Fixed BIAS in a tube microphone.
- Thread starter vmanj
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moamps
Well-known member
I would add one resistor of say 100kohm in series with the potentiometer slider and would increase that capacitor to 100nF. Your tube filament power supply should be very well stabilized and filtered.
A cathode resistor is not required.
A cathode resistor is not required.
... And the tube is running at maximum gain, which I'm not 1000000% sure is what you want, in a mic...
RuudNL
Well-known member
If you decouple the cathode resistor, the tube is also running at maximum gain.
I would add an extra (high value) resistor between the wiper of the potentiometer and the capacitor, for extra filtering.
I would add an extra (high value) resistor between the wiper of the potentiometer and the capacitor, for extra filtering.
For example like this?
With resistor and capacitor looks like u67 and c12 circuits.
I wanted to do without this capacitor and resistor.
If it is for filtration, then I have it well filtered by the heater voltage - 6.3V.
But maybe this capacitor and resistor are needed not only for filtering, but for something else?
With resistor and capacitor looks like u67 and c12 circuits.
I wanted to do without this capacitor and resistor.
If it is for filtration, then I have it well filtered by the heater voltage - 6.3V.
But maybe this capacitor and resistor are needed not only for filtering, but for something else?
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C12 is cathode bias with filtering note the voltage drop across R8(most of the current is from the cathode of the tube)
The voltage to the grid is filtered by the 2uf after the 1meg.
Some fixed bias microphone use a - filiment supply and use a voltage divider for the grid voltage
or you could do something like the M250b
or
The voltage to the grid is filtered by the 2uf after the 1meg.
Some fixed bias microphone use a - filiment supply and use a voltage divider for the grid voltage
or you could do something like the M250b
or
Matador
Well-known member
Why not try it without the cap and resistor and see what happens?
dmp
Well-known member
In this design the 6.3V needs to be filtered extensively after rectifying in order to have low enough ripple to not be audible, since it is directly on the tube grid. In a U67 DIY build, starting with 12V AC, you need to use a choke PI filter to get 6.3v AC out without too much voltage drop (from RC filtering). Meaning, you may or may not have enough filtering on your 6.3v supply for this design. Typical mic power supplies do not have enough filtering on the H+
The 0.01 MF is a band aid to filter a little more at the bias junction and will be needed or not depending on how much ripple your 6.3V has (We don't know), but it doesn't do any harm except take up a little space. The frequency effect is negligible since the connecting 510M is so large.
Keep at it though - in my experience a fixed bias tube mic is a audible change (and awesome) but is so difficult (due to H+ filtering) there are not many builds (vs C12 clones).
If you do manage the fixed gain - get rid of 1000 pf to the capsule!
Switch the grid connection to the other side (that currently goes to ground). The tube grid (at ~ -1.5v) can provide the capsule reference. The backplate can stay connected to the polarizing voltage (~60v). See the U67 schematic.
dmp
Well-known member
C800G = cathode bias mic?
The heater is DC in the u67 (if you did AC you would have a pretty loud 60 Hz signal on your tube).
"Filtering" means creating a 6.3V DC heater supply with low ripple.
A fixed bias mic (like the U67) grounds the cathode and puts a small negative voltage on the grid. When the small negative voltage is derived from the heater supply, it needs to have very low ripple or else it will be amplified to the mic output.
Filtering a DC voltage is filtering a DC voltage, regardless of what it's used for. Unless there's some "magic voodoo" i'm yet unaware of?
http://www.foxaudioresearch.ca/Sony c800 schematic.jpg
http://www.foxaudioresearch.ca/Sony c800 schematic.jpg
dmp
Well-known member
Yes
I would get rid of the trimmer in the final build also (replace with two resistors instead). Find the resistances you need and then swap them in for the trimmer.
It's clear.
I tried to connect the capsule according to this scheme and I got a quiet and bad sound ..
For some reason, the sound from the back of the capsule is louder than from the front.
I have a cardioid capsule and this is very strange ..
And he began to work in antiphase with other microphones ..
I have tried replacing the resistors (polarizing capsule and mesh resistor) with 1G and there has been no significant improvement.
I had to go back to the old scheme.
I don't understand why this is happening ??
Just a trimer allows you to adjust the bias over a wide range.
And how can the trimer interfere, will it somehow affect the sound?
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dmp
Well-known member
strange. Does your capsule have a membrane on the front and back? The 'backplane' is the metal center, it is not the membrane on the back, if you have a dual sided capsule. You are just swapping the connections of the bias and grid.
Just for long term durability. You can use a trimmer
