SCT700 Mods ?

[Rixsta]

Hey people

Just re-seen this thread and most of its over my head!
However, Iv'e been thinking of getting Electro Harmonix 12AY7  tube to mod this Mic.

Anyone recommend anything else I should do ? I'm feeling the mic is lacking mid-range energy and the U47 sounds nicer in the mids compared to this microphone.

Perhaps my best bet is to upgrade this mic into the U47 circuits the FET or Valve versions, in which case if anyone knows of a good Kit I can get please let me know
Cheers

 

[ln76d]

There was a kit from ioaudio it was mk7 or something like that.
Didn't tested, but i remember that schematic looked pretty decent.
You can do what ever you want (ok, almost) with use of original boards - only size of the tube is limiting.
Try any simple plate follower with sub mini pentode or triode - 5840/EF732, 6s6b, 6112 or any other.
Plate resistor 100-120k, cathode potentiometer 4.7k (set to get the full low end and highest sensitivity possible), 200M 1st. grid resistor to ground, diaphragm connect direct to 1st grid,  polarisation voltage to backplate (filtering cap 10nF-22nF), output cap in the 0.5-1uF range, polarisation voltage divider double 1Mohm. After setting bias, try to bypass cathode with 22uF-100uF capacitor and compare what you like better. If you are not feeling well with setting B+ voltage and setting polarisation voltage, then convert psu to 120V, then double 1M divider will give you 60V of polarisation voltage. You should be happy with the result

 

[RuudNL]

A 6S6B with the correct cathode resistor works very well in the SCT700.
Changed the power supply to +120 V.
The result is much better than with the original cheap Chinese 12AX7!
Self noise is at a very low level.

 

[ln76d]

A 6S6B with the correct cathode resistor works very well in the SCT700.
Changed the power supply to +120 V.
The result is much better than with the original cheap Chinese 12AX7!
Self noise is at a very low level.

Rather you don't have in mind to fit 6s6b to exact original sct700 circuit and just change cathode resistor and lower polarisation voltage?!?!
6S6B - yeah, like the most of proper working sub mini tubes which fit mike circuits tubes - in case of noise

 

[RuudNL]

Rather you don't have in mind to fit 6s6b to exact original sct700 circuit and just change cathode resistor and lower polarisation voltage?!?!

I didn't mention all other changes. I think only the PCB and the transformer are original!  ;D

 

[Rixsta]

Hey
Thanks for info about Mk7, that one looks good maybe it will fit nicely in this SCT700
Would a EC81 or Electro Harmonix 12AY7  be a drop in replacement for this microphone ? and which one is most recommended

Thanks a lot for this info very helpfull!

 

[ln76d]

Not sure what you ask about?
Replacement for Mk7 or original circuit?
For ecc81 and pretty good original CCDA mod i paste you link to schematic on the other page.
For MK7 ecc81 or any 12.7 double triode isn't proper.
I wrote you schematic for the one of the best options, which you can simply make with this microphone donor
Not one thing - original circuit have limitation with lack of space fo bigger tubes.
If you want to use original boards and transfimer shielding can then there's even hard to place 12a.7 tubes with use noval socket.
For this i had to drill shielding cover to fit tube tip inside.  Direct soldering to the board of these tubes is weak idea.
For that - Sub mini tubes are the best

 

[Rixsta]

Thanks

I wanted to know if I can directly replace the original valve in the SCT700 with Electro Harmonix 12AY7 Tube
without needing to modify the circuit ?  I don't know much about valves yet
I have not much money the next months to buy parts so decided just a valve mod and maybe a capacitor or two to improve SCT700.
No direct soldering ?  I do have a good solder sucker   yes theres not much space in the microphone.

