Apex 460 mod - Tim Campbell's capsule

Hi guys,

I've just ordered an Alctron HST-11A (Apex 460) and I'm still trying to decide which way I'm going to go upgrade wise. Has anyone here actually used one of Tim Campbell's C12 capsules with an Apex 460? I did some searching and couldn't find opinions of people who had actually done it. I've already got a CM-2480 transformer and was thinking of simply following this guide.

http://www.allthisnoise.com/All_This_Noise/Apex_mod.html

Would this guide work well with Tim's capsule? My goal is to get the mic as close to the C12 as possible. I'm pretty confident with a soldering iron and have successfully built myself a GSSL comp but I'm still learning when it comes to going from schematic to PCB. Can anyone tell me if this guide will achieve my goal as far as emulating the C12 circuit goes?

Thanks,

Kris

It's going to be more like an ELA M circuit but it'll sound good with Tim's capsule, it's amazing capsule. But you have to modify the PSU to provide for 55-60V polarization voltage (which means 110-120V B+). Learned that the "hard way" myself. You'll find means to do that searching here. Changing R7 to 1.8K (ELA M value)  starts to make sense also with 110 B+. All the capacitor swaps may not be necessary. When you get the mic check first what's in there, stock parts might be fine, they've changed through the years.

Thanks for the tips. I might hold off on buying parts until it arrives. Is that the ELA M 251E you're talking about? I'm not familiar with Telefunken mics at all. I didn't even know the Apex 460 could be modded to be like one. I might have to look into that.

Thanks again,

Kris

I've heard a couple of Apex460's modded with Tim's capsule (and 6072tube + possibly new transformer) - and I'm pretty sure that this has the highest bang-for-the-buck factor in current microphone technology...

Jakob E.

Really? No other modifications to the circuit? That's interesting of you to say. I'm fairly set on Tim's capsule after all the great things I've read about it, I already have a few 6072 NOS tubes and a CM-2480 transformer.

Perhaps I'll just switch the tube, transformer and capsule first and go from there. Will I still need to modify the PSU though?

Kris.

I've just recently done this: http://www.groupdiy.com/index.php?topic=47764.0

There's only one VERY important thing in modding this mic: the capsule. And Tim's capsule is an amazing, timeless piece of art. Compared to the more or less chinese K67 variations with edge termination, Tim's capsule is on another lever alltogether and will make your mic something you will use for the rest of your life.

The rest: the circuit, the tube etc. wont matter that much, but you just have to make sure that the circuit is well balanced (tube&transformer impedance etc)

I used TAB/AMI T14 and GE6072A and changed all the components to match the original C12/ELAM251 circuit. Remember to change the zener diodes in the PSU to 60V so the B+ will be around 120V.

Difference between Alctron and C12 is that C12 is a plate follower and Alctron is a cathode follower circuit. If you use CM2480 you can change it to plate follower. It wont make a huge difference, though. But i'll just say that all the classic german mic designs were plate follower circuits. 

 

Ta for the info! Plenty for me to read in your thread  .

So the Zener diodes in the PSU should be all I need to change?

As I said in the OP, I'm still very much learning. what's the difference between a cathode follower and a plate follower and how would I change this mic from one to the other?

Many thanks,

Kris

Enchilada said:

Ta for the info! Plenty for me to read in your thread  .

So the Zener diodes in the PSU should be all I need to change?

As I said in the OP, I'm still very much learning. what's the difference between a cathode follower and a plate follower and how would I change this mic from one to the other?

Many thanks,

Kris

Click to expand...

Yes, to change the B+ voltage to 120V, all you need to do is to change the two zener diodes to 60V zeners.

Difference between plate and cathode follower circuit is easy to check from the schematics. (compare Apex 460 and ElaM251) See how the tube and the transformer are connected together.

Recent China mics might be plate followers stock. Check this thread.

http://www.groupdiy.com/index.php?topic=47314.0

And like others have stated, a great capsule like CT12 will make a good mic with any decent circuit.

Good point about the power supply, yes it needs to be set to 120V HT for the mod.

btw: shouldn't this thread go into the "Project Specific Discussions"/"microphones" section?

Jakob E.

I didn't know that those sections existed. If all then I have no problem with a mod moving the thread.

Quick question, you said that the mod from the aforementioned "Apex 460 to C12" website will end up more like the ELA M251. If I were to follow the mods there would I end up with a plate follower circuit?

Enchilada said:

Quick question, you said that the mod from the aforementioned "Apex 460 to C12" website will end up more like the ELA M251. If I were to follow the mods there would I end up with a plate follower circuit?

Click to expand...

quick one: no. Also IIRC it doesn't mention PSU voltage drop either.

telefunk said:

quick one: no. Also IIRC it doesn't mention PSU voltage drop either.

Click to expand...

