Rixsta

SCT700 Mods ?
« on: June 30, 2017, 07:32:29 AM »
Hi
Just wondering, does anyone have any ideas of some quick mods for this microphone ? transformers, capacitors ?
I think id like to change some caps, the mic is lacking some sheen. I do have a Thiersch Blue Line capsule in which is a little dark but I think a little mod on the caps would probably make things a bit nicer.



RuudNL

Re: SCT700 Mods ?
« Reply #1 on: June 30, 2017, 07:35:29 AM »
Did you keep the original 12AX7? (The one in my SCT700 was crap...)
There is a solution for every problem!

http://www.vansteenisaudio.nl

Rixsta

Re: SCT700 Mods ?
« Reply #2 on: June 30, 2017, 12:49:19 PM »
Thanks yes its the original :) I find this tone a bit muffled its nice but lacking clarity top.
my friend said that russian tubes are great but not sure what tube to solder in.

RuudNL

Re: SCT700 Mods ?
« Reply #3 on: June 30, 2017, 01:20:02 PM »
Are you happy with the self noise of the microphone as it is now?
I changed the tube and now the noise level is more than 10 dB lower than with the original Chinese 12AX7!
There is a solution for every problem!

http://www.vansteenisaudio.nl

Rixsta

Re: SCT700 Mods ?
« Reply #4 on: June 30, 2017, 01:58:21 PM »
yeah self noise is fine its just overall tone, I can tell that the Thiersch Blue Line is a step up in sound quality, original capsule is brighter which suits the circuit. the Thiersch is darker and the circuit is darker so its double dark lol so Im hoping either valve or capacitor change will get a bit more brightness on it.

Other than that the Thiersch really is a great sounding Capsule, Ive been thinking I might try it in my M-Audio Nova which is a phantom power transistor mic. (cheap but good for price)

Ill be recording more intimate guitar music so I think better noise level would be a good idea!


Re: SCT700 Mods ?
« Reply #5 on: June 30, 2017, 11:52:20 PM »
Did you keep the original 12AX7? (The one in my SCT700 was crap...)

12AX7 is a high mu triode and doesn't behave well at 100 megohm or higher impedance on the grid.  Most 12AX7's are very microphonic.  Try a 6072 or 12AY7.  You may lose some gain but pick up dynamic range and signal to noise..

Rixsta

Re: SCT700 Mods ?
« Reply #6 on: July 01, 2017, 05:02:42 PM »
Thanks for that, that's fine my audio interface is very sensitive anyway so loosing some gain is fine.

RuudNL

Re: SCT700 Mods ?
« Reply #7 on: July 01, 2017, 05:09:40 PM »
the Thiersch is darker and the circuit is darker so its double dark lol so Im hoping either valve or capacitor change will get a bit more brightness on it.
Don't expect miracles from changing capacitors.
(IMHO most of the effect of changing capacitors is 'between the ears'...  :P You KNOW that you have changed something, so you want to hear it!)
There might be some effect on the sound, but it won't make a dull sounding microphone bright!
There is a solution for every problem!

http://www.vansteenisaudio.nl

Rixsta

Re: SCT700 Mods ?
« Reply #8 on: July 14, 2017, 04:42:38 AM »
RuudNL   : which tube did you use ?
Thanks

 going to decide which one to buy!

RuudNL

Re: SCT700 Mods ?
« Reply #9 on: July 14, 2017, 12:36:00 PM »
I had something like 20 pcs. ECC83 (12AX7) tubes here, so I measured them all and selected the one with the lowest noise.
The one that is inside the microphone now isn't very microphonic. Brand name is unreadable...
But yes, the 12AX7 isn't the best tube for a microphone! (It wouldn't be my first choice if I made a microphone myself.)
There is a solution for every problem!

http://www.vansteenisaudio.nl


mhelin

Re: SCT700 Mods ?
« Reply #10 on: November 03, 2017, 03:56:58 AM »
I'm going to swap capsule to AA AK47 and try AA's CM48T kind of fixed, or "forced" like late Oliver Archut called it, cathode bias with 5654W tube. The circuit uses OA's U47 alternate circuit:
http://www.tab-funkenwerk.com/id85.html

Just don't know if the 100 ohm R2 and 29 ohms R3 are good for biasing this tube.

Btw. There in one SCT 700 on German ebay left (got the other one from this seller, ask shipping if outside Germany):

http://www.ebay.de/itm/t-bone-SCT-700-Studio-Rohren-Grosmembran-Mikrofon-TOP-Zustand/253232305916
Mikko

ln76d

Re: SCT700 Mods ?
« Reply #11 on: November 03, 2017, 06:31:30 AM »
I'm going to swap capsule to AA AK47 and try AA's CM48T kind of fixed, or "forced" like late Oliver Archut called it, cathode bias with 5654W tube. The circuit uses OA's U47 alternate circuit:
http://www.tab-funkenwerk.com/id85.html

Just don't know if the 100 ohm R2 and 29 ohms R3 are good for biasing this tube.

Btw. There in one SCT 700 on German ebay left (got the other one from this seller, ask shipping if outside Germany):

http://www.ebay.de/itm/t-bone-SCT-700-Studio-Rohren-Grosmembran-Mikrofon-TOP-Zustand/253232305916

Tried similar things with 6ak5/5654 tubes - unfortunately doesn't works good. Much better is just to make simple bias and put cap from cathode to ground and 100M from grid to ground. I would use potentiometer to set correct bias (catch the sweetspot ;) ). 6ak5, 6au6, 408a pentodes works much better with higher transformer ratio than 7:1.

