Official C12 Clone - Build and Support Thread

[Matador]

Ok. But, with the stock Alcatron capsule and a good 6025 tube, turning the B+ trimmer still does nothing to the voltage. Is this a problem?

Another question. I'm getting hum when I listen. When I touch the top of the capsule or another part, the hum goes away, but I can't hear the capsule. Could I have a problem? Could it be the same problem with a cap (C10 or C13?) as described in earlier posts?

Sorry. Ha Ha. Chunger calls himself a newbie....I'm the real deal!

Ok let's tackle these one at a time:
1) Please remove the mike from the equation.  Unplug it from the PSU, then place a 180K resistor across the B+ and ground connections.  You can stick the leads right in the 7-pin XLR connector and then fire up the PSU.  Is the B+ adjustable?  If so, you have problems in the mike (leading to part 2).
2) Per my previous post:

It sounds like a grounding issue to me.

Please confirm the following:

1) On your 7-pin XLR connector into the mike, that you have whatever wire is carrying ground also bridges over to the brass lug that connects with the XLR screw.  You can clearly see it in Chunger's build pic:

2) That you "loosened" the XLR screw which actually tightens it against the bottom housing.

If you don't have a solid ground path in the mike, then the tube will not draw any current, and thus the B+ will sit high and appear to have little adjustment.  Make sure you have less than 1 ohm resistance between the ground lug in the PSU (pin5 of the 5-pin PSU connector, labeled "P8" I think) and everywhere inside the mike with everything plugged in (POWER OFF!!!!!!) and the outer tube sleeve removed.

The fact that you have hum changes when you touch the parts leads me to believe you have a bum ground path.

 

[kcatthedog]

Hi Matador,

thx,

no mike plugged in.

I was taking readings from the 5 pin PSU block , like in the thread picture. I have a new  Sperry metre and followed the settings in the pictures.

I have sent cat5 pictures of top and bottom of pcp and he has suggested my soldering is suspect ; probable true .

also, after doing the build of the selector, I realized I had put two wrong resisters  on, so I unsoldered them and did install them on the board, all legs were contacting pads.

Trimmers are correctly placed.

kcat

Hi everyone,

My voltage readings are off.  I tested the trimmers before installing them and believe they are correctly placed and I have reviewed the entire pcp:

b+  28.7
H      6.3
B        .31
P    22.1

Typically I can turn the trimmer screws many times to little effect.

Suggestions ?

Thx !

Need more input.  Is this without a mike installed?  Start at the basics...no mike installed, what are all of the voltages as measured right at the PSU 5-pin terminal block?

I understand how to determine where the blue and red wires go on the t14 transformer but still am unsure of how to determine the positive and negative leads.  There is no explanation in the thread.

In this design, such a distinction doesn't matter.  Once you know primary from secondary you are good.

EDIT:  this statement is true unless you consider matched pairs of mikes.  The phase on both should be consistent to each other.  Easy way to tell is to set up two of them, and then clap a few times (providing something near an impulse response) while recording on your DAW.  Zoom way in to the sample level, and you should see the transients move in the same direction on both channels.  If one "goes up" while the other "goes down" you should swap the secondary wires on one of the mikes.

 

[kcatthedog]

So, I still do not get correct readings from the power supply.

In terms of trouble shooting, what is next, removing pcp and checking electrical connections with my metre from the bottom of the board ?

In the interim , I started the mike pcp build and made certain I only applied solder to pad , dabbing until it was melting/flowing to ensure a good solder. Stopped  at wires as I need to buy some, should those wire be 18 or 22 gauge ?

thx!

kcat

 

[thedude]

Mic finally put together an workin. But it seems to be an issue with the PSU voltages. When i tested it with the stock capsule and test tube the voltages where ok. When the Tim Campbell  capsule and the my NOS tube was installed the B+ voltage dropped from 120 to 104.

The problem is that i can't trim it up higher than 114 with the TC capsule. Is that an issue, the mic is sounding great. Maybe a little bit more distortion than i expected but in a good way.

Any ideas, where do i start troubleshooting?

 

[Splunge]

I'm trying to build the ELAM/C12 hybrid version of this project. While tracing the circuit to determine exactly how to attach the capsule for this version, I notice that on the PCB (ver. 1.1A), 'FC' is attached to ground and 'RC' is attached to the junction of R16 and C11, but on the schematic (C12_v3) it is shown as exactly the opposite. So which way is correct?