 

[ln76d]

If you already have a 12ay7?!? Anyway it wouldn't be any huge improvement if you will keep original circuit. A little bit lower output and little bit less distortions. Still direct soldering 12a.7 isn't best option. Definately it isn't so easy to remove these tubes (double sided boards), so be aware that you can damage tracks. For the ccda original circuit, which is the worst thing (except maybe cheap 12ax7) in this microphone, you could at least try to rework PSU and use sub mini double triode like 6111, 6112, 6N16 etc. Better for direct soldering definately and always easier to remove if there will be a need.  Minimum - change the capsule connection - compare my schematic with sct700 which you can find in google.
To be honest, spend a little more time and do yourself a favor, build decent microphone with it - otherwise it's like decorating the dumpster with flowers

 

[Rixsta]

Thanks
Finally after some months, im recording new album here at home. so am going to try and modify this microphone, Im not new to electronics but im new to microphones.
Am going to buy the 12AY7 don't have one yet.
http://www.jj-electronic.com/en/12ay7

Don't have the money to upgrade to MK7 or U47 Fet sadly, unless anyone knows a used circuit for sale cheap.
anyway...
My basic problem with this microphone is actually that it is lacking in midrange, bass is very strong perhaps too strong overall sound with Thiersch blue line capsule is very nice however. The U47 sound in youtube videos and wav file samples sounds much more open in the midrange and foreward.

Maybe I will try your circuit schematic you sent me ln76d (thanks for that but Im lacking the time and money at the moment so..
At this stage step by step instructions will only work for my tired and confused mind!  swapping a tube is easy swapping a capacitor, adding a trimmer variable resistor to set bias is ok although saying that.

Voltage over C1 is  = 142v
The voltage between the pin 1 of valve and C1 is 29V
The voltage between the pin 1 of valve and ground/earth is 104V
Zener diode in PSU measure 74v from chassis to Zener.

 

[Rixsta]

So I have installed 62V Zener diodes in the power supply as its safer for the Thiersch capsule and I have replaced the original Chinese valve with a JJ 12AY7.  I have not set bias yet
Can anyone tell me do I set the bias with my multi meter from pin one of valve to where ?  ground or C1 +

Thanks for any newbie help

In regard to the sound quality, the microphone sounds more open now and there is no distortion to be heard! which the original valve did produce, so great!. However gone are the nice midrange and treble the sound is a little darker and less shine at the top end.
Is this because I have not set the bias yet or ?  :  I think possibly the heater looks a bit dimmer which I can understand as I have lowered power supply from 148  to 122 volts

 

[kingkorg]

I never needed to change bias when i modded those mics. I don't think lowest possible THD is the way to go for tube mics. But that is just me. I did adjust bias once for lowest THD and got a tube mic that sounds 100% like FET mic with that same capsule.

Did you get rid of RF filtering caps on that mic? I am not sure if it's the same as Nady1050. If so, you should remove them. They can cause HF roll off.

What kind of output capacitor does this mic have, and value?

There are several ways to controll low end on tube mic, at different stages. But all of that is allready out there. True rabbit hole

You can find topics on moding Apex 460 and see which component does what.

If this is the same mic (Nady 1050), you can follow instructions here. You don't need to change transformer. Stock one is great, you will just get a bit lower output with it.

http://advancedaudio-europe.com/forum/viewtopic.php?p=72#.WtPzcoDRYXM

You need to measure frequency response of circuit in your mic. If it's flat that would say that Thiersch capsule just sounds that way.

 

[RuudNL]

The heater isn't infuenced by the lower plate voltage!!!
Heater output from the power supply is something like 6.7 V. (7806 + diode)
There is some voltage loss in the cable, so at the microphone it will be around 6 V.
(Especially with the Chinese cable, with the thin wires!  ;D )
In principle the frequency response of the circuit isn't changed because of the lower plate voltage, although the capsule can behave slightly different.
But 148 V (meaning: 74 V polarisation voltage!) was too high anyway!
And yes, as kingkorg already explained, most of the sound is in the capsule!
(The elements that are most responsible for the 'sound' are 1-the capsule, 2-the tube, 3-the transformer.)
Capacitors will make a little difference too, but not so much.

Just a thought: if you put in so much time and effort, wouldn't it be better to just buy a better microphone?...

 

[Rixsta]

I never needed to change bias when i modded those mics. I don't think lowest possible THD is the way to go for tube mics. But that is just me. I did adjust bias once for lowest THD and got a tube mic that sounds 100% like FET mic with that same capsule.

Did you get rid of RF filtering caps on that mic? I am not sure if it's the same as Nady1050. If so, you should remove them. They can cause HF roll off.

