DIY Tube Mic project

[thor.zmt]

Grid = -3,1V (and -520mV when I hold the mic)

I don't think you can directly measure the grid voltage.

Your meter is 1MOhm or 10MOhm, Even a rare and expensive 1GOhm precision DC Meter is a bit dodgy.

Thor

 

[Emmathom]

I don't think you can directly measure the grid voltage.

Your meter is 1MOhm or 10MOhm, Even a rare and expensive 1GOhm precision DC Meter is a bit dodgy.

Thor

Of course ! I forgot that "détail"

 

[Emmathom]

Last measurements :

cathode R = 112Ω
B+ = 93v
I lowered the load (from 33KΩ to 47KΩ) but because of the 96v zener minus 3v IRF +93v is the max. volatge I can get without having to add one more zener (6v ? 10v ?) > would be great if I can avoid to disassemble the circuit once again

I will add a 12v z because things have to be done proprely... and so B+ can be raised to 95v or even 100v

 

[MicUlli]

You may use your multimeter in series with a 1G resistor. Of course calibration is required (use a 9V battery)..

 

[Emmathom]

(4*24v zeners already there) + 9,1v added today

B+ = 98v (can go now up to 100v > does NOT change the plate voltage so much...)
Plate = 73v
Polar. capsule = 63v
Cathode = 362mV (362mV/112Ω = 3,23mA)
R load = 47KΩ

Will now install the Tx out and soon do some audio tests...

 

[thor.zmt]

(4*24v zeners already there) + 9,1v added today

B+ = 98v (can go now up to 100v > does change the plate voltage so...)
Plate = 73v
Polar. capsule = 63v
Cathode = 362mV (362mV/112Ω = 3,23mA)
R load = 47KΩ

Looks good.

Will now install the Tx out and soon do some audio tests...

I suspect it will hum like.... something.

Thor

 

[Emmathom]

I suspect it will hum like.... something.

Thor

Hum ? 100Hz ? because of the 29mV in the ground ?

 

[thor.zmt]

Hum ? 100Hz ? because of the 29mV in the ground ?

50Hz - beacuses voltages change when you pick up the Mic.

Thor

 

[Emmathom]

Yes I get a "nice" 50Hz hum but I tried without the mic body > will put it on and come back
I think it will be better but not perfect > what should I do to get rid of this hum ? I got no scope to measure AC in the HV nor in the heater voltage...

I was think about this shielded 5 wires cable : maybe use two cables tied together > one XLR "classic" shielded cable + one (not shielded and outside the XLR cable) with 2 wires just for heater & HV... but anyway the ground will be common !

 

[rock soderstrom]

Yes I get a "nice" 50Hz hum but I tried without the mic body > will put it on and come back

Just to avoid any misunderstandings. You always need a closed body (=Faradays cage) with good contact to ground or your mic will hum like hell.

 

[Emmathom]

Just to avoid any misunderstandings. You always need a closed body (=Faradays cage) with good contact to ground or your mic will hum like hell.

That's just what the mic did : hum like hell, even if the headbasket was installed (I thought it would be sufficient)
I will (tomorrow) also shorten the Tx wires to the strict minimum and put the mic body for test n°2

What about using 2 different cables (tied together) ? one shielded classic XLR and a 2 wires for HV & heater ? (but on the other hand, currents are so weak that I doubt it would change anything...)

 

[rock soderstrom]

That's just what the mic did : hum like hell, even if the headbasket was installed (I thought it would be sufficient)

No, that's not enough, the electronics must also be shielded, as you know now.

What about using 2 different cables (tied together) ? one shielded classic XLR and a 2 wires for HV & heater ? (but on the other hand, currents are so weak that I doubt it would change anything...)

This is not necessary, a clean signal routing and a good grounding scheme in a standard cable are sufficient.

 

[Emmathom]

This is not necessary, a clean signal routing and a good grounding scheme in a standard cable are sufficient.

I repost my (last) cable wiring...

 

[rock soderstrom]

I repost my (last) cable wiring...

I do not use the 100R + 100nF and I connect the cable shield and the ground from the PSU output (B+ and filament together) at the post near to XLR input. The PE is connect to a separate M4 screw according to German safety standards.

 

[Emmathom]

...and I connect the cable shield and the ground from the PSU output (B+ and filament together) at the post near to XLR input.

Yes the shield and the ground (wire to mic) are connected together to ground PSU (common for heater & HV)

The PE is connect to a separate M4 screw according to German safety standards.

You mean to another "location" in the chassis ? why not group them at the same location ?

@thor.zmt told me to ground the PE thru 100nF + 100R...

 

[rock soderstrom]

You mean to another "location" in the chassis ? why not group them at the same location ?

Not allowed in Germany.

 

[Emmathom]

Something I don't understand with NTE10/3...
It's supposed to be a 1:10 ratio 200Ω > 20KΩ (or 1:3 depends on secondary wiring), so shouldn't it be wired reversed ? primary at secondary and vice versa ? to lower tube output impedance by a 10 factor
Like so to get 10:1 :
Red output tube (thru 1uF MKP) / Blue (not connected) / Black ground
White XLR pin 2 / Yellow XLR pin 3

 

[rock soderstrom]

Something I don't understand with NTE10/3...
It's supposed to be a 1:10 ratio 200Ω > 20KΩ (or 1:3 depends on secondary wiring), so shouldn't it be wired reversed ? primary at secondary and vice versa ? to lower tube output impedance by a 10 factor

Sure, the transformer is operated in reverse (10:1) here. Who says otherwise?

Like so to get 10:1 :
Red output tube (thru 1uF MKP) / Blue (not connected) / Black ground
White XLR pin 2 / Yellow XLR pin 3

Correct

 

[Emmathom]

Ok so Thor made a mistake (see attached) and I wired (yesterday) the Tx in the wrong way !
Some people use it in a Mic pre, so wired in the right way (1:10)

So I will do :
Red output tube (thru 1uF MKP) / Blue (not connected) / Black ground
White XLR pin 2 / Yellow XLR pin 3

 

[Emmathom]

Here the last schematic
Capsule = 63v to backplate / signal front diaphragm