A Tube Mic

[MagnetoSound]

Thanks. Does anyone see what advantages/disadvantages there are of using this method compared to bypassing Ra with a cap to reduce HF gain?

Isn't it the same thing, when Ra has 1uF behind it?

It's my understanding that the effect is actually the same, but because it sees a path to ground either directly, or in the case of bypassing Ra, via B+. I think I see what you mean about the path through the 1uF though; it's effect would also be the same, but I have a feeling it would also be the same without that 1uF cap there.

Bypassing Ra means we can draw dynamic curves for different frequencies ...

You've still got 47.5k behind Ra as well. Although this acts as a dropper, it also forms a filter with the cap, to remove any noise picked up in the cable. Some mics bother with this, some don't. If your supply is clean enough and your cable well-screened, you may well do without it.

If you took out the 1u but left the resistor, I think bypassing Ra would be an interesting experiment. You might even play with the values of the two resistors a little bit, to alter the shunting effect?

... but I'm not sure how something similar would apply to tying that cap from the plate straight to ground. At the moment, the only way I can see of re-calculating the gain is to consider the loading of that cap on Rp.

That would be my assumption too.

 

[analag]

should i buy more 6AU6?
thanks

6AH6 is the final tube I used with excellent results. Screen and suppressor grid should be tied to plate.

 

[3nity]

Thanks Sir.
Update: working on the body at the moment.
thinking twice before buying a capsule holder.
Thanks

 

[mista min]

What should I do with the 500pF capacitor if I wanted to use a capsule without the HF rise?  Thinking of using a Chinese K47 copy for it.

Should I just leave it out?

 

[pasarski]

...just leave it out...

 

[Blue Jinn]

Is it possible to use this circuit with a single sided capsule? I'm assuming just eliminate the two .1uF capacitor and the 250Megohm resistor otherwise follow the schematic the same?

 

[MagnetoSound]

Is it possible to use this circuit with a single sided capsule? I'm assuming just eliminate the two .1uF capacitor and the 250Megohm resistor otherwise follow the schematic the same?

You will need to provide an efficient AC ground path for the backplate.

In this circuit the ground path is via the rear diaphragm - without it the capsule will not work, unless it is bypassed.

The quietest solution is to tie the second .1uF (the one normally connected to the rear diaphragm) to the backplate. This retains the filtering provided by the 1G resistor.

 

[Blue Jinn]

Thanks!

The PCB for the Aurycle kit mic looks like it could be adapted for this circuit.