What kind of output capacitor does this mic have, and value?

There are several ways to controll low end on tube mic, at different stages. But all of that is allready out there. True rabbit hole

You can find topics on moding Apex 460 and see which component does what.

If this is the same mic (Nady 1050), you can follow instructions here. You don't need to change transformer. Stock one is great, you will just get a bit lower output with it.

http://advancedaudio-europe.com/forum/viewtopic.php?p=72#.WtPzcoDRYXM

You need to measure frequency response of circuit in your mic. If it's flat that would say that Thiersch capsule just sounds that way.

With the amplifiers I have built over the years the higher bias within reason not only seems to sound better as more current is running through the transistors but also the overall dynamics and power/punch of the amplifier would improve, I wonder if its the same for the bias on the valves or if im way off the mark ?

Im going to get rid of those filtering caps today
The schematic

Ive seen someone say that upgrading C8 (output cap I guess) to 2uf or 2.5uf or something after removing C9 and C10 was a good idea.

Thanks for info about Apex 460 I noticed its circuit is very similar!
Your right I could do with measuring the freq response but have no gear to do this at moment.

The heater isn't infuenced by the lower plate voltage!!!
Heater output from the power supply is something like 6.7 V. (7806 + diode)
There is some voltage loss in the cable, so at the microphone it will be around 6 V.
(Especially with the Chinese cable, with the thin wires!  ;D )
In principle the frequency response of the circuit isn't changed because of the lower plate voltage, although the capsule can behave slightly different.
But 148 V (meaning: 74 V polarisation voltage!) was too high anyway!
And yes, as kingkorg already explained, most of the sound is in the capsule!
(The elements that are most responsible for the 'sound' are 1-the capsule, 2-the tube, 3-the transformer.)
Capacitors will make a little difference too, but not so much.

Just a thought: if you put in so much time and effort, wouldn't it be better to just buy a better microphone?...

Thanks for this info I think its making sense
Yes the wires do not look the best lol
I know its strange that the frequency response should not be change just changing the valve and the PSU voltage but it has and perhaps technically the response is still the same but to my ears a loss of mid and trebble.

But I will set bias later, please if you know is it between pin 1 and where I need to measure the voltage ?

And about buying a better microphone, there is no money! we are in debt already so can do no spending!

Thanks a lot people! and your help RuudNl has been very helpful
Cheers

 

[RuudNL]

Measure the anode (plate) voltage between pin1 of the tube (+) and ground (- , any metal part of the microphone).
This voltage should be roughly halfway the supply voltage of this stage.
In that case you can get the maximum voltage swing (max. SPL without significant distortion).

 

[kingkorg]

Remove c7 and lower c6 to something in order of 20uF to lower bass response. If that is too low, try 47uF. C8 should be uograded to non electrolytic.

 

[Rixsta]

Measure the anode (plate) voltage between pin1 of the tube (+) and ground (- , any metal part of the microphone).
This voltage should be roughly halfway the supply voltage of this stage.
In that case you can get the maximum voltage swing (max. SPL without significant distortion).

PSU is 122v
Thanks a lot. Anode voltage is 72v so ill try lower value of R7. So voltage is around 61v and see what happens.

 

[MagnetoSound]

Remove c7 and lower c6 to something in order of 20uF to lower bass response. If that is too low, try 47uF. C8 should be uograded to non electrolytic.

There is no need to remove C7. That is there to help the top end.

 

[Rixsta]

Remove c7 and lower c6 to something in order of 20uF to lower bass response. If that is too low, try 47uF. C8 should be uograded to non electrolytic.

Thanks I have
10x 22nf metallised poly caps 630v  could use some in parallel for C7
1x polystyrene foil 1000pf for C4
2x 1UF metallised poly film for C8 could use the other for C1 if its in audio path
Thanks for info ill have some tweaking fun 

 

[Rixsta]

Ok Ill give this a go also and report back presuming all safe to just remove those and try.  I think its nice to adjust to taste even if its only by my own ears lol
Also Ive noticed my microphone has a C5 on the top PCB yet there is no C5 in the schematic above perhaps I missed something!