 

[Reverend]

Ok. But, with the stock Alcatron capsule and a good 6025 tube, turning the B+ trimmer still does nothing to the voltage. Is this a problem?
1)Hello Matador, No, when I strapped the 180k resistor between B+ and ground, the B+ trimmer pot was still not adjusting anything.
2)I didn't want work on the possible grounding issue in the mic until this psu thing is resolved.

Another question. I'm getting hum when I listen. When I touch the top of the capsule or another part, the hum goes away, but I can't hear the capsule. Could I have a problem? Could it be the same problem with a cap (C10 or C13?) as described in earlier posts?

Sorry. Ha Ha. Chunger calls himself a newbie....I'm the real deal!

Ok let's tackle these one at a time:
1) Please remove the mike from the equation.  Unplug it from the PSU, then place a 180K resistor across the B+ and ground connections.  You can stick the leads right in the 7-pin XLR connector and then fire up the PSU.  Is the B+ adjustable?  If so, you have problems in the mike (leading to part 2).
2) Per my previous post:

It sounds like a grounding issue to me.

Please confirm the following:

1) On your 7-pin XLR connector into the mike, that you have whatever wire is carrying ground also bridges over to the brass lug that connects with the XLR screw.  You can clearly see it in Chunger's build pic:

2) That you "loosened" the XLR screw which actually tightens it against the bottom housing.

If you don't have a solid ground path in the mike, then the tube will not draw any current, and thus the B+ will sit high and appear to have little adjustment.  Make sure you have less than 1 ohm resistance between the ground lug in the PSU (pin5 of the 5-pin PSU connector, labeled "P8" I think) and everywhere inside the mike with everything plugged in (POWER OFF!!!!!!) and the outer tube sleeve removed.

The fact that you have hum changes when you touch the parts leads me to believe you have a bum ground path.

 

[Matador]

I'm trying to build the ELAM/C12 hybrid version of this project. While tracing the circuit to determine exactly how to attach the capsule for this version, I notice that on the PCB (ver. 1.1A), 'FC' is attached to ground and 'RC' is attached to the junction of R16 and C11, but on the schematic (C12_v3) it is shown as exactly the opposite. So which way is correct?

The PCB is correct.  FC goes to ground.

 

[Reverend]

OK. With the mic unplugged, I put a 180k resistor between B+ and Ground. The trim pot did not do anything - no adjustment possible. The voltage was quite high. 195 Volts. Since it was cheap, I've already ordered another trim pot off the BOM from Mouser.

 

[teacat]

Finally got the funds together and ordered the parts package. So, I already have a late 50's ge 5-star 6201, and am thinking of using it instead of buying a nos 6072 for now. So, all I have to do is change R17 to 47K? Has anyone done this yet?  What's the diff sonically?

I think R17 should be lowered to about 47K.  With -1V bias, idle current looks to be about 1.25mA with a 6201.  R1 and R2 being 100K should still be fine and give adjustment room around the 120V point.  Plate should be sitting at about 60V at idle (same as 6072).

I'm not sure about how the sonics will be different:  the main reason to switch to a 6201 is availability of high quality tubes at this date.  The 6201 has more robust internal construction than the 6072, so it might be quieter (all other things being equal).  With a lower R17, output impedance will be lowered which should make the amp slightly "faster" (for lack of a better term).

I have some parts in transit to me and I'll build a version with this tube.  I'll post my exact tweaks if you don't want to experiment (or be a guinea pig) in this case.

 

[Matador]

OK. With the mic unplugged, I put a 180k resistor between B+ and Ground. The trim pot did not do anything - no adjustment possible. The voltage was quite high. 195 Volts. Since it was cheap, I've already ordered another trim pot off the BOM from Mouser.

With the 180K resistor still in:  tell me the voltages at:
1) The junction of R1 and C1
2) The junction of R1, R2, and C2
3) The junction of R2, R4, and C3

 

[Reverend]

OK. With the mic unplugged, I put a 180k resistor between B+ and Ground. The trim pot did not do anything - no adjustment possible. The voltage was quite high. 195 Volts. Since it was cheap, I've already ordered another trim pot off the BOM from Mouser.

