SCT700 Mods ?

[RuudNL]

Also Ive noticed my microphone has a C5 on the top PCB yet there is no C5 in the schematic

C5 is not on the PCB. There is also a R9, that is not on the PCB.
They probably use the same PCB for other (multipattern) microphones.
I think in that case R9/C5 will form the filtering for the back diaphragm voltage.

 

[Rixsta]

C5 is on my PCB its in the SCT800 but also in my SCT700 for some reason, will upload pictures later.

So we think its a filtering cap ?  schematic below...

https://en.audiofanzine.com/large-diaphragm-condenser-tube-microphone/the-t-bone/SCT800/medias/pictures/a.play,m.1158954.html

Interesting update, working with C6 and C7
Microphone had lost its power removing them, the only way I can explain is that with C6 especially and also tried 470uf for fun, sound is more upfront, gain higher. Distortion about the same, its almost like turning a compressor on which sounds nice. Without C6 and 7 sound is weaker but perhaps more authentic, however I could also Say the same with the caps there is more depth to my voice. To my ears there is more than just frequency response changes going on there!.

 

[Rixsta]

Interesting that anode voltage was about 72v and can't seem to lower it lower than 65v using trimmer at this point my 5k trimmer instead of (R7)  had reached the ohms level.

 

[kingkorg]

And that is why i have switches on some of my mics, to turn on and of some of elements in them, and manipulate sound Might be the way to go for you as well.

You did lose some gain removing those caps, but you gained more headroom.

 

[Rixsta]

great idea, Ill order a toggle switch and sort this, I also have an idea to use this for valve hotswapping.
https://www.ebay.co.uk/itm/1-X-Zocalo-noval-PCB-B9A-9-Pins-dorados-gold-ceramic-socket/201605682234?hash=item2ef0a2943a:g:EIsAAOSwojRYSmB9

Above the PCB or below depending on what works in order to allow myself to change valves between 12AX7 and 12AY7 at will, I like both for varying reasons so will be nice to hot swap. Ill either have to move the circuit up and fit it underneath bending the pins of socket or put socket above the PCB upside down attaching wires.

 

[Rixsta]

Question is which 12AX7 would be best  Electro Harmonix, J.J, or Tung-sol 12AX7 which im interested in as Iv'e heard people say its good for vocal microphone.

 

[RuudNL]

C5 is on my PCB its in the SCT800 but also in my SCT700 for some reason.
So we think its a filtering cap ?

Exactly what I wrote:

"Non connecté sur le SCT800.
Utilisé pour les micros à multidirectivité."

Although the components may be there, they have no function in this microphone.

 

[Rixsta]

Interesting, thanks
Yep there is a cap there for sure

 

[RuudNL]

It is probably easier to stuff all components and use the same PCB for different microphones.
But, as you can see on the schematic, C5 and R9 are just 'being there', without any connection to the rest of the circuit.

 

[Rixsta]

Another question to those who have been and can be kind

I have just put in a ECC83 (12AX7 EH) Electro-Harmonix
for some reason I cannot get anode voltage below 88v adjusting R7, and 72v on the other valve.
124v is on pin 6

 

[RuudNL]

So that is a voltage of 36 V (124-88) over a resistor of 110K.ohm (100+10K.ohm), meaning an anode current of only 0.32 mA.
That is a very low value!
Ideally you should have a current in the order of (a little over) 0.5 mA, that will set the anode voltage to about half of the supply voltage. A different anode voltage will work, but the maximum SPL without significant distortion will be lower.

 

[Rixsta]

Great, I think I understand this and yes its about 30v over the 100+10k i see how you get current rating via resistance and voltage. ok so.
Is there any way I can increase the mA up to atleast 0.5ma ?  and get voltage right.
You had said to put a 5k resistor instead of R7, I have done this and when it reaches 0ohms I can still only get the voltage down to 88v.  so perhaps I should change value of R2 or R1 ?

 

[kingkorg]

We had debate about r2 value in thread abiut tiny russian tubes. I use 50k at r2 with them. Best way to test this is to place a pot instead of r2(100k) and r7(20k) and experiment.

You can as well insert full range signal (song) from your audio interface and listen how different values affect sound of the citcuit

 

[Rixsta]

great idea thanks for that, ill try get my bias current higher, is it safe to go over 50mA ? Im guessing it depends on what the tube specifications are ?
inserting audio is a good idea because I can use the interface also to measure signal freq response but also can hear any changes to the full audio spectrum.

 

[kingkorg]

I am more concerned about overheating power supply transformer.

 

[Rixsta]

Ok yes, it does look a bit small and cheap lol

 

[kingkorg]

Here it is, found it. I was wondering about same stuff regarding russian tube. The same questions you ask. Was lucky to get some fenomenal answers, i think you should read it.

By the way, i just measured one of my mics with 50k resistor. Current is 1ma with it, power supply gets warmer, but i wouldn't go higher than that. You might see in this thread that c800 runs at 1.5mA.

https://groupdiy.com/index.php?topic=67870.0

 

[MagnetoSound]

great idea thanks for that, ill try get my bias current higher, is it safe to go over 50mA ?

You don't mean 50mA do you?

 

[Rixsta]

Here it is, found it. I was wondering about same stuff regarding russian tube. The same questions you ask. Was lucky to get some fenomenal answers, i think you should read it.

By the way, i just measured one of my mics with 50k resistor. Current is 1ma with it, power supply gets warmer, but i wouldn't go higher than that. You might see in this thread that c800 runs at 1.5mA.

https://groupdiy.com/index.php?topic=67870.0

Thanks was an interesting read, ok so lower resistance = higher bias mA .
Out of interest are you measuring your current direct with tester or are you measuring voltage and resistance to calculate your mA.  Sorry for newbie questions but never used my multimeter to measure mA before.

I'll send some picks of what I've done think you guys might laugh lol

 

[Rixsta]

You don't mean 50mA do you?

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