Official C12 Clone - Build and Support Thread

[Matador]

I may be stating the obvious, but are all of the trimmer pot values verified and installed in the correct positions?  Seems odd that the B+ would only have a few volts of adjustment range regardless of where the final voltage lands . . . and that the bias adjustment would be very touchy as that was quite a gradual change on my sample build per turn of the pot.

omg.. i'm a dickhead for this but I thought all Adj pots were the same value.. simple mistake..

In Bias I have installed W502
In B+ I have installed W202
In Heater I have installed W104

Is there an easy way to remove these components because I'm having real difficulties desoldering the three legs at the same time.

Cheers

Thanks Chunger, I was going to suggest this check next. 

I've had (limited) success removing these by turning the board upside down, then wiping the iron quickly across the three pins while applying sideways pressure on the pot body.  You can do something similar with a chuck of solder wick or wire that has been tinned.  If you can heat all three joints they should come out easily, then you can suck out the solder from the plated through holes with some fresh wick and some liquid flux or paste.

If you end up cutting them off, you can substitute in temporary resistors across the two outside holes using Ohm's law to your advantage.  This should get you going until new trimmers arrive, as these voltages aren't particularly critical:

1) Heater - average loss is 0.2V (via I*R loss) by the time the voltage makes it to the tube pin.  This means you want 6.5V at the supply side.  Output heater voltage is set by the equation Vout = 1.25*(1+(R12+R10)/R9).  Since R12 is 1K and R9 is 392, then the pot R12 should be set to 646 ohms.  A 640 ohm resistor would be very close.
2) Bias - total return current is the plate current + the bleeder resistor R6.  At 120V, R6 bleeds 0.25mA.  The plate current at -1V bias at B+ 120V is 0.7mA, so we want 1V across R3 with 1mA, which means that R3 should be 1K ohms.  10% bias voltage up or down won't hurt things for testing.

R4 is more difficult, but you can get close by testing with your 180K resistor, trimming to 120V, then cutting out the pot and measuring the resistance between pins 2 and 3.  Then substitute this with a fixed resistance.

Or, barring ALL of this, just get them out or buy new trimmers:  they are only a few bucks for all of them.

 

[KMB-AUDIO]

I may be stating the obvious, but are all of the trimmer pot values verified and installed in the correct positions?  Seems odd that the B+ would only have a few volts of adjustment range regardless of where the final voltage lands . . . and that the bias adjustment would be very touchy as that was quite a gradual change on my sample build per turn of the pot.

omg.. i'm a dickhead for this but I thought all Adj pots were the same value.. simple mistake..

In Bias I have installed W502
In B+ I have installed W202
In Heater I have installed W104

Is there an easy way to remove these components because I'm having real difficulties desoldering the three legs at the same time.

Cheers

I think you have the bias pot in the correct position, looks like the B+ and heater are reversed.

 

[chunger]

I know that it is an expensive investment, but the Hakko 808 desoldering tool would make short work of swapping out those pots.  I've been able to consistently suck out all of the solder in seconds and just have the part fall out of the pcb when desoldering with that tool.

Sooner or later in DIY, you'll wish you had it. . . well, in this instance, it would appear to be sooner.  I seem to need it more often than I would care to admit.

 

[Winetree]

+1  Hakko 808 can't DIY without it.

 

[JessJackson]

Thanks for everyones help, Just ordered the hakko (i borrowed my friends but it removed the tin on the circuit board at the same time... I was able to salvage the exsisting trimmers, I had to cut a couple of the pins but just used a small extension of wire and some heat shrink and the trimmers live off the board (for now).... was annoying b/c my build is as neat as changers / or pretty close.

So The Results are in...

1st 5 Star GE Tube

B+ = 121.69 (Adj pot fully minimum) @ PS
Heater = 6.30 @ Tube Base
Bias = -1.00 @ PS
Plate = 48.54 @ OP Cap

Am I good to go?

 

[Matador]

Thanks for everyones help, Just ordered the hakko (i borrowed my friends but it removed the tin on the circuit board at the same time... I was able to salvage the exsisting trimmers, I had to cut a couple of the pins but just used a small extension of wire and some heat shrink and the trimmers live off the board (for now).... was annoying b/c my build is as neat as changers / or pretty close.

So The Results are in...

