Tube mic plate / anode current

[Khron]

I'm probably showing my (still relative) noob-ness by asking this, but...

What might the reasons be (perhaps apart from power dissipation?) for running impedance converters (triodes) at the minuscule currents that seem to be customary (in the region of 1mA or so)?

 

[rock soderstrom]

I'm probably showing my (still relative) noob-ness by asking this, but...

What might the reasons be (perhaps apart from power dissipation?) for running impedance converters (triodes) at the minuscule currents that seem to be customary (in the region of 1mA or so)?

Higher currents generate more noise AFAIK. The increased grid current also plays a role.

Edit: check this nice wikipedia link - noise types

https://en.wikipedia.org/wiki/Noise_(electronics)#Quantification

 

[MagnetoSound]

Yes, lower noise, lower rp, less Miller effect.

 

[Khron]

That makes sense, so far. Is there any (practical) lower limit for the idle bias current, then?

 

[MagnetoSound]

There are a few factors there. You need adequate voltage drop across the plate resistor to provide the required output swing (headroom).

Bias current also affects plate impedance and linearity. Distortion and freq response will suffer. It mostly comes down to the sound.

The load lines for any given tube will give clues as to how low you can go, but your ears will decide. If you want to experiment, you can force bias the cathode past the norms of ~1.0 - 2.0v and see whether you like it.

 

[rock soderstrom]

Bias current also affects plate impedance and linearity. Distortion and freq response will suffer. It mostly comes down to the sound.

The load lines for any given tube will give clues as to how low you can go, but your ears will decide. If you want to experiment, you can force bias the cathode past the norms of ~1.0 - 2.0v and see whether you like it.

This is consistent with my empirical experience. Regardless of the application, preamp tubes sound better when a little more current is flowing IMHO.

I once tested this extensively in a microphone preamp in the V1 position. There were audible differences.

In a mic headamp, with this background, you are looking for the best compromise between noise, headroom and sound.

 

[Khron]

Is headroom really that great of an issue, though? How far from the rails can it be said that a tube starts overly-distorting? And/or is there some "usual" gain figure tubes end up at, in mic headamps (that then gets attenuated via the step-down output transformer)?

Let's use the U67 as an example: sensitivity spec (cardioid) is 20mV/Pa, max SPL 116dB, which translates to a max output of about -7dBu / 0.346Vrms. Transformer ratio is about 12:1 if memory serves, so plate output works out to around 4.15Vrms. And that's from a 210V B+. Doesn't sound like a whole lot to me

 

[MagnetoSound]

Is headroom really that great of an issue, though? How far from the rails can it be said that a tube starts overly-distorting? And/or is there some "usual" gain figure tubes end up at, in mic headamps (that then gets attenuated via the step-down output transformer)?

Let's use the U67 as an example: sensitivity spec (cardioid) is 20mV/Pa, max SPL 116dB, which translates to a max output of about -7dBu / 0.346Vrms. Transformer ratio is about 12:1 if memory serves, so plate output works out to around 4.15Vrms. And that's from a 210V B+. Doesn't sound like a whole lot to me

You asked if there was any practical lower limit.

At zero plate current, there is no drop across Ra and therefore nothing to modulate with your signal. Ergo, there must be some drop at idle in order to superimpose your signal onto the DC. That’s your short answer - there must be some current in the tube or the tube doesn’t work! The ‘practical lower limit’ is greater than zero. The actual practical lower limit depends on other factors …

As mentioned already, the operating point affects impedance and linearity. Following on from there, you need the tube to sound good, and that means finding an operating point that preserves the timbre of the capsule and does not introduce any significant (unpleasant) distortion. These are the deciding factors for the most part, and as most of us have discovered, using your ears is the best way to fine tune a microphone tube.

The fact that you find significant voltage drop across the anode resistor is not really what I was getting at. As already discussed, tubes are biased to a point at which they sound good, are quiet, and present a workable output impedance.

 

[MagnetoSound]

In a mic headamp, with this background, you are looking for the best compromise between noise, headroom and sound.

This is it.

 

[Tubetec]

A long stable lifespan can be expected from tubes run at low current ,
a definite plus in a tube mic .

The very high grid resistances involved wont work so well at higher plate voltages with higher current tubes, meaning less tubes will pass the reverse grid current and noise tests ,

Extra headroom might also require a larger transformer ,
that might be inconvenient ,

Heat is the enemy of electronics .
Then you have the inevitable warm air currents drifting past the membrane to worry about ,
Thermally bonding the tube to the chassis is possible , but most certainly at a cost in terms of microphony and weird resonances .

Where higher headroom is a requirement a cathode follower does the job admirably ,

Just one more point regarding heat came to mind ,
a useful side effect of a tube mic is it tends to opperate warm to the touch some way above ambient temperature , this very much reduces any chance of condensation forming up on the membrane and plates .

I know there will be cries and screams of anguish when I say this but the AKG 414 was always a mic I avoided especially on vocals , sometimes depending on atmospheric conditions they became noisy ,
The studio itself was shadowed by mountains on all sides , when it rained it poured ,often for days on end , even by Irish standards the rainfall and humidity was massive .