Tube / valve biasing for microphone applications

[Khron]

Despite having hung around here for quite some time, the whole tube deal is more or less a bit of a mystery to me - the biasing of them, to be more specific.

From what i've read especially recently, most(?) triodes (or triode-wired pentodes), in condenser mic impedance-conversion applications, tend to be biased at significantly lower than "normal" plate currents.

Some starting questions would then be:

- What sort of plate currents are we aiming for? The average seems to float around 1mA or so.
- Is the resulting plate voltage relevant at all? Should we aim for mid-supply (B+) or so, as with JFETs, perhaps?
- Is there any rule-of-thumb how far from the supply rails (B+ and ground) clipping occurs, or is that to be determined empirically / experimentally (if it varies from triode to triode)?
- I've seen certain cathode voltage values mentioned in the context of biasing mic tubes, but i couldn't really figure any rhyme or reason for those values - what gives?

 

[ruffrecords]

Tube biasing is most often achieved using what is called cathode biasing. The current flowing in a cathode resistor causes a voltage drop that sets the bias point. The value of the cathode resistor depends very much on the chosen plate current and the tube characteristics. Most tube microphones use an output transformer. A ratio of 10:1 is not uncommon. This means that even if the output were to reach 0dBu, the plate voltage would need to swing only just over 20V peak to peak so from this point of view the actual plate voltage is not critical. Designers are more likely to choose plate voltage for minimum distortion. A tube quiescent current of 1mA means a peak to peak current swing of 1mA which, via a 10:1 transformer implies a 10mA peak to peak or 3.5mA rms current in the output load which is +8dBu in a 600 ohm load.

I have never designed a tube microphone so the above is simply my take on your observations.

Cheers

Ian

 

[moamps]

Some of the most expensive microphones use fixed bias such as U47, U67, C12.
When selecting the operating point, the current of the first grid and the return voltage (related to the permissible maximum resistance at the input) are also very important.
I think this topic has been covered few times in this forum.

 

[ruffrecords]

Some of the most expensive microphones use fixed bias such as U47, U67, C12.
When selecting the operating point, the current of the first grid and the return voltage (related to the permissible maximum resistance at the input) are also very important.
I think this topic has been covered few times in this forum.

I looked at the C12 schematic. No evidence of fixed bias there. U47 schematic shows a mixture of cathode and fixed bias.. U67 does have fixed bias.

Cheers

Ian

 

[Gus]

U47 is way more fixed bias than cathode bias about 39mA "flows" around the tube to the 29ohm resistor compared to the under 1mA plate current
So the 29ohms has little to do with cathode bias

The C12 has been written about, it supplies a fixed filtered - voltage to the grid by moving the reference ground for the grid to a connection of a resistor in series with the negative side of the power supply. so it is a mix of cathode bias from the resistor mixed with the filtered -negative(adds a different time constant to the cathode voltage node).

There are even microphone circuits with contact bias.

Some M Neumanns use a voltage divider on the + heater supply to reference the grid lower than the cathode to bias it this needs a low noise heater supply

What you select for tube operating points depend on what you want

There are subtitles to setting up a tube microphone circuit they look simple however you need to think about a few things.

The ELA M250 251 uses cathode bias the 49c M does etc.

 

[ruffrecords]

So, the elephant is the room is what, if anything , is the benefit of using fixed bias in a microphone?

Cheers

Ian

 

[moamps]

Some M Neumanns use a voltage divider on the + heater supply to reference the grid lower than the cathode to bias it this needs a low noise heater supply

KM53a, KM54a, KM56, KM253, KM254, KM256, M49, M50, M249, M250,M269, SM2, SM23

 

[Matt Nolan]

So, the elephant is the room is what, if anything , is the benefit of using fixed bias in a microphone?

Cheers

Ian

The avoidance of a relatively large value and hence most likely electrolytic capacitor in the cathode for sonic and space reasons?

