AKG C12 A / B Version Build

[Matt Nolan]

Yes. Go for it.

 

[Spencerleehorton]

Hope this doesn't sound like a silly question but just thought of it.
With using the 2 x 6.8k and 5.6k so I can get the correct current for both mics does this mean if I only use one mic the current and voltage will be wrong?
If so is there a way I can switch between or do I have to just build 2 B+ pcbs?

Regards

Spence.

 

[Matt Nolan]

The simplest and fail-safe way would be to have 2 PCBs - or at least 2 sets of RC, bleeder, lorlin. Though even then, you might get still a slight rise in B+ with one mic not connected as there will be a little less load on the transformer. A slight over-voltage is no biggie though.

The other way would be to switch in a 50k dummy mic load on the B+ when you don't have a mic connected. In order to make this fool-proof, I think you need one 3PDT switch per side. First pair of poles switch the mains, in parallel - so if either or both are on then the transformers get energised, but if both are off then everything is off. The next pair of poles are wired separately to switch each heater supply on and off. The third pair of poles are wired separately to flip the B+ outputs between mic socket or 50k dummy load. That's probably illegal. You probably shouldn't have mains and secondary stuff going through / wired to the same switch. Maybe better to only switch stuff on the secondary side. You'll have to have a separate master switch for mains on and off. Not as neat, but much safer!

 

[Spencerleehorton]

Ok, I think I'm going to go with building 2 x B+.
Could I have a switch on the AC feeding the second B+? I have a nice 250v switch that could handle the 188V AC or would it be better to switch at a lower voltage? Safer anyways.
Like you say there will be a slight rise  in the voltage when using just one mic but most of the time I would be using them as a pair anyways.
Need to get just one working first so I'm going to stick with what I've got B+ wise at the moment as I'm getting a perfect 120v.
Seems there's no easy way to do this, and it's making me think I should just build 2 independent supply's!.

Regards

Spence.

 

[tubestation]

Hello Spence,
Congratulations to Your nice Bodybuild, it s really nice to see a " Real" DIY Concept !
You already invented a lot of Energy and Time to this nice Project  and I suggest to go the Step to build 2 separate Powersupplys. There are a few Reasons to have Independent  Power for each Mic.  With one for both Mics , If You have  something going wrong in the Powersupply or in one Mic, both Mics will fail. If You are Independent You still have the clean Signal from the  other Mic. The Crosstalk will be much less with at least 2 Independent B+ Rails. To split the B+ dc Rails may  cause heavy Crosstalk. You can use one Transformer for both B+ but why  ?  and You still have the  Problem that in  Case of a B+  Failure in  one Micchain  both Mics are affected.  Do You want to save some Money or is it a Space Problem ? All this Resistor Dummys and Switch Aktion for the different Loads  for One or Two Mics  If You  split  B+, is not very elegant , I think... If You forget  to Switch from Two to One Mic ,when using just One,  less Current is pullend  that  ends in a too high Plate and more critical a  too high Capsulebiasvoltage and the diaphragms may get sucked  into the Backplate.......and You have to take Care to connect the 2 Mics always to the correct Plug because One Tube may pull  a small different  Platecurrent. . probably not a huge  Difference  but maybe  enough to  interrupt Your Stereoimage.........  Of course it s possible but why ? I think every  possible Error  will happen Sometime, most of the Time during a important Recordingsession ......
To have 2 Independend Powersupplys have the Advantage to use the 2 Mics at different Locations , or in a  huge Recording Room  You don t need  a superlong Multistrandcable  if You want to place the Mics in different Corners of the Room. 
Maybe a Friend  wants  to rent one Mic, You still can use the other One.
I understand that to build a second Case for the other Powersupply is more Work but in the End in Relation to the already  done Aktion  its not a big Story......
Greatings  and Congratulations  again for Your nice Build !
Lothar

 

[Spencerleehorton]

Hi tube station,

I totally agree with all your points, I will start looking for two psi cases.
I need to just get one of these Mics working first, still waiting on the 100R pots to arrive before I can start firing up the H+.
Need to finish the capsule mount as well as wire up the 7 pin connector when it arrives as well.
Once it's all working I will then nickel plate and polish.
On the home straight now!!!

Regards

Spence.

 

[Spencerleehorton]

Few more pics as I get closer!!
Got 7 pin sockets on now.

 

[Spencerleehorton]

Bit unconventional but I've messed around with how I'm going to create the capsule mount and at the moment this is what  I've done.
Isolates fine that's the important thing.

 

[Spencerleehorton]

Just need to order a couple of capsules and also consider to replace these nos akg transformers as I did read that they weren't great on the bottom end?
Any advice on options for txf would be great.

Regards

Spence

 

[Matt Nolan]

Just need to order a couple of capsules and also consider to replace these nos akg transformers as I did read that they weren't great on the bottom end?
Any advice on options for txf would be great.

