Gyraf G7 - t.bone SCT-800 Mod

[fillitalli]

Hi,

I want to share some PCB designs to update a t.bone SCT-800 tubemic to Gyrafs G7. The advantage is to get a realy cheap set of everything you need: microphone body, power supply box, microphone mount, all caples and a case. I got one in ebay for 115 Euro and decided to do the pcbs.

No sooner said than done and now, I think, I have some problems. I measured some voltages on some points and was a little bit confused. There are no voltages like in the Gyraf schematic. Then, I tried the mic in the cardioid mode and it sounds really great. But I don't understand why. All voltages are wrong.

Maybe somebody can help me?

In the next picture you can see all measured voltages.

The PSU voltages without connecting the mic are:

Heater: 6,2 VDC
HV: 149,2 VDC
Cardioid: 74,2 VDC

And if the mic is connected, then the situation chages a lot.

All Informations are in the following pdf

Mic front

Mic back

PSU


Here are some informations about the parts:

PSU first option:

If you want to use the t.bone transformer, then use the smaller PCB in the PDF. Now you have to add 150kOhm for R1, 22kOhm for R2 and change the first capacitor vor higher voltage, because the t.bone transformer gives 300 VDC.

1x 470uF/400V
1x 470uF/200V


PSU second option:

If you don't want to use the t.bone transformer, then use the bigger PCB in the PDF. Now you have to remove the 230V/115V switch, add a wire bridge for R1 and the following parts

1x Gerth 1x9V, 4.8VA - Gerth 421.09-1
1x Gerth 1x15V, 3.6VA - Gerth 387.15-1

1x 470uF/200V
1x 470uF/200V

Mic:

RK-12 Microphone Capsule
Electro Harmonix EF86EH
Lundahl 1538XL

1 GOhm Vishay/Dale
2.2uF and 1uF MCAP Mundorf Audio Capacitor Mundorf Audio Capacitor

7-Pole Connector:

1 --> Heater Voltage (brown)
2 --> High Voltage (yellow)
3 --> Pattern - This is the only wire, which you have to add
4 --> GND
5 --> -OUT (black) - already connected to XLR jack
6 --> +OUT (green) - already connected to XLR jack
7 --> GND

 

[CJ]

try loading the supply without the mic and see what you get,
by loading, i mean put a bleeder resistor across the B+,

when a supply has no load, caps will charge to peak value,

when you load it, it should come down 10 or 20 percent,

try a 50K 1 watt if you have it, or 2 100K in parallel





report back your findings immediately,

 

[fillitalli]

Ok, I found a 47k resistor and plugged between HighV and Ground. The voltage drop to 110,3 VDC.

I updated some informations in my main post above.

 

[CJ]

cool!

how does that mic sound?

don't worry about the 13 volts on the capsule back, you do not need to measure that, when you do, leakage from the meter screws up the reading, so we just measure before the big resistor and use that as it wll be very close to the capsule voltage,

 

[fillitalli]

Why cool??? I thought, there must be 160VDC on the HighV and 80VDC on Pattern.

The Mic sounds really good. I compared it to the following two configurations:

Neumann TLM103 --> Gyraf G9 --> Gyraf GPultec --> RME Fireface800

and

The new t.bone Mic Mod --> RME Fireface800

The new Mic is more wider and more in the face.

 

[CJ]

ahh, so you first posted the Gyraf pdf with values you wanted, and now you have a drop in voltage that you did not want? little confused on what values you were shooting for,

so maybe a problem with the power supply?

is anything getting hot on the PSU?

put an ohmeter on the B+ line of the mic with power off of course, allow caps to drain, and tell me what you get,

 

[CJ]

ok, looks like R1 is wrong,

150K will cause too much of a voltage drop,

maybe 15K?

 

[fillitalli]

Maybe my english is confused 

My aim was to build a mic mod. My starting point was the gyraf G7 schematic. So I designed the PCBs and build them up. Then I measured the voltages and I wonder about the results. In gyrafs schematic there are 160VDC at B+, 80V at capsule centre. I have 113.9 VDC at B+, and 0.6 VDC at capsule centre.

 

[fillitalli]

No, I haven't 150k. I use the second PSU option with 10k. I have updated the PDF. Now there are all informations. The voltages in the PDF are the actually measured.

 

[fillitalli]

I added an ohmmeter and it started at 5MOhm and falls more and more. And slower and slower. Now I have 207kOhm.

 

[CJ]

that seems reasonable, you were charging a cap with the meter, or a cap was discharging as you took the reading,

if it sounds good and nothing is getting hot, i would just leave it,

 

[fillitalli]

Ok thanks. I will leave it. Now other people can use to update t.bone microphones.

I will add some noise measurements and sound examples.

 

[gigid]

hello people,
I have just bought a sct800 and I'm looking for advices,

I have two valves, a Mullard ecc81 and a Genalex ecc83
I was thinking to install the Getwin k47 capsule (it's cheaper than Maiku)

any help would be really appreciated

I have got lost reading this topic


thank you

Gigi