 

[abbey road d enfer]

The avoidance of a relatively large value and hence most likely electrolytic capacitor in the cathode for sonic and space reasons?

I believe the main objective reason is that electrolytic caps were not reliable enough considering the temperature in the mic's body.
It seems to be confirmed by the fact that mics that used the smaller and cooler-running nuvistors used cathode bias.
KM series did not, probably for lack of room in the small body.
The avoidance of electrolytic caps there has been later diagnosed by self-proclaimed experts as a means of avoiding the dreadful and terribly damageable sonic effects of phase-shift at low frequencies.

 

[Matt Nolan]

I believe the main objective reason is that electrolytic caps were not reliable enough considering the temperature in the mic's body.
It seems to be confirmed by the fact that mics that used the smaller and cooler-running nuvistors used cathode bias.
KM series did not, probably for lack of room in the small body.
The avoidance of electrolytic caps there has been later diagnosed by self-proclaimed experts as a means of avoiding the dreadful and terribly damageable sonic effects of phase-shift at low frequencies.

Perhaps more a concern about multiple -3dB points adding up all the way from the mic to the speaker?

I'm not sure about the heat argument for avoiding electrolytics - they still appear often as DC blocking caps between the anode and the transformer.

 

[vmanj]

I welcome everyone.

Instead of creating a new topic, I decided to write in this one.

In my microphone, according to the Sony с-800g circuit, the anode voltage is 230v.
Anode resistor 100k, cathode 1k.
Later I lowered the anode voltage to 120v.
Now the bias of the tube has changed.

Is it possible to return the bias that was at 230v with a cathode resistor. anode voltage?

 

[abbey road d enfer]

Later I lowered the anode voltage to 120v.
Now the bias of the tube has changed.

I reckon you mean the anode current has changed.

Is it possible to return the bias that was at 230v with a cathode resistor. anode voltage?

It is quite possible to make the tube run at the same current than previously, but it would almost certainly involve replacing both the cathode and anode resistors. Not changing the anode resistor would certainly result in decreasing significantly the plate voltage. In all cases, it would result in less headroom.

 

[vmanj]

I understand, thanks.

 

[luckythedog]

My 2p worth is that mic circuits are special because the grid circuit is typically ultra high impedance to avoid lf roll-off with capsule capacitance. I recently became curious about how such valve mic circuits can work despite apparently 'breaking' design rules about max Rg, sometimes as high a 1Gohm or more typically hundreds of Mohm.

Take say a 6027 or ECC81/12AT7 in a classic cathode bias circuit, say with Rk 2.7k Ra 100k and B+120V. If one floats the grid completely for test purposes, the valve appears to conduct about 0.45mA or so. So there must exist an internal grid - cathode potential at which there is true zero grid current - after all the grid is floating. This potential is tricky to measure directly using the sort of equipment we can generally access, but valve curves suggest its about -1V or so

By choosing cathode resistor carefully, one can arrange it so that floating grid is at ground potential. It then doesn't matter what value grid leak resistor Rg we use because there is no potential across it. So we can use a very high value. Ta Da.

To keep the circuit stable we can choose a very high value Rg to avoid the risk of grid blocking (a 2nd stable operating point at much higher Ia).

In this way some valve types valves seem to have a 'happy' self setting operating point and in mic circuits that appears the place to aim design bias. Perhaps for these reasons.

Just my 2p worth!

T

PS curves for floating grid aren't published

 

[abbey road d enfer]

It then doesn't matter what value grid leak resistor Rg we use because there is no potential across it. So we can use a very high value. Ta Da.

One thing that will matter is whatever galvanic leak there in the socket and the circuit. That makes for a circuit that's hard to produce industrially.

In this way some valve types valves seem to have a 'happy' self setting operating point and in mic circuits that appears the place to aim design bias.