With the 180K resistor still in:  tell me the voltages at:
1) The junction of R1 and C1
2) The junction of R1, R2, and C2
3) The junction of R2, R4, and C3

180k resistor is still in. So, for #1) do I touch black from my DMM to the ground and the red to the lead coming off of the resistor R1 towards C1?
#2) Black on ground and red on the lead coming off of R1 going toward C2?

 

[Melodeath00]

Quick question...

Is the original C12 power supply regulated or non-regulated? What is the modified Alctron PSU?

 

[Matador]

180k resistor is still in. So, for #1) do I touch black from my DMM to the ground and the red to the lead coming off of the resistor R1 towards C1?
#2) Black on ground and red on the lead coming off of R1 going toward C2?

Ground always stays ground.  If black is ground, and red is your test probe, then touch it in sequence at the places I mentioned above.

 

[Matador]

Quick question...

Is the original C12 power supply regulated or non-regulated? What is the modified Alctron PSU?

Non-regulated, but heavily filtered.  I don't understand the second question.

 

[Reverend]

180k resistor is still in. So, for #1) do I touch black from my DMM to the ground and the red to the lead coming off of the resistor R1 towards C1?
#2) Black on ground and red on the lead coming off of R1 going toward C2?

Ground always stays ground.  If black is ground, and red is your test probe, then touch it in sequence at the places I mentioned above.

OK. I understand the black to ground thing. Should I touch the red to the lead coming out of the resistor R1 on the side going to C2? And so forth? Thanks and sorry.

 

[Melodeath00]

Quick question...

Is the original C12 power supply regulated or non-regulated? What is the modified Alctron PSU?

Non-regulated, but heavily filtered.  I don't understand the second question.

Second question is in regards to your PCB for the PSU for this project. Is it the same? Non-reg, but heavily filtered? Thanks!

 

[Matador]

180k resistor is still in. So, for #1) do I touch black from my DMM to the ground and the red to the lead coming off of the resistor R1 towards C1?
#2) Black on ground and red on the lead coming off of R1 going toward C2?

Ground always stays ground.  If black is ground, and red is your test probe, then touch it in sequence at the places I mentioned above.

OK. I understand the black to ground thing. Should I touch the red to the lead coming out of the resistor R1 on the side going to C2? And so forth? Thanks and sorry.

That's right.

 

[Matador]

Quick question...

Is the original C12 power supply regulated or non-regulated? What is the modified Alctron PSU?

Non-regulated, but heavily filtered.  I don't understand the second question.

Second question is in regards to your PCB for the PSU for this project. Is it the same? Non-reg, but heavily filtered? Thanks!

Oh I see what you meant.

My PSU is identical to the original with the following exceptions:
1) Linear regulator (LM317) for heater in mine, C12 original used Zener reference
2) Full-wave rectifier for B+ in mine, C12 used half-wave rectifier

Both are filtered but non-regulated B+ supply.  Polarization supply and bias supply is identical.

 

[Reverend]

OK. With the mic unplugged, I put a 180k resistor between B+ and Ground. The trim pot did not do anything - no adjustment possible. The voltage was quite high. 195 Volts. Since it was cheap, I've already ordered another trim pot off the BOM from Mouser.

With the 180K resistor still in:  tell me the voltages at:
1) The junction of R1 and C1
2) The junction of R1, R2, and C2
3) The junction of R2, R4, and C3

1) R1 and C1  Not getting a good reading. It flashes several different readings but very quickly goes to 1 (left side 1 on DMM read out) and stays there.
2) R1 and C2 Same thing as #1...no real reading 196 for a split second then back to one
    R2 and C2  no real reading.....one
3) R2 and C3  no real reading...one
    R4 and C3  no real reading again except when I touch the top of the trim pot (R4), it goes to zeros

One more thing. R5 to C5 - 1.2
Heater 6.3
Bias -1.1

 

[cariocaman85]

Hi,

I'm really happy with these C12, but I'm wondering if some Deoxit applied on the tube pins (which are a little bit oxydated, 50 years old nos) might improve the signal/noise ratio?

I currently have deoxit d100L, but I was about to buy deoxit gold, which seems to be better for this application (no lubricant I believe).

What is your opinion?

Thanks,

Rémi