KrIVIUM2323

Re: SCT700 Mods ?
« Reply #12 on: November 03, 2017, 08:39:42 AM »
Hi,
Piotr, when you said the fixed bias approach didn t worked for you is this related to noise issues?
If yes is this related to tube (noise induced by tube geometry/build) or psu related noise?

ln76d

Re: SCT700 Mods ?
« Reply #13 on: November 03, 2017, 09:20:54 AM »
No, it isn't related to noise but to low output level and weak frequency response especially in low range. I tried different plate resistance to manipulate the output impedance but this also didn't gave me much better results. Pure cardioid and higher polarisation voltage didn't bring the output to a decent range. Much better is Oliver design with EF80/EF800 for that kind of biasing.

ln76d

Re: SCT700 Mods ?
« Reply #14 on: November 03, 2017, 09:39:25 AM »
BTW. by occasion of all these tests i made my S16 microphone which sounds lovely. It's my favourite microphone for bass guitar tracking. Also really great for vocal tracking.  It is also easy to build with SCT 700 PCB and 5654 tube. you can even try with original transformer, but 10:1 would be better ;)
https://groupdiy.com/index.php?topic=62423.0
Few notes for cardioid version with other capsules than "teflon" CK12:
- use one plate  resistor (120k) instead R6/R7
- don't install C3
- try lower value of R4 like 100M-470M

Pretty simple and effective ;)

fsalter

Re: SCT700 Mods ?
« Reply #15 on: November 03, 2017, 10:56:23 AM »
I have a couple of M-Audio Nova's that i got on eBay dirt cheap, and put in some Mic-Parts K47 capsules in them and what was a decent sounding mic is now in a different class altogether.

yeah self noise is fine its just overall tone, I can tell that the Thiersch Blue Line is a step up in sound quality, original capsule is brighter which suits the circuit. the Thiersch is darker and the circuit is darker so its double dark lol so Im hoping either valve or capacitor change will get a bit more brightness on it.

Other than that the Thiersch really is a great sounding Capsule, Ive been thinking I might try it in my M-Audio Nova which is a phantom power transistor mic. (cheap but good for price)

Ill be recording more intimate guitar music so I think better noise level would be a good idea!

ln76d

Re: SCT700 Mods ?
« Reply #16 on: November 03, 2017, 01:12:26 PM »
I have a couple of M-Audio Nova's that i got on eBay dirt cheap, and put in some Mic-Parts K47 capsules in them and what was a decent sounding mic is now in a different class altogether.

You wouldn't believe how this microphone can sound after modifications. For both capsules - and yes - original chinese k67 is much better choice for this microphone than chinese k47 ;)
For k67 there's a need to build extra de-emphasis feedback network.
For both capsules circuit modification change a lot and except de-emphasis mod, there's a lot to change inside.
First thing is FET biasing, which is completely unbiased in this microphone, what cause really bad sounding hi-mid and hi frequencies range. There's also few things more to do ;)
Why k67 is better? - because headbasket bumps a 2-4k range where the chinese k47 is not so superior and boosted too much.
You can extend capsule saddle mount, to move capsule higher to more flat mesh area - this helps a little.

mhelin

Re: SCT700 Mods ?
« Reply #17 on: November 06, 2017, 02:51:43 AM »
It appears also Dave @AA tried fixed cathode bias with 5654W tube first:
https://www.gearslutz.com/board/attachments/low-end-theory/553061d1461090211-affordable-tube-microphone-cm48t-schematic.pdf

However, it seems they since went for self biasing with this tube:
https://www.gearslutz.com/board/12466100-post129.html
"The CM48t circuit is designed around the 7 pin sub-miniature GE/JAN 5654w that will fit in our CM48fet/CM87 body with modern high quality components. ... We used the passive polarization circuit used in the original U48 but we changed the tube bias from fixed bias to the more modern self bias."

ln76d:
 How about increasing the cathode bypass cap (C2 3.3uF) to make the biasing stiffer (closer to how U47 is biasing)? Also isn't there any other way to decrease the gain than using the fixed bias, I guess to do that the cathode should be biased a little bit less positive compared to the grid voltage. So why not use a LED (red one with  around 1.8V diode drop) or series of regular 0.7V diodes for that purpose? As you know the oldest tube mics used battery biasing there, no LED's were available then.

 
« Last Edit: November 06, 2017, 02:57:28 AM by mhelin »
Mikko

ln76d

Re: SCT700 Mods ?
« Reply #18 on: November 06, 2017, 05:00:10 AM »
Still AAM mike have low output (see specs) and there's a question - why it's much cheaper than their CM47? ;)

Increasing bypass cap doesn't make the biasing stiffer, you will lose low end, and we need to increase the gain, not decrease.
Not much fan of using any diode for biasing and the values - with that kind of voltage as you specified effect will be the same as with  resistor - poor gain and poor response.
You can also try SELA circuit - it's amazing - really ;)
https://groupdiy.com/index.php?topic=62601.0

mhelin

Re: SCT700 Mods ?
« Reply #19 on: November 10, 2017, 04:14:51 PM »
SELA is interesting (quite a low B+ voltage though). Anyway, I'll first try a 5670W dual triode tube which is shorter but has different pinout and heater supply voltage. It can be socketed, and if you turn it around 180 degress, drill a new hole for the pin 5, connect it to old pin 5 on socket, and swap the anode and cathode for the old pins 1 and 3 (swap the connections to them) you can use it if you mod the PSU to output 6.3V (or rather 5.0V for U47 style). I haven't though checked the heater voltage on SCT700 yet (it could be 6.3V as well). 5670W is used on Manley Reference Silver, and it's the 6.3V version of 407a which also can be used  to replace VF14 if properly connected (according to rumours on the internet).
« Last Edit: November 10, 2017, 04:18:24 PM by mhelin »
Mikko