Modding a t.bone SCT-800

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Modding a t.bone SCT-800

Postby tvbode » Mon Feb 16, 2015 10:57 am

Hello,

I have a t.bone SCT-800 tube mic, tha basically is another Apex 460 variation, but with a different PCB layout and – more important - the 2nd half of the tube is not in use, the cathode follower stage is omitted. I found the following circuit online, and as far as I could check against, it is exactly the circuit of my mic, except for one ceramic cap labeled “1.5” that is connected between pin 1 and pin 2 of the tube socket (as can be seen on the picture from recording hacks).

http://www.twin-x.com/groupdiy/albums/userpics/SCT800.JPG
http://cdn.recordinghacks.com/images//mic_extras/tbone/SCT800-PCB.jpg

So my original question was if I could still use the Apex 460 mod kit. But you already answered to my email that it will not work without the cathode follower stage.

I'm sure it would be no big deal for someone with more DIY experience than me to reactivate it. But I'm quite a newbie to gear modding, and the problem is, the tube socket is mounted on a daughter PCB and it looks like all the pins of the 2nd tube stage are directly soldered to a big ground trace. Changing the circuit would call for serious trace cutting and point to point rewiring. The mic is not that bad in the stock version,I would just like to see what more I could get out of it, but I would like to keep the upgrades simple, and if that means keeping the single tube stage, so be it. Otherwise I'm afraid I end up with something totally messed up.

So while the complete mod kit is out of question, I might still be able to perform some upgrades. First, I guess I would still benefit from the CK12 capsule. But what else would still work for me?

As I understand from another thread in this forum without the cathode follower stage C6 + C7 should NOT be omitted?

But omitting C9 and C10 and upgrading C8 to a 2.2uF foil cap should still gain some advantages?

Swap the stock 12AX7 tube to a 6072 is also still advised, I guess. What about a 12AT7 or even a 12AV7, as those have an even lower output impedance compared to the 12ax7 than the 6072. Could they bring me back in the ballpark where I could use the lower ratio transformer from your mod kit? Or do you maybe have transformers (probably with a higher ratio) that would suite that single tube stage circuit better?

What is that 1.5 ceramic cap between pin 1 and 2 good for? Should I leave it in?

Any other changes in components (low esr-elkos maybe) or values, that makes sense?

As I said, I'm a newbie to this. Any advice would be greatly appreciated. I would really love to get as close as possible to the “Advanced Audio” sound as I could. But I'm on a tight budget, the SCT-800 is already in my mic locker, and the Apex 460 are hard to find over here in Germany.

Many thanks in advance,
best regards,

Thomas
tvbode
 
Posts: 2
Joined: Tue Feb 10, 2015 2:00 am

Re: Modding a t.bone SCT-800

Postby Dave Thomas » Thu Feb 19, 2015 4:46 pm

Hi Thomas, thanks for the inquiry.

Yes, the SCT-800 was originally imported into North American and Europe from Feilo Microphones in China. These microphones came with an economy 32mm K67 type capsule with more rise at 3-5khz and less rise at 10khz than our AK12.

The circuit works best "as is" with a 12AT7 tube and good transformer with a 10:1 ratio. If you use a 6072a the circuit becomes very similar to the ELA M251 circuit and it requires an output transformer with a 14:1 ratio.

The stock circuit requires a cathode bypass capacitor so you should fit it with a good 200 ufd electrolytic bypassed with a .01 metal film or poly cap which will improve things a bit.

The SCT 800 is a cardiod only but by adding a 9 pattern power supply and in the microphone a 51 meg blocking resistor, a .01 bypass capacitor can have 9 patterns.
The 51 meg is soldered to pin 3 of the 7 pin connector and to the rear diaphragm with the .01 bypassing the back diaphragm to ground as far as audio is concerned.

The SCT-800 circuit and the ELA M251 circuit are nearly identical for all practical purposes. The capacitor across 1 and 2 (1 being the plate and 2 being the grid) increases the "Miller effect" which states, as the internal capacitance of a tube between its signal grid and plate increase the more the High Frequency response will roll-off. The capacitance between the grid and the plate whether internal or external is multiplied by the gain of the circuit. The ELA M251 accomplished this by putting a larger capacitor across the output transformer to tame the HF rise.

The gain of at 12AT7 is 60 so the 1.5pf in parallel with the internal 1.8 pf X 60 = 200pf which will start to roll out the high end response at 10khz to compensate for the rise in the CK12 and AK12's rise in response above 10khz.

