what I'm looking for.

I'm hoping to build a AKG c12 from scratch.

Which xformer are you using for this?

What are your pattern select resistor calculations for a c12?

Have you built this circuit and does it work?

Much love and thanks for putting this together

Michael
Oxford
England

 

[kilmister]

Hi Michael!

Yeah, I've build couple of these with Antek AN-05T120 transformers.
For my C12 clone I used resistor values straight from AKG schematic, 400k's IIRC.

Glad to see my simple design is useful.

Cheers,
Paavo

 

[muffy1975]

is there anywhere in europe that supplies the

AN-05T120?

which resistors have you for R4 R14 R15 R16 R13 R18?

And the zener is 6.3v?

Thanks if you have time to clear this bit up.

Again this would be for the c12 design

Michael

ps this PSU looks genius and very respectful of the original

 

[muffy1975]

also,

which switch are you using. Is it a 1 pole 12 position lorlin?

 

[muffy1975]

I'm beginning to be able to compare your PSU to the original for resistor values.

Is R4 1m2?

Tell me i'm close ;D

Any detail of how you connected the switch would be great

 

[kilmister]

Personaly I don't like Lorlins. I use Grayhill 1x10 switches, but you can use whatever you like just lock it in 9 positions. Switch is connected with wires to screw terminal.

R4 and R13 are 1M2 (R10, R20 in AKG schemo) For C12 build zener is not needed at all so you can just leave it out. If you want to use it (might raise noise), it should be 120V. Please refer to http://www.reuk.co.uk/Zener-Diode-Voltage-Regulator.htm
Use Ohms law to get values for R1-3 to achieve little over 120VDC in case yo want use zener. Without zener you'll like to have 120VDC, +/- 5 volts is fine.

R14-16 and R18 depends of your AC voltage from transformar and your tube filament current. You have to use ohms law to calculate them. Just make sure you don't drop voltage too much before LM317. I feed 12,6VAC to rectifier so there's plenty of voltage to drop with R14-16 and LM317. I also use R18 (I used 10R ircc) to remove LM317 noise. For C12 type of mic it's not necessary and you can put jemper instead. R18 is intended for U47 types of mics where filament voltage have to be very very noisefree.

Cheers,
Paavo

 

[muffy1975]

Hey dude,

you've been too kind getting back to me. If you have any more time, could you tell which resistors you used in position R1 R2 R3 and R14 R15 R16. I see in Finland you have the same 220-240v power that we have in the UK and I shall use the same AN-05T120 as you did.

I don't understand ohms law in this application. Trust me, I wish I did. I sometimes wonder about my IQ these days :

At a guess I would use

R1 5k o.6w
R2 5k 0.6w
r3 27kk 0.6w

but thats just because the AKG schematic says that.

As for

R14
R15
R16

I'm at a loss to understand the maths.

I don't however expect you to show me anymore than you have.

Thanks again

Michael

 

[kilmister]

You really should learn how to use ohms law R=U/I.

First thing you need to know is total current of B+. At starting point you can assume it is 1mA and calculate R1-R3 values with that. Antek trafo gives to secondaries 120VAC. After rectifier and fisrt filttering cap you have DC which is about 1.3 times AC voltage = 156V. Now we know that voltage drop haves to be 36 volts. Let's use mr Ohm: R=36V/0.001A=36000ohm=36k
Due there is 3 resistors in series we can divide 36k/3=12k. What I suggest is R1-2 are 10k and R3 16k

When you have actually mic ready, you connect it to your psu and measure B+ voltage at the node of R3 and C4. Our target is nearby of 120V. If voltage isn't there (+/- 5volts or so it's not that accurate) you have to measure R3 voltage drop to determine current flowing to circuit and calculate new value for R3.

-Paavo

 

[muffy1975]

Hey there,

great stuff. Getting closer.

I'm trying to match the original schematic to your PSU outs.

your out

1= AKG pin 3

your out

2= AKG pin 4

your pin

3= AKG pin 5

your

4= AKG pin 6

your

5= AKG pin 8


IS that correct? What voltage should we see on your pin 2?

Thanks again

Michael

 

[kilmister]

In my schemo pins at connector P4 are:

1 - Polarization
2 - Bias (negative voltage, adjustable with VR1)
3 - B+
4 - 0V
5 - Filament

-Paavo

 

[pyjaman]

Hi, I like the Idea and thought myself about that kind of pcb. just a suggestion to make it more "universal" : you could add a jumper to link heaters  ground and HV  ground, so one can choose to link both supplies 0volts whether in the PSU or in the microphone .

Laurent.

 

[muffy1975]

Paavo,

gonna be a pain the ass. :-*

What is the maths to calculate the resistors on the 6.3v line? ???

Are they all 7W resistors?

Nearly there. :-X

Oh and who do you order your Antek from?Do you get in from the USA? ;D

 

[kilmister]

Thanks for your suggestion Laurent. One question tought: what's a benefit to run two separate grounds to mic?

Muffy:
edit: By no means to be rude, my answer to your resistor value question is:
Sorry about that, I read wrong post while answering. Anyway the answer is same.
R=U/I
P=U^2/R=RI^2
P=UI
You'll never learn how to use ohms law if you don't have to use it.

I've ordered trafos straight from Antek. It's overkill, but cheap one. I would like to know what trafos other people has used in their psu's?

-Paavo

 

[muffy1975]

Oh I'm glad you are not too frustrated with me. Ok here goes

I'm gonna  presume 1MA for the the two 6.3v taps.

So we have 12.6V going into the bridge.

You said this will increase by 30% ( 1.3 ).

That equals 12.6 times 1.3=16.38V

10V needs to be dropped by the resistors.

10/0.001ma=10,000 ohms.

IS that first bit correct?

I'm making a few assumptions ( MA and the diode voltage etc )

 

[kilmister]

Your math is cool but assumption with current flow not.
I suppose you are gonna to use 6072 tube.
Check datasheet: http://tdsl.duncanamps.com/show.php?des=6072
You can see that 6072 has two filaments in series and center tap between them. C12 uses only one filament so it eats 175mA.

Also you need couple volts more before LM317 so voltage drop should be lower like 6-7 volts. If you want put resistor also after regulator (it's not crucial in C12 builds but dosen't harm neither) you want drop to be even lower to achieve bit higher voltage output fom regulator.

-Paavo

 

[muffy1975]

oh Paavo,

you're like St Jerome.

Ok,

let's do 5V at 175MA

5/0.175=28ohms.

28 ohms.

So maybe three 10 ohm resistors for R14 and R15 and R16.

Say 3 watts each ( power rating )?

Thanks again

Your student

Michael

 

[kilmister]

Looks good.
However if drop is too much for some reason you allways can use jumper instead last resistor.

-Paavo

 

[muffy1975]

Hey there again,

just getting around to finishing the PSU for a c12 clone

If using a lorlin 12 position switch, looking at the right hand side of the circuit board, is the top right hand connection connected to the centre pin of the lorlin and the other 9 positions connected to 1-9 etc?

thanks for any help

michael
oxford

 

[kilmister]

Hey!

See picture attached. It shows connection for center pin. Positions 1-9 connects are left side from that.

-Paavo