Regards

Spence

Looking good!

What's the trafo ratio? 4:1 ish? For sparkly clean, you could use a Lundahl. They have something at 5:1 for sure, maybe a 4:1 option also. I was really impressed with some AEG Moscow re-winds of old AKG laminations from ebay seller archtopkay2002 - there were 10:1 and 5:1 available. I don't see any up there at the moment but it might be worth asking. Or have a look at Sowter's range.

 

[Spencerleehorton]

The transformers I have are from archtoplay,  so they should be ok right?

 

[Spencerleehorton]

100R pot just arrived and they are 1/10 of a watt!!!!!
Ordered some 5w 100R, little bit of a set back!!

 

[Spencerleehorton]

just got hold of a 100R 5w pot, will start the powering up of H+!!!!
have got two temp cases and have all the txf i need for 2 builds, just need to get this H+ right, will dial in the ohms then replace with resistor.
not long now!!!

regards

Spence.

 

[Spencerleehorton]

Ok, no smoke so far! Got 5.4v on H+ with a 45 ohm resistor as dummy load.
Is this looking ok? I can't get any lower as this is at full 100R

Regards

Spence.

 

[Spencerleehorton]

Well these are my findings with mic and valve connected.

Front capsule = 5v ?
Cathode = 6.9v
Backplate = 5v
Plate = 206v
XLR pin 3 = 102v ( this is the polar pattern voltage)
H+ = 5v

So it's all looking rather weird to say the least!!!
I've obviously got several wrong connections, need to try and have another look without the distraction of my kids!!!

Regards

Spence.

 

[Matt Nolan]

Your front capsule should read 0V.
You won't get an effective reading from the backplate, you should measure it at the divider tap (i.e. before the 200Meg).
It doesn't look like the valve is conducting - your B+ is super-high and the cathode voltage is way low.

I'd have a good careful look around. Can you measure a voltage at the grid?

 

[tubestation]

Hello Spence,
Did You check the Powersupply with the 50 k Ohm Dummyresistor in the B+ Chain ?
If Your Powersupplyvoltages are in the Ballpark  with the Dummyresistors instaled, You  probably have a wiring Error or a defektive  Tube that doesn t conduct Current. Do You have a Socket for the Nuvistor ? If so You could  remove the Tube and install the Dummyresistors to the Socket. I don t know If the  Tube likes  206 Volts Platevoltage, maybe You blowed  it??? 
http://www.datasheetarchive.com/dl/Scans-024/ScansEX80388.pdf
I am not sure If You can meassure the  Cathodevoltage without they Tube installed ?
How can it happen to have 5 Volts at the Frontdiagramm  If You  are  connected to 0 Volt ? Did You Check all Your 0 Volt  Connections for 0 Ohm Continue  ?  Maybe You have a  bad Connection that Results in a Offset ?  5 Volt for H + is a  little low, no ? Try to adjust this Voltage first , with the Tube instaled , meassure  at the Socket.
The Mic Cable is Ok ?
No wiring Error in the Cable ? ( probably not because the Voltages seem to be in Place )
As Matt  mentioned,  you should measure at the divider tap (i.e. before the 200Meg) for  Backplatevoltage , as the Inputimpedance of Your Meter is Way too low to meassure at such a high Resistance. The 200 M Ohm and the Inputresistor create a Voltagedivider, That s  why only 5 Volt reach Your Meter. You  would need a Elektrometer to meassure at such a high  Resistance.
I also wanted to ask how did You Build The beautifull Miccases, but that s probably the wrong Moment
now.
Sorry , I can t say more or be a better Help.
Good Luck
Greatings,
Lothar

 

[Spencerleehorton]

Hi,


Yes I need to take out tube and sort everything with load resistors first.
It could just be a simple socket wiring mismatch , it's happened before.
Will give it a good look tonight!
I made the bodies from brass, I cut out each piece and braised the front back and sides together and the bottom, then cut a hole out for the socket. The head basket pushes in and is retained with a screw each side.
The capsule mount plate slots neatly into the head basket, the capsule mount I made from a bit of plastic and brass bent into shape then put a m3 screw through but used a piece of heatshrink to isolate the screw and nut from touching anything. The when the capsule is mounted using ties it helps to isolate even more.
I'm thinking I could make some u47fet's  a little bit bigger in this style so for getting nicely in for toms!

Regards

 

[Matt Nolan]

You could take the Nuvistor out and put an 18k resistor in place of its anode to cathode path. You should get approximate voltages with that if all the wiring is OK.

5V is perfect for the heaters. AKG were under-heating the Nuvistor. It's right there on the schematic on page 1 of this thread.

 

[Spencerleehorton]

On the schematic which side is the front capsule? Is it the side which is down to Gnd?
And can I just connect the front capsule just to get it to work? And the backplate I meant as well.

Regards

Spence.