In the C12 AKG did not reduce the natural rise of the capsule but they also used fixed bias which lowers the plate output resistance by 1/2.

If you use our AK12 capsule and leave the capacitor in you will get a more C251 like curve or if you remove the capacitor it will have a more C12 type curve.

In the ELA M251 SCT800 circuit with it the de-emphasis capacitor across pins 1&2 the HF response will vary depending on the gain of the tube used. More with a 12AT7 less with a 6072a and even more with a 12AX7.

In our circuit we fit the capacitor between the plate of the first stage and grid of the 2nd stage which offers more buffering. It is not effected by different tube gains in this position.

I believe John Peluso used the same body and ELA M251 circuit with a 6072a tube in his original 22 251 type microphone. He also had a capacitor between 1 and 2 in his 22 251.


The 2 stage CCDA circuit that we use in our microphones will work with a 12AT7 or 6072a which was the tube used in the original C12 and ELA M251. The CCDA circuit provides and even lower output impedance than the C12 or even the U47.

The first stage uses self bias like the ELA M251 but it has no problem driving the Cathode follower output circuit that has and impedance 20 times lower than the 6072a with fixed bias.


It is a very simple change to make:-

1- disconnect pins 6,7 & 8 from ground. If you heat the socket leg with a soldering iron you can pull them off ground 1 by 1. Worse case is you cut them away and tack a small wire to the remain socket leg.

2-make sure pins 4&5 are tied together and pin 5 is not tied to ground. Also make sure pin 9 in connected to ground.

3- jumper pin 1 to pin 7

4- connect pin 6 to the other side of the plate resistor which is usually a 100K going to pin 1.

5-connect pin 8 to ground through a 270k resistor and tie a 2.2/250v capacitor to pin 8. The other side of this capacitor feeds the red wire of the output transformer.

6-You can remove the plate output capacitor connected to pin 1.


With the CCDA circuit you can easily drive our 6.5:1 iron core dual bobbin transformer fashioned after the BV8 or our BV18 which has a 8:1 ratio and is a slightly larger and lower ratio version of the transformer in the original C12.

Cheers, Dave

tvbode wrote:Hello,

I have a t.bone SCT-800 tube mic, tha basically is another Apex 460 variation, but with a different PCB layout and – more important - the 2nd half of the tube is not in use, the cathode follower stage is omitted. I found the following circuit online, and as far as I could check against, it is exactly the circuit of my mic, except for one ceramic cap labeled “1.5” that is connected between pin 1 and pin 2 of the tube socket (as can be seen on the picture from recording hacks).

http://www.twin-x.com/groupdiy/albums/userpics/SCT800.JPG
http://cdn.recordinghacks.com/images//mic_extras/tbone/SCT800-PCB.jpg

So my original question was if I could still use the Apex 460 mod kit. But you already answered to my email that it will not work without the cathode follower stage.

I'm sure it would be no big deal for someone with more DIY experience than me to reactivate it. But I'm quite a newbie to gear modding, and the problem is, the tube socket is mounted on a daughter PCB and it looks like all the pins of the 2nd tube stage are directly soldered to a big ground trace. Changing the circuit would call for serious trace cutting and point to point rewiring. The mic is not that bad in the stock version,I would just like to see what more I could get out of it, but I would like to keep the upgrades simple, and if that means keeping the single tube stage, so be it. Otherwise I'm afraid I end up with something totally messed up.

So while the complete mod kit is out of question, I might still be able to perform some upgrades. First, I guess I would still benefit from the CK12 capsule. But what else would still work for me?

As I understand from another thread in this forum without the cathode follower stage C6 + C7 should NOT be omitted?

But omitting C9 and C10 and upgrading C8 to a 2.2uF foil cap should still gain some advantages?

Swap the stock 12AX7 tube to a 6072 is also still advised, I guess. What about a 12AT7 or even a 12AV7, as those have an even lower output impedance compared to the 12ax7 than the 6072. Could they bring me back in the ballpark where I could use the lower ratio transformer from your mod kit? Or do you maybe have transformers (probably with a higher ratio) that would suite that single tube stage circuit better?

What is that 1.5 ceramic cap between pin 1 and 2 good for? Should I leave it in?

Any other changes in components (low esr-elkos maybe) or values, that makes sense?

