I just realized Advanced Audio's forum is down, and most of the pages can't be accessed even through the Wayback Machine. That forum was a source of some valuable information, as Dave Thomas is very open person and willing to share his knowledge and findings. This way i'd also like to thank Dave for all the info he selflessly shared.
So i managed to dig up this post at least on moding t.bone sct800. Not very likely, but someone might want to go from plate follower to cathode follower.
There was also a quite useful thread on moding Nady 1050, but that one is gone now.
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
So i managed to dig up this post at least on moding t.bone sct800. Not very likely, but someone might want to go from plate follower to cathode follower.
There was also a quite useful thread on moding Nady 1050, but that one is gone now.
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