T.Bone SCT800 modifications - C12 and other type circuits.

[ELM]

Point is optimal. Polarisation voltage too.
There's need to better drop voltage.

Ok Guys!
What do you think about simple voltage divider after rectifier?
Then it will be 150V. So two 1k resistor and three 47uF should be enough.

Your probably right.
I've already changed the four caps for quallity 82uf/400v. That could never be bad i thought.

 

[ln76d]

Another idea, which doesn't hurt anything, and it's independent from the final circuit.
Change C6 and C1 for 100nF, add 100nF in place of R4, if you fit under board -  add 100nF in parallel to (probably C8) last capacitor in heater supply. You can also change voltage regulator from 7806 to 7805 to lower heater voltage a little.

Here's schematic with voltage divider.

Values of resistors are optional. If you already have 47k (1W-3W) you can use it. For R3 also you can try it but maybe lower would be better.

If there's something unclear, let me know.

BTW.

If there's a bridge between two pins on 7pin xlr socket (for high voltage not ground!) then remove it.

 

[ELM]

Thanks for the schematic! I'l try it out!
I actually found all the components needed in my garage! 

 

[ELM]

I´ve rebuilt psu after scematic but nope, modified psu is still noisier than original.
I used 47K for r1 and "c3. R3 is now 30K.
B+: 117V
heater: 5,7V

I can ´t let go of the thought that the problem is about the lower capsule polarisation not being optimal for the original capsule?
Lower capsule polarisation lower the sensitivity and makes SNR worse, right?
Maybe a m7/k47 makes less noise than a k67 at 60V?
have your friend Maliq tried with a m7/k47?
I´d really love to keep the better sound of the modified mic but get rid of the noise!

 

[ln76d]

Capsule don't make noise by itself.
Same capsule (32mm alctron) is also runing from phantom at 46-47V volt polarisation.
Works without problem.
SNR is better with high polarisation voltage but it not change noise generated by anything, it makes only higher output level.
I gived you tip how to rise polarisation voltage without changing tube operating point.
Be patient, we will find source of noise (Zener probably - What Wattage it is?)
Some good pics of microphone circuit as also PSU circuit will be helpful.
With mic connected, measure voltage inside PSU after rectifier and after each resistor.
This definately will be helpfull. 
Did you used 100nF?
Heater looks good Did you changed regulator?

 

[ELM]

Capsule don't make noise by itself.
Same capsule (32mm alctron) is also runing from phantom at 46-47V volt polarisation.
Works without problem.
SNR is better with high polarisation voltage but it not change noise generated by anything, it makes only higher output level.
I gived you tip how to rise polarisation voltage without changing tube operating point.
Be patient, we will find source of noise (Zener probably - What Wattage it is?)
Some good pics of microphone circuit as also PSU circuit will be helpful.
With mic connected, measure voltage inside PSU after rectifier and after each resistor.
This definately will be helpfull. 
Did you used 100nF?
Heater looks good Did you changed regulator?

Hi!
Im not in a hurry with this project, and Im a paitient guy.
Im very greatful for all help and I enjoy this trouble shooting process, Im learning stuff!
My point was that maybe the psu is not creating noise but the lower output signal from the capsule makes you hear noise that always was there, even with the unmodded psu.
Btw, the tip how to rise the polarisation voltage? Did i miss it?
Yes i changed heater regulator.
Yes i used 100nf film caps.
Zeners are 62V 1,3W Vishay.
B+dropped to 114V w mic conected.
Some pictures :

 

[ELM]

Psu

 

[ELM]

Mic

 

[ln76d]

Hi!
Im not in a hurry with this project, and Im a paitient guy.
Im very greatful for all help and I enjoy this trouble shooting process, Im learning stuff!
My point was that maybe the psu is not creating noise but the lower output signal from the capsule makes you hear noise that always was there, even with the unmodded psu.
Btw, the tip how to rise the polarisation voltage? Did i miss it?
Yes i changed heater regulator.
Yes i used 100nf film caps.
Zeners are 62V 1,3W Vishay.
B+dropped to 114V w mic conected.
Some pictures :

Soon i will investigate everything, but two pics more would be helpfull - pic of 5pin xlr connections inside of PSU and pic of second side of PCB (without tube installed) in microphone circuit.
For polarisation voltage i will repeat myself (maybe it wasn't so obvious when am reading it again)

If you would like to see what you can squeeze from original capsule, then try mentiond cap from plate to ground as also you could change R3 for 1.3M resistor (optional you can use 1.5M).
After resistor change, you need focus on low end response, if it doesn't change then is good
It should improve signal/noise ratio so the output will be a little higher.

