Why do mics always use pentodes as triodes?

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zebra50

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Jun 4, 2004
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I was wondering...

Tube microphone circuits always seem to use pentodes wired as triodes?
(Examples: Neumann ELA M 027, U47, Sony C37, C800, Gyraf G7, Royer mod circuits, Lomo, Oktava MKL2500 etc etc.)

Am I wrong? Are there any exceptions?

And are their reasons why we never see them wired as pentodes?

Always we see grids 2 & 3 tied to the anode or cathode. Should it be possible to lower the noise by supplying stabilised voltage to grid 2?

Yet generations of talented designers seem to have gone for the 'as triode' approach. Is there anything inherently unsuitable about other approaches?



:?:
 
[quote author="Larrchild"]Capsules are square-law in frequency response.
The Miller-Capacitance of triodes counters this rise and all is well, again.[/quote]

When I first read this I didn't get it and thought you were talking about compensatory distortion. After a hasty reply I withdrew the musings on that. But having thought of it I wonder if there is a rationale there, if the polarity was right and the load at the plate was frequency-compensated. Hmmm...
 
The part of the deleted post I might have preserved was Why use a triode-connected pentode instead of a triode?

Another reason to use a triode per se is the lower noise (absence of "partition noise").
 
Tube condenser mikes are a German invention.

Germans used pentodes for everything.

So did several US board makers. I've seen a board that was all 6J7, mostly triode strapped.

After 1937, triode development stalled. The good thinking went into pentodes.

After 1942, triodes were mostly made as dual-triodes. What do you do with the extra?

Big studios need spare tubes. The fewer types of tubes used, the less cash tied up in the spares closet. Since a Pentode can do triode work, while a triode can't do pentode work, just use pentodes.

You usually don't use a pentode for a low-noise input. Partition noise.

(Though because the most-improved tubes were the pentodes, you can get pretty low noise with a hot pentode, comparable to the average triode.)

You don't use a pentode for a cathode-follower: it winds up working triode anyway (unless you have a floating battery for G2).

> Capsules are square-law in frequency response.

I don't even know what you mean here.

For maximum output, flat response, the head-amp is just an infinite impedance.

It can also be a NFB capacitor. But if that's what you want, you put in a capacitor.

Miller effect depends on gain, on output loading, and tubes generally do not have so much gain to avoid response changes with loading. So you usually won't want plate-grid C coming into it.
 
Thanks for the insights, particularly the historical aspect. It's interesting and fun to try and think like the engineers of the past.

>>You don't use a pentode for a cathode-follower: it winds up working triode anyway (unless you have a floating battery for G2).

It was this particular idea that had set me off thinking about this. I wondered how much mileage could be got from a well filtered G2 supply. It was this article that set me off:

http://www.glass-ware.com/tubecircuits/Easy_Cathode_Followers.html

and the statement "This is one advantage a pentode enjoys: grid 2 only draws a fraction of the current that the plate does, yet it effectively overwhelms the plate in current controlling ability, which means that regulating the voltage to grid 2 yields the same benefit as regulating the whole B+ power supply."
 
Sorry PRR, I inadvertently put "freq response" in place of signal response, regarding square law ..ma bad. My contention was that the Miller capacitance would work for you in that case.
 
CJ good to see you!

I do remember laughter.

Which in turn reminds me (lot of that lately going on) the wonderful passage in one of the Fawlty Towers episodes, Basil saying "ah well, then there's your life mate..."
 
> It was this article that set me off

I hate writing which says "noise" without saying what KIND of noise. (And John should know better.)

He apparently means power supply ripple and cross-talk. In context of a mike head-amp, you just filter the crap off.

Splitting current between plate and screen increases random hiss "noise", and more than you'd think from the mere diversion of current.
 
Thanks PRR - that helps to clarify that article.


Here's a related thread on how to wire the pentode as triode.

http://www.groupdiy.com/index.php?topic=10491
 
Germans used distant mic in mono.
Things and mics progressed into US' close mic Miles at Van Gelders, so Rein switched over to triode, remember now?
.
 
Also... When using a pentode as a triode, what is the quickest / best way to draw up a load line etc. if the data sheet does not contain the 'static anode' plot for operation in triode mode.

Lots of data sheets show this type of plot...

SampleTubeData1.gif


but not the more useful (at least to me) average anode characteristics:

SampleTubeData2.gif


These are actually both from the 6Z1P data sheet (tube used by Lomo and Oktava). But lots of data sheets don't show the second type of plot.

It's probably a simple answer, but I currently have just enough tube knowledge to get myself into trouble.
 
I'm thinking the same thing. Curve tracing for mic tube operating points wouldn't need any fancy pulse techniques, could be done at hand crank speeds.. I might be inclined to build an adjustable CCS for the anode supply and measure the a-k voltage to plot each curve in current steps rather than try to change the voltage and measure the current; just seems easier and built-in current limiting for the gassy ones ;-) A bench supply could feed the grid.

It would be easy from there to add whatever degree of automation suits you.
I'd like to add an audio output to check for microphonics and maybe even put a cheap capsule on it for grins.

Cheers!
Michael
 
If you have money and no time (increasingly less likely these days!) I can recommend the Amplitrex. I've only used it so far in the stand-alone mode but it is quite flexible when under computer control (windows machines only).
 
> using a pentode as a triode, what is the quickest / best way to draw up a load line

There are points on the "pentode" curves equivalent to the triode curves: where Vp equals the Vg2 used in the plot.

You also know that all curves (usually) pass through 0,0.

You know triode plate lines are approximately straight, tending to Child's Law, and sometimes aberrant at low voltage. %$#@! it, draw straight lines to 0,0.

The plot you cite is for Vg2=120V, so dot your Vp=120V points. Sketch straight parallel lines.

tri-pent.gif


My plot says in Triode, Vg1=zero and Vp=100V gives 17mA. Your triode cite gives 16mA. This is better agreement than any two real tubes.

As you get away from the plotted Vg2 points, error increases. In particular you can't get a decent small-signal value for Mu(g1-p). You already have an approximation: for Vg2=120V it takes -4V on G1 to cause cut-off, so you know it is in the area of 120/4= 30.
 
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