SCT700 Mods ?

[kingkorg]

I measured it actually for the first time today, as that is the mic that gets my psu hotter. But in that thread someone actually calculated it for me and was right.

 

[Rixsta]

Oh cool that's really good of them definitely helpful people around here, although I'm still in a state of confusion lol

Ive taken out R2 and put a 100k variable in there when I turn him the anode voltage goes up ..hmmm urrrmmm hmmm

 

[Rixsta]

Here you can see my Thiersch blue line capsule which Im very glad to have here and I have modified the microphone so that I can swap valves at will without having to unsolder them by using this.

Resistor from the front of PCB had to be moved to the back to fit the large capacitor on front!

 

[kingkorg]

Great. I like how you found a way to fit tube socket. That capsule is beautiful!

 

[Rixsta]

Hey thanks
So I got a little confused
I took out R2 and put a 100k variable in there instead and when I turn him down to say 50k the anode voltage goes up to like 90v  yet I need to get the anode voltage down to half the PSU voltage I think ?  Is it that if the voltage increases the bias level goes down ?
I tried to actual put my 100k variable in series with the original 100k to make 200k and I still could not get the voltage to go down to 60v.

Ok so Im a bit lost here so if anyone can help would be appreciated

Cheers!

 

[Rixsta]

anyone got any ideas on my Bias issues above ? am I missing something ?  or being dumb ?

 

[RuudNL]

If you can't get the plate voltage down (=plate current up), this could have two reasons:

- The value of the cathode resistor is too high (grid voltage too negative)
- The tube has a very low emission

 

[Rixsta]

hello!
Is the cathode resistor R8 ?  interesting in the schematic it mentions 200k as well as the 270k I have.
Also is it possible to increase value of C1 ?

 

[RuudNL]

R7 (2K7)
You could increase the value of C1, but don't expect any audible changes.

 

[Rixsta]

Ok thankyou, I had another capacitor laying around which I can use instead of C1 (needed a smaller cap to fit in space)

Other than tweaking this R7 which other way should I be getting Plate voltage down and current up.

 

[Banzai]

Ok thankyou, I had another capacitor laying around which I can use instead of C1 (needed a smaller cap to fit in space)

Other than tweaking this R7 which other way should I be getting Plate voltage down and current up.

You can't do both. Plate voltage down = plate current down

If you want higher plate voltage (higher current), use a smaller plate resistor. If you want lower plate voltage (lower current), use a bigger plate resistor.

 

[kingkorg]

If you want to remove that silly logo and letters you can use acetone. They come right off.

 

[mhelin]

Took this mic and did some mods, not too many. though Just reworked the capsule connection a'la U47, and removed the output filter caps. The grid resistor is 200Mohms which is ok for this tube (12AX7,  less than 100 MOhms would be better though), and my tube seems to be OK, no extra noise, no need to replace. Mic now sounds pretty good and breathes with the vocals, though might be a little bit sibilant. It may be capsule though. Could try adding a small plate to  ground capsule (100-200pf) to fix that, or replace the capsule with a better one.  Anyway a simple mod, though not all are so lucky with the quality of the tube.

Now the tube, the 5670W dual triode (assume it's 407A derivated tube, and some people use 407A to replace VF14) might be good choice as it's short (can fit in with the socket), and got low noise. Also if you parallel the 5670 triode sections you could use it without the CF stage.