 

[ELM]

Thanks! I´ve tried to change R3 in mic to 1,5M but it did´t lower noise so I went back to 1M again.
Then I rebuilt the PSU back to original circuit but kept the 100nf film caps and the 47K (R1,2,3). I then changed back to original zeners and the noise dropped significantly! the modded mic/psu is still a little noisier than the original but differens is very small.
B+:190V
Yes, you were right early on, its the voltage drop and zeners  causing trouble. 
I find this very interesting, changing the zeners seems to be the standard mod around the web on different (but probably the same) chines tube mics. Am I the only one that gets higher noise levels?
About the mic xlr: there two pins grounded, the one in the middle an one more, I check later.

 

[ln76d]

For XLR i'm interested are there's jumper between two pins for B+.
Also you can rewire cable a little.
Some chinese mikes have ca. 200V transformers (different models).
This one have 300V, it's harder to drop more than 150V.
You can try higher wattage Zeners and there should be need to find better values of resistors for current optimisation.

 

[ELM]

Ok, i´ts time for some following up!
This has been a very fun project and i´ve learned a lot!
What i´ve done:
Microphone circuit rebuilt after ”Maliq mod” scematic, all components changed for higher quality.
Original capsule changed to a Thiersch M7 red line.
Omni switch added.
Original transformer changed to a Lundahl LL1951 (recomended by Pär Lundahl), used in 14:1 configuration.
Mic body painted to dampen body ringings.
Inner layer of mesh removed from head basket.
PSU rebuilt after first recomendation, all caps changed, heater voltage lowered.

I´ve spent quite som time trying to get lower noise levels but don´t think I achieved much improvement. On the other hand the noice seems to be very dependent on the tube used. Another thing that makes the modified mic sound noisier is that it is much more detailed and sensitive and therefore picks up more background noice. That said, I dont think it is a noisy microphone but if anyone knows the most efficient way to reduce noice, feel free to inform me! Regulated PSU? Point to point hi Z section? Tube hunt?
Thank you In76d!
I love how It sounds! 
Have listen, the dull one is the original (crappy mp3, but you get it…):
https://soundcloud.com/inkluder/t-bone-stc800

 

[ln76d]

Change zener Diodes to 100V (can be original) and 35V to get 135V. 100V from the side where's ground connection.
Try resistors 68k, 48K, 33k  (from left to right on the PCB).
Zener noise should disappear
You will get little bit higher level.
Anyway 14:1 ratio is in my opinion a little to high, i would rather go for 11:1 (like original transformer) or 10:1.

 

[ELM]

Thank´s, I will look into the PSU later.
I did some calculations (don´t ask) and found that 14:1 would give me the optimal impedance ratio between the mic and my 1,2k input preamp.
What are the disadvantages with a to high ratio output trafo? It would make life easier for the tube, right? Noise level i guess.
Btw, the Lundahl changed the sound a lot compared to the original, lot´s more of detail and clarity. Not what everyone is looking for maybe, but i liked it. The high end air is so pleasant.

 

[ln76d]

Thank´s, I will look into the PSU later.
I did some calculations (don´t ask) and found that 14:1 would give me the optimal impedance ratio between the mic and my 1,2k input preamp.
What are the disadvantages with a to high ratio output trafo? It would make life easier for the tube, right? Noise level i guess.
Btw, the Lundahl changed the sound a lot compared to the original, lot´s more of detail and clarity. Not what everyone is looking for maybe, but i liked it. The high end air is so pleasant.

Yeap, it's better for tube, also higher s/n due to higher level.
That more air is rather difference in headroom and different output impedance in work with your preamp than difference between transformers properties.

 

[ELM]

Forums without pictures are boring.
Original vs modded.

 

[ELM]

The other side.

 

[Rishisv]

@ ln76d

I just found this thread because I had the idea to modify my SCT 800 to a more C12 type mic. I bought a Mic&Mod APEX 460 mod kit with a C12 type capsule and a Electro Harmonix 6072 tube (unfortunately before I read this thread) . Do you see a chance to achive something in the direction, this thread was originally pointing (I'm quite sure, there are still lot's of SCT 800 owners with a similar goal.
Thanks in advance

 

[gigid]

Hi

I'm looking for info about mod this microphone, I have got a bit lost in this topic

thank you

Probably you wouldn't hear much difference between AMI and chinese transformer. Both are clean linear.
Some will argue with core size etc and low end response.
I would leave transformer alone especially if you want to put it in original circuit with original capsule.
You want to test transformer, buy cheap tiny neutrik NT10/3 and see do you hear difference.
You will fit it everywhere.
I really like small beyerdynamic transformers in tube microphones or bigger funkwerk leipzig (these are hard to find with higher ratio).
If you want to keep original capsule i can draw new schematic for you, but you will be tester of it, because i didn't tested it by myself.  It's about tuning few parts value for best performance. Do you have skills to solder it?
It would be using U87 style deemphasis to damp HF of k67.

My overall idea of modding tube microphones is making it warmer, more neutral with deeper low end than you will find in most of chinese and many even better "modern" tube mikes.
Anyway it's hard to make with k67 without deemphasis circuit.
Usual people are satisfied from my work. Mostly am using proven parts, usual so-called "vintage" in these parts of the circuit where it needs.