SDC Tube Mic. KM54 style conversion

[0dbfs]

Hi, I have a pair of SDC Tube Mic's exactly like these:
http://www.carillonaudio.com/products/tube/26/carillon-tube-100-kit/

I have purchased AMI Bv13 / T13 transformers and am planning to rebuild these into something different with fixed bias, 5840 tube, and AMI's T13 transformer.
I wanted to make them like a KM54 but there a couple issues with doing that.

1. The KM54 is a nickle skinned diaphragm.
2. The capsule in the KM54 headbasket is isolated such that both the diaphragm and back-plate are electrically isolated from chassis.
3. In the KM54 the backplate feeds signal to the grid and sits about 2.5V above GND while the diaphragm is biased with 40VDC as well as what looks like an NFB path from the plate.
Anyway, the doner-mic has the backplate connected to chassis/GND in the headbasket so it's not isolated.

I have started working this schematic up with (hopefully) some clever ideas borrowed from other mic's. I'll have to work out some component values but you should get the general idea.

I also am interested in using other capsules, or perhaps some sort of adapter to fit other capsules.

Any comments are appreciated!

Best,
jb

 

[grantlack]

hadn't even seen the T13, might have to pick one up for comparison down the road.  fwiw, i remember reading a psw post saying there had been little perceived difference in gold/mylar kk54 reskins- not that anything you'll find will have the same chambering anyway.
if you want to remain faithful with isolation, the rode sdc capsule could work.  20mm OD, so you might be able to fit it into the existing housing with space for an insulator. there's plenty of wiggle room when nested  back-to-back with an mxl capsule.  i'll be doing this test myself soon, i've got one with a mangled rear chamber and it's just begging to get chopped.

 

[bezen4uk]

Hi,
You need isolated capsule and I'll recommend old Gefell M94.
You need good capsule to meet "Bv13/5840/KM54" challenge, so I'll recommend old Gefell M94.
You need nickel capsule - why not to use old Gefell M94.
What I missed - chinese mics (and capsules especially) usually don't worth trafo/tube modding.

 

[mjk]

Hi,
You need isolated capsule and I'll recommend old Gefell M94.
You need good capsule to meet "Bv13/5840/KM54" challenge, so I'll recommend old Gefell M94.
You need nickel capsule - why not to use old Gefell M94.
What I missed - chinese mics (and capsules especially) usually don't worth trafo/tube modding.

Body tube size on M94 is much easier to DIY. Also the compatible M93 omni is available. Great sounding capsules.

 

[0dbfs]

Yes the T13's are in. I've got four for u87 builds and an extra set to experiment with in these mic's.
If I can source and fit KM54 capsules, M94's, M70's or something similar I will need to add a "spring clip" type of contraption similar to those 691/582 bodies for isolated backplate and capsule. That sounds do-able.

I disassembled the capsule on one of these mic's to get a better look. The diaphragm is grounded and the backplate is polarized with about 43-45V.

I don't think there is much to do with the capsule except maybe customizing the back-plate by milling or perhaps adding additional delay/resonance chambers behind the backplate. There is some room there and modifying the chamber space would surely change the acoustic damping behind the capsule. I think it's likely that modifying that space could be quite significant regarding sonics and will require experimentation to better understand.

Sorry, I didn't take pictures while the capsule was apart but it is a very simple and interesting contraption held together with pressure from the back threaded disk. It's basically like this:

----------------------
front grill
thin teflon ring
gold sputtered diaphragm
thin mylar "o-ring / spacer"
milled brass backplate chamber inserted in an isolation ring/cup
signal pin connected to backplate sticking out the back of capsule
threaded disk open in center for isolated signal pin
----------------------

The diaphragm is connected to chassis and at 0V. Backplate is charged via a divider from B+. Standard plate out with a coupling cap between backplate and grid. Both grid and backplate have 1G to gnd. Self bias. Have not checked out the PSU but am assuming that it's a typical zener based B+ and maybe a 317 in the H+ which measures 5.8V at the heater. No dropping resistor in the mic body.

While the mic sounds ok stock I have other SDC's available that are better. I think this mic can be better in a unique way. My goal is to identify the strengths and make this one as good as or better than what I have otherwise available in that department which is basically a bunch of 184's...

A capsule upgrade isn't out of the question either. Especially if I can sort out a capsule adapter of sorts. I have seen M94's for a couple hundred as well as M70's which are supposed to be available new but not sure on pricing. I can probably experiment with 184 and 185 capsules if I can sort out mounting them.

The construction of the stock capsule may also lend itself to further experimentation with other diaphragm material if I can source it. The capsule assembly in this mic is much simpler than the Debenham, Robinson & Stebbings capsule and looks decidedly open to modification.

Cheers,
jb

 

[Marik]

I have started working this schematic up with (hopefully) some clever ideas borrowed from other mic's. I'll have to work out some component values but you should get the general idea.

You have "signal" side of the capsule shunted with a capacitor.

Best, M

 

[0dbfs]

Right. Thanks! I guess there is a 15-20k after that on the km54.

Cheers,
jb

 

[Marik]

Also, 5840 is not a directly heated tube, but you have cathode connected to heater...

Best, M

 

[0dbfs]

Marik, thanks for pointing that out. I had not considered the direct/indirect differences between 701 and 5840. I had intended that node to be a clean dc source with the heater acting as resistor such that I can calculate a certain voltage to be present at the cathode which added to the grid voltage would bias up at about -1.6V @ grid wrt cathode. Maxes mk7 uses this approach and works well.

I will have to read up a bit on the differences between direct/indirect to understand those details.

This is mostly a learning experiment and I hope to better these mic's by whatever means ends up working but don't mind at all taking the longer path around to get to that point. I can always use more magic microphones

There never seem to be enough of those to go around once you start handing them out in session

Cheers,
Jonathan

 

[bezen4uk]

AC701 is indirectly heated tube:
http://www.radiomuseum.org/tubes/tube_ac701.html
There is no problem with 5840 usage.