DIY Tube Mic project

[Emmathom]

I "loose" 11v thru E-352 so 90v before and (last measurement) 79v after...
Grid voltage is at -2,34v
Cathode : 840mV

I noticed also that I get 160mV between PSU ground & mic ground...

 

[Emmathom]

@thor.zmt you didn't comment the possibility to put 2 x 1N4148 (for ex.) in serie to get 1,2v at cathode...

 

[thor.zmt]

Earth is not yet linked...

Put 100R/100nF in parallel between chassis and earth.

Thor

 

[thor.zmt]

I "loose" 11v thru E-352 so 90v before and (last measurement) 79v after...

That is a bit on the low side. Maybe lower the Cathode R.

We like it to be appx 60V

I noticed also that I get 160mV between PSU ground & mic ground...

Brrhhh.... That's wrong.

Thor

 

[thor.zmt]

@thor.zmt you didn't comment the possibility to put 2 x 1N4148 (for ex.) in serie to get 1,2v at cathode...

You can do that.

But why would you put a non-linear device in a connection where signal dependent current flows and the voltage across the non-linear device is in series with the input reference voltage?

Are you trying to make a distortion pedal?

Thor

 

[Emmathom]

Put 100R/100nF in parallel between chassis and earth.

Thor

100R (1w) + 100nF (MKP10 > I have several in stock)

 

[Emmathom]

That is a bit on the low side. Maybe lower the Cathode R.

We like it to be appx 60V



Brrhhh.... That's wrong.

Thor

Ground is connected to shield at PSU side (only) : should I connect it to shield at mic side too ?
Lower the cathode R to... 200Ω ? 180Ω ?
Adding 1KΩ in // with 235Ω = 190Ω

 

[Emmathom]

- 1KΩ in // with 235Ω R cathode (188Ω measured)
- earth to chassis thru 100R + 100nF
- shield connected to ground inside the mic

B+ = 90v
Polar. = 57,4v
Plate = 73v
Cathode = 612mV (with 188Ω to ground)*
Grid = -330mV

*612mV / 188Ω = 3,255mA

28mV between ground PSU and ground Mic (5 meters cable rolled up) > 160mV previously

... and I forgot to say that the transformer is not connected so the 1uF coupling cap is connected to nothing...

 

[Emmathom]

BTW the HV doesn't get down to 0v...
After 20mn Hv powered off 27v are still present : I should put a charge R after the last 1000uF (56KΩ ?) to "empty" the HV in the circuit...

 

[thor.zmt]

Lower the cathode R to... 200Ω ? 180Ω ?
Adding 1KΩ in // with 235Ω = 190Ω

120 maybe?

We are shooting for 60V on the Anode.

Thor

 

[thor.zmt]

BTW the HV doesn't get down to 0v...
After 20mn Hv powered off 27v are still present : I should put a charge R after the last 1000uF (56KΩ ?) to "empty" the HV in the circuit...

Sure.

Thor

 

[thor.zmt]

*612mV / 188Ω = 3,255mA

Yes, CRD's mostly have a mildly negative impedance.

So more voltage across the CRD = less current. Based on Datasheet, at 30V (and 60V at anode) we will have appx. 3mA.

Thor

 

[Emmathom]

I'm gonna try 330Ω // 235Ω = 137Ω at cathode and 39KΩ@1w for output load (90v/39KΩ = 2,3mA)

 

[Emmathom]

Measurements this morning with R cathode 135Ω and R load 33KΩ@1w on HV
* earth connected to ground chassis thru 100Ω + 100nF
* ground connected to mic enclosure & shield

B+ = 90v
Polar. = 58v
Plate = 76v 74,5v
Cathode = 430mV (so 3,18mA)
Grid = -3,2v (but -600mV when I hold the mic)

still 29mV between PSU ground & mic ground, even when HV is not ON (only heater ON)

The cathode R has "lost" 100Ω (so 235Ω -42%) but plate volt. doesn't react so much...
Should I still decrease cathode R to 100Ω ? or less ?

 

[Emmathom]

The polar. is ok (+58v)
The current thru the tube is ok (3,18mA)

What's the problem to get +75v on plate ? since it can handle max. +120v
and about max. power dissip. : 75*3,18mA = 239mW (max. datasheet : 120v*7,5mA = 920mW)

 

[thor.zmt]

The cathode R has "lost" 100Ω (so 235Ω -42%) but plate volt. doesn't react so much...

Yes, the grid leak bias takes over.

I guess 75V will have to do.

Should I still decrease cathode R to 100Ω ? or less ?

Put 120 Ohm. Leave at that.

Thor

 

[thor.zmt]

The polar. is ok (+58v)
The current thru the tube is ok (3,18mA)

What's the problem to get +75v on plate ? since it can handle max. +120v

No big problem, just the voltage across the CRD is a bit low. Woy might want to bump up +B to 95V or 100V, to give it some more voltage to work with

Thor

 

[Emmathom]

Yes, the grid leak bias takes over.

I guess 75V will have to do.



Put 120 Ohm. Leave at that.

Thor

Ok but what about these 29mV on ground ?? not normal...

Anyway I will install the NTE10/3 and do some audio tests...

Should I put 1GΩ from grid to ground ?

 

[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
Polar. = +58v
Plate = +74v
K = +364mV (so 3,25mA)
Grid = -3,1V (and -520mV when I hold the mic)
still 29mV between PSU ground & mic ground...

Should I put 1GΩ from grid to ground ?

 

[thor.zmt]

Should I put 1GΩ from grid to ground ?

No, the tube Bias will be wrong.

Thor