There are many examples of vintage audio gear using grid-current bais (that's what this scheme is called) with grid resistors in the 5-20 Megohms range.
It requires proper QC for putting that in an industrial piece. That was relatively easy in the days of "tubes rule", but not that much today.

curves for floating grid aren't published

Of course, because grid current is a parameter that's hard to control.

 

[luckythedog]

One thing that will matter is whatever galvanic leak there in the socket and the circuit. That makes for a circuit that's hard to produce industrially.

There are many examples of vintage audio gear using grid-current bais (that's what this scheme is called) with grid resistors in the 5-20 Megohms range.
It requires proper QC for putting that in an industrial piece. That was relatively easy in the days of "tubes rule", but not that much today.

Of course, because grid current is a parameter that's hard to control.

Thanks AFAIK grid-current bias relied upon there being grid current. But in the bias arrangement described at the 'happy' operating point there is none. I think the typical mic circuit with ultra high Rg exploits this.

T

 

[abbey road d enfer]

Thanks AFAIK grid-current bias relied upon there being grid current. But in the bias arrangement described at the 'happy' operating point there is none. I think the typical mic circuit with ultra high Rg exploits this.

T

There's always grid current. You can't prevent the odd electron reaching the grid instead of the plate. In the real world, there is nothing like an infinite resistance, so not connecting a grid resistor does not mean that there is no galvanic continuity.
In tube mics, the tube is almost always underheated with a low plate voltage, so the grid current can be very small. There are examples of tube mics that use 7 gigaohm grid resistors but most manyfacturers tend to restrict to less than 500Meg, for repeatability.

 

[Murdock]

Don’t know if it is of any use for this discussion but I found the east german M50 equivalent, the M15, interesting in that it doesn‘t use a grid resistor. And a few other things i havent seen in another mic.

http://mikrosammler.de/images/8/8e/M15Sch-1.png

http://mikrosammler.de/M15/16

 

[abbey road d enfer]

Don’t know if it is of any use for this discussion but I found the east german M50 equivalent, the M15, interesting in that it doesn‘t use a grid resistor. And a few other things i havent seen in another mic.

http://mikrosammler.de/images/8/8e/M15Sch-1.png

http://mikrosammler.de/M15/16

Well, it takes someone much cleverer than me to figure out what the designer was thinking. This circuit is "idiosyncratic", with positive FB between plate and cath via the xfmr primary.

 

[luckythedog]

There's always grid current. You can't prevent the odd electron reaching the grid instead of the plate. In the real world, there is nothing like an infinite resistance, so not connecting a grid resistor does not mean that there is no galvanic continuity.
In tube mics, the tube is almost always underheated with a low plate voltage, so the grid current can be very small. There are examples of tube mics that use 7 gigaohm grid resistors but most manyfacturers tend to restrict to less than 500Meg, for repeatability.

Thanks. Several things contribute to grid current. Some positive and some negative in current direction. For example, emission from a hot grid is opposite direction to electron collisions with passing cathode current. For some valve types there can be equilibrium at an operating point where these cancel and nett grid current is 'zero'. If we float the grid or use very high Rg we force this condition. Because then grid current is 'zero' by definition. For triodes commonly used in mics this operating point seems to be usable where anode Va is around 70V, and Ia about 0.5mA. I am curious whether heater current (cathode temperature) might affect the equilibrium and so control operating point, and hence why heaters can sometimes be cool or adjustable in some mic circuits?

Don’t know if it is of any use for this discussion but I found the east german M50 equivalent, the M15, interesting in that it doesn‘t use a grid resistor. And a few other things i havent seen in another mic.

http://mikrosammler.de/images/8/8e/M15Sch-1.png

http://mikrosammler.de/M15/16

Interesting thanks. EC92 has same curves as ECC81/12AT7 of course. In principle Rk even at 10k doesn't set the operating point in this circuit, the valve will find its 'happy' point.

I recently did some tests with an ECC81 in a similar floating grid arrangement and it was stable. Not advocating, just saying.