1st 5 Star GE Tube

B+ = 121.69 (Adj pot fully minimum) @ PS
Heater = 6.30 @ Tube Base
Bias = -1.00 @ PS
Plate = 48.54 @ OP Cap

Am I good to go?

Spot on!  8) Your tube is idling at (121.69-48.54) / 100K = 0.73mA which is exactly what the tube curves suggest under those bias conditions.

 

[JessJackson]

Great Matador!

Things get even more interesting.

Above post is measured with Fluke 115

I made same test with Extech MN42

B+ = 111.0
Bias = -0.54
Heater = 6.32
Plate = 45.5

Both meters give me 10.01v exactly tested on a 9v battery.. so odd

I dont know why but I've never trusted the fluke although its the more expensive meter. Both have new batteries

 

[Biasrocks]

B+ = 111.0
Bias = -0.54
Heater = 6.32
Plate = 45.5

Plate seems low.

Your plate voltage should be closer to 60-70 volts with a healthy tube.

Mark

 

[Matador]

B+ = 111.0
Bias = -0.54
Heater = 6.32
Plate = 45.5

Plate seems low.

Your plate voltage should be closer to 60-70 volts with a healthy tube.

Mark

Plate voltage will be up or down by 20% depending on the tube.

Remember the chart I post earlier:

50V is right in the average for a typical tube.  If the tube in question pulls slightly less current, quiescent plate voltage is higher.

As for the meters:  keep in mind that the meters work by placing a large(ish) resistance in parallel with whatever point is being measured, and measuring the current shunted through this resistance.

If the circuit being measured is low impedance (like a battery), then the impedance of the meter doesn't matter much.  If I try to measure a bias voltage on a capsule (supplied from a ~gigaohm impedance), the circuit impedance is 4 orders of magnitude higher than the meter and the meter "takes over".  This is why the polarization voltage collapses when you try to measure it with a typical 1M to 10M DVM).

Plate supply's sit in the middle:  for this design, the supply is "highish impedance", having several hundred kohms in series with the supply.  From my earlier post, we are talking about supplying 1mA at 120V which means the PSU output impedance looks like a 120K resistor.  If your meter is 1M, then it adds 0.12mA or 12% error current into the equation.  A 10M DVM adds 1.2%.  Obviously for these kids of measurements, the higher impedance meters will give more accurate results.

Most Fluke's are 10M input impedance, so it's best to find the meter with the highest impedance for these kids of measurements.

 

[Biasrocks]

Both of my microphones are at ~67-68VDC on the plate. GE 5 Star tubes.

Also from Klaus Heyne comes this.

"The plate voltage of a healthy 6072 tube working inside a healthy C12 should be around 65-80 VDC."

http://repforums.prosoundweb.com/index.php?topic=33288.msg493077#msg493077

Regards,
Mark

 

[Matador]

Both of my microphones are at ~67-68VDC on the plate. GE 5 Star tubes.

Also from Klaus Heyne comes this.

"The plate voltage of a healthy 6072 tube working inside a healthy C12 should be around 65-80 VDC."

http://repforums.prosoundweb.com/index.php?topic=33288.msg493077#msg493077

Regards,
Mark

This may imply that AKG might trim the bias resistor more towards the -1.5V to -2V range.  -1V (at least based on my prototyping and experimentation) gave a good compromise between signal swing, input headroom, and linearity.

Does anyone have an original they can measure?

 

[JessJackson]

Thanks Matador, you are in fact correct... its all basic arithmetic and there is no reason that a tube output should be higher than the calculation end point.

Find attached 6072 Data Sheet

 

[Ericbazaar]

I have a question! : On my microphone varies the B + voltage. I set the Voltage to 120V and 1hour later ist the Voltage higher at 121V. Is it bad when the voltage fluctuates between 119 and 121V? And Thanks Chunger for sending me a new T14!!

 

[chunger]

I have a question! : On my microphone varies the B + voltage. I set the Voltage to 120V and 1hour later ist the Voltage higher at 121V. Is it bad when the voltage fluctuates between 119 and 121V? And Thanks Chunger for sending me a new T14!!

As I understand it, the B+ section of the power supply is a passive setup without a regulator, so the B+ will fluctuate a bit according to fluctuations in the line voltage.  This is normal.

 

[Pip]

Hi:

First post to this forum after being a voyer for awhile.

Just finished this build and wanted to say how rewarding it was, mic sounds great (still has stock capsule Tim Campbell capsule on order) all went well. Hats off and much appreciation to Matador, Chunger and all others who contributed. THANK YOU FOR LEADING US FOLLOWERS!