That's good, I've read that the CM2480 is better suited to the cathode follower circuit. I'll make sure I don't forget the 60V Zener diodes in the PSU.

telefunk said:

Enchilada said:

Quick question, you said that the mod from the aforementioned "Apex 460 to C12" website will end up more like the ELA M251. If I were to follow the mods there would I end up with a plate follower circuit?

Click to expand...

quick one: no. Also IIRC it doesn't mention PSU voltage drop either.

Click to expand...

Actually, I think, the part: "some disassembly" are instructions for removing the cathode follower. It's hard to see because he only talks about  wire T1 etc. that are not shown in the schematic but removing the R8 is sure sign of cathode follower removal, ain't it?

But like I said, your mic might be plate follower stock, these mics have changed. CM2480 is not a great match with 6072 when plate fed, but will probably work ok. I would go for a cheaper tube though, 12AT7 (NOS of course), which would also be a better match for your trafo.

pasarski said:

Actually, I think, the part: "some disassembly" are instructions for removing the cathode follower. It's hard to see because he only talks about  wire T1 etc. that are not shown in the schematic but removing the R8 is sure sign of cathode follower removal, ain't it?

But like I said, your mic might be plate follower stock, these mics have changed. CM2480 is not a great match with 6072 when plate fed, but will probably work ok. I would go for a cheaper tube though, 12AT7 (NOS of course), which would also be a better match for your trafo.

Click to expand...

I do have a couple of NOS RCA 12AT7 tubes. I may try on of them if I end up doing this mod. To start with I'm starting to think I should keep it simple. Just change the Zener diodes in the PSU, Tim's Capsule, GE 6072A and the CM2480.

Sorry, now I checked the schematic  :: You're right Pasarski, of course , that mod includes cathode follower removal. You have the before/after schematic both of them so it's very easy to compare. All Apex 460's i've seen have the same cathode follower circuit. The new version of the T.Bone SCT800 is another story.

Sure the original Apex 460 has the C8 but it is connected to cathode not plate, thus making it a cathode follower circuit.

As you can see in this schematic: http://www.allthisnoise.com/media/apex460-c12-mod.jpg

The "right" side of the tube is shut down by connecting the pins to ground. There is also a mistake, heater voltage is not 12v, but 6,3V. Also, R7, I used the AKG-C12 value of 2,7K. You can see all this in my build thread, there's a little schemo i drew.

I actually have one mic with 6072A-CM2480 combo with plate follower circuit and it works pretty fine to me.

This is what our member Burdij has to say about the subject:

"The Cinemag CM-2480 is a 10.5:1 transformer that is small enough to fit into the rather confined space in a C-12 body, has two bobbins so it inherently cancels stray fields and is recommended by Cinemag as a replacement for the the T14 type transformer used in the original.

The important concept when using a transformer to couple a vacuum tube plate to a low impedance output is to achieve an impedance on the transformer primary that approaches the the impedance of the tube plate paralleled with the impedance of the plate load resistor and series impedance of the power source. Triode plate resistance is significantly lower than a pentode would be. For the 6072a the plate resistance in a class a amplifier circuit is approximately 25K ohms and the plate load in the standard C-12 circuit is 100K. Calculating the total resistance the load would be about 20K. If the transformer is going to drive a 200 ohm output load multiplying by the square of the turns ratio gives about 22K, a fairly good match to the calculated plate resistance."

Did a pair of Apex 460's up for a friend of mine. 

* Tim Campbell's capsules
* Switched circuit to plate follower
* upgraded a few caps
* 120v B+
* Oliver's T14 output transformers
* 12AV7 tubes (NOS GE's) -- don't have any 12AY7's at the moment, so just grabbed a couple of these
* removed inner layer of mesh in headgrill

Shockingly good mics.... truly stellar.  I have a pair of C12 clones that Scott Hampton built for me a decade ago that use original brass CK12 capsules, and these were every bit as good.  Highly recommended.

JC

rascalseven said:

Did a pair of Apex 460's up for a friend of mine. 

* Tim Campbell's capsules
* Switched circuit to plate follower
* upgraded a few caps
* 120v B+
* Oliver's T14 output transformers
* 12AV7 tubes (NOS GE's) -- don't have any 12AY7's at the moment, so just grabbed a couple of these
* removed inner layer of mesh in headgrill

Shockingly good mics.... truly stellar.  I have a pair of C12 clones that Scott Hampton built for me a decade ago that use original brass CK12 capsules, and these were every bit as good.  Highly recommended.

JC

Click to expand...

Mine is identical to the ones you've built, except the tube. I used the 6072A. You're right, amazing mics.

Waiting for my Tim C capsule .......I made a ela m251 point to point circuit using a CV465 instead of AC701 using a T14 from the 60s....also did a studio projects TB1 kinda C12 circuit, both circuits sound very similar cant wait to get Tims capsule.....loving the C12 ish sound rather than a U47 ish sound (MK47 with cheap K47)

all the best.