As I said, I'm a newbie to this. Any advice would be greatly appreciated. I would really love to get as close as possible to the “Advanced Audio” sound as I could. But I'm on a tight budget, the SCT-800 is already in my mic locker, and the Apex 460 are hard to find over here in Germany.

Many thanks in advance,
best regards,

Thomas
Dave Thomas
 
Posts: 172
Joined: Mon Dec 30, 2013 2:46 pm

Re: Modding a t.bone SCT-800

Postby tvbode » Fri Feb 20, 2015 1:21 pm

Dear Dave,

thanks a lot. I know from other threads that you are very helpful and go to great detailed, but *WOW*, this is way over my expectation. Besides answering my questions I even got a “history lesson” about the difference between the C12 and the ELA M 251 plus a practical explanation of the Miller-effect for free... ;)

Seriously, your informations are really helpful. So there are quite a few different directions I could go.

And though I'm really thankful for your step-by-step explanation on how to convert the mic to a CCDA circuit and activate the multi pattern function, as I wrote, I would like to keep my first mic mod as simple as possible.

I really am a bit afraid messing with the tube socket daughter PCB and those ground traces. And a new power supply would add to the costs, too. So I guess I'll stay with the “classic” single tube stage design and the cardioid only pattern. I have the feeling I better safe that money and maybe go for another mod with a real Apex 460 later of just safe up for on of your AA mics, instead of building more or less a completely new mic out of the SCT 800.

OK, so to sum up what is safe for me to do (as far as I understand):

- change the capsule to one of your AK12

- upgrade the cathode bypass elko (c6) from 100 to 200uf plus bypass it with a 0.01 foil cap (c7), as well as upgrade c8 to a 2.2 uf foil cap. Any recommendations for good brands? I probably would go for Panasonic FC or FR for the elko and WIMA for the foil, as they are often recommended as high quality “no frills” caps - at least in applications like recapping older gear.

- get rid of c9 and c10

- replace the zener diodes in the psu to some with 62V value.


What I just did is, I tried out two different 12AT7 I already had at hand. Those gave me about 2.5db less gain but more topend slowly starting to rise at about 3kHz up to 1.5 db at about 12kHz, compared to the stock 12AX7. I'm guess this is where the Miller-effect comes into play. (BTW, interesting side observation: the EH 12AT7WA rolled of the low end down from 70Hz a few db compared to both the RFT ECC81 and the stock 12AX7).
So maybe I should try your AK12 capsule first and than decide if I keep the de-emphasis cap between pin 1 and 2, or if I take that one out (or even just lower the value to get somewhere in between a C12 and ELA M 251).

There are still two questions left, though:

- I have no idea what transformer is equipped in the stock version (I guess it's not the same than that in the Apex mics, since the Apex has the CCDA stage and though needs a different – lower – ratio). The SCT 800 is a one stage design and is equipped with a 12AX7 that has an even higher output impedance than a 6072, so if I understand things right, the transformer should already have a pretty high ratio. The Apex stock transformer seems (at least regarding to Brian Fox) to be a pretty good performer, but I don't know if that is true to the SCT 800 transformer, too. But there is no T14 style transformer in the price range of your BV8 or BV11, is there?

- For what tube should I go? The 6072A is closer to the original designs, has a lower gain (with the benefit of more headroom), the 12AT7 has a lower output impedance and I could probably get away even easier with the stock transformer. Or do I mix something up here? :?

Again, many many thanks for being so helpful, I highly appreciate it.

All the best,
Thomas
tvbode
 
Posts: 2
Joined: Tue Feb 10, 2015 2:00 am

Re: Modding a t.bone SCT-800

Postby BernatVM » Thu Aug 31, 2017 6:14 am

I just modded my sct-800 with your ak12 capsule and bv11 transformer. I also changed the tube for an electro-harmonix 6072a and implemented the CCDA circuit. The sound is very good (huge improvement from stock one), i will be doing some testing in a session this week.

Just notice one thing: in your explanation you say:
4- connect pin 6 to the other side of the plate resistor which is usually a 100K going to pin 1.

Doing that resulted on a very noisy sound and volume changes on my unit. After some researching I compared my mic with an apex 460 and noticed pin 6 should go to the other side of the 2n resistor on the plate trace (10k 1/2 w in my case). Now the problem is solved, sounds amazing!

Image
BernatVM
 
Posts: 1
Joined: Tue May 16, 2017 7:12 am


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