A few points of possible interest, maybe. I used the Carvin Guitars version of the Alctron HT-11A as a donor. The only difference between it and the one in the thread is the power transformer was mounted on PC board and needed to be remounted to metal case, two holes drilled and some 6/32 screws and nylock nuts later voila. It should also be mentioned that the output wire pairs on my transformer were reversed to those in the build thread pics and not labeled so make sure you check before hooking up! Be sure to attach ground wire as part of this process as well.

As to grounding. I can not stress enough how with this and all mic projects ground integrity is really important so do yourself a favor and really keep on it!

I did have problem with B+ voltage level getting past 113 VDC measured at PSU with both 180K dummy load and Mic attached. I will work on this looking at Matadors suggestion of changing the values on resistors 1 & 2. All seemed to work and mic sounds great. Also no puffs of smoke and blue sparks always a bonus. I will take any and all suggestions on this  and will keep forum informed. I plan to go back at this when Tim Campbells capsule arrives. Stay tuned.

I used a NOS JAN 6072 Phillips USA tube. I have mic rated GE 5 Star 6072 tubes but the glass envelope is bigger on them and as space is a premium and the Phillips tube so far sounds good with no artifacts so be it. No difference by the way with either tube and the B+ issue.

Thats it for now. Once again thank you all for a rewarding build.

 

[micaddict]

Welcome!

Thanks for your contribution and yes, I have a feeling we we will stay tuned. 

 

[Matador]

A few notes based on feedback received from several people:

1) I am changing the BOM/schematic to increase the values of R1 and R2 slightly up to 100K for those using 6072A tubes.  This will give about 20V more adjustment room to B+ for differing tube types (including the stock 12AX7 tube which many are using for testing purposes).
2) Pip pointed out that the schematic indications for front capsule and rear capsule are reversed:  I'll fix this.  The silkscreen on the mike PCB is however verified correct. 
3) On recommendation from Kidvybes, I have ordered a handful of GE and JAN 6201 (12AT7WA and 12AT7WC) tubes, as you can get nice deals on these for NOS 5-star grade material:  I got mine from TC Tubes for about $25 each.  These tubes feature triple micas and extra support rods so microphonics should be excellent.  These tubes are higher mu so a few circuit tweaks will be in order:  mostly lowering R17 down to make the gain about the same as compared to a 12AY7/6072A.  Bias will also need to be tweaked so that the idling point is about the same.  These tubes draw much more quiescent current at idle so tweaks to R1 and R2 may be necessary as well.

 

[micaddict]

OK I didn't want to unnecessarily pollute the thread, but now you've brought it up yourself, here goes. 

http://tubedata.tubes.se/sheets/093/6/6072.pdf
http://tubedata.tubes.se/sheets/093/6/6201.pdf
http://www.thegearpage.net/board/archive/index.php/t-157299.html
http://www.gearslutz.com/board/high-end/669136-bock-251-now-using-6201-tube.html

As a warm up, for those interested.

 

[Gilgamesh]

Apologies if this has been covered somewhere else already, but what is the maximum diameter for c12 to comfortably fit in the Alctron body?

 

[JessJackson]

A few notes based on feedback received from several people:

1) I am changing the BOM/schematic to increase the values of R1 and R2 slightly up to 100K for those using 6072A tubes.  This will give about 20V more adjustment room to B+ for differing tube types (including the stock 12AX7 tube which many are using for testing purposes).
2) Pip pointed out that the schematic indications for front capsule and rear capsule are reversed:  I'll fix this.  The silkscreen on the mike PCB is however verified correct.
3) On recommendation from Kidvybes, I have ordered a handful of GE and JAN 6201 (12AT7WA and 12AT7WC) tubes, as you can get nice deals on these for NOS 5-star grade material:  I got mine from TC Tubes for about $25 each.  These tubes feature triple micas and extra support rods so microphonics should be excellent.  These tubes are higher mu so a few circuit tweaks will be in order:  mostly lowering R17 down to make the gain about the same as compared to a 12AY7/6072A.  Bias will also need to be tweaked so that the idling point is about the same.  These tubes draw much more quiescent current at idle so tweaks to R1 and R2 may be necessary as well.

Thanks for this info

I just ordered some 71-RN60D-F-100K for R1 and R2 as per your suggestion.