DIY Tube Mic project
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thor.zmt
Well-known member
thor.zmt
Well-known member
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
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thor.zmt
Well-known member
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
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100R (1w) + 100nF (MKP10 > I have several in stock)
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
- 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...
- 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...
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thor.zmt
Well-known member
thor.zmt
Well-known member
thor.zmt
Well-known member
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
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 ?
* earth connected to ground chassis thru 100Ω + 100nF
* ground connected to mic enclosure & shield
B+ = 90v
Polar. = 58v
Plate =
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 ?
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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)
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)
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thor.zmt
Well-known member
Yes, the grid leak bias takes over.
I guess 75V will have to do.
Put 120 Ohm. Leave at that.
Thor
thor.zmt
Well-known member
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
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 ?
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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 ?
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 ?
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thor.zmt
Well-known member
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