Power supply?

Vagodeoz

Vagodeoz

One-Man-Band
What are those power supplies that comes with some mikes. Why aren't they just phantom powered?
I know some tube mikes has the tube in it, but is it like that with all of them? If it has a power supply it's a tube mike?
 
I think its generally the case that a tube mic will have a power supply to power the tube, since I would imagine it would require current much greater than what phantom power provides. I've never seen a non-tube mic with its own supply, but there are almost always exceptions to norms :p
 
AT makes a couple tube mics that use phantom, but beyond that I believe most tube mics that are currently made require a separate power supply.
 
The sensitivity of a capacitor microphone is proportional to the polarising voltage used so some manufacturers (such as DPA and B&K) use a power supply to give a polarising voltage of 130v.

So no, if a microphone requires its own power supply it isn't definitely a tube microphone, but it might be...:)
 
most likely it's a tube... btw those AT's that run on phantom power are starved plate designs so caveate emptor...
 
dementedchord said:
most likely it's a tube... btw those AT's that run on phantom power are starved plate designs so caveate emptor...
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All the phantom-powered tube mics I've ever seen use subminiature tubes. Those tubes are designed to run at a lower voltage (about 22-30VDC in the case of the 6418 tube that the AT3060 uses), so while the AT circuits may use a low plate voltage, they are not starved plate designs.
 
There are a few preamps that provide 130V, so an external supply wouldn't always be needed for the DPAs ;)

AT3060 is not starved plate. Specified plate voltages per the datasheet are 15V and 22.5V, with a maximum of 30V.
 
anybody got a link to a data sheet for that tube???? understand that it's designed for lower voltage but not sure that disqualifies it as starved... hard to get those electrons boilling off at 20V it seems to me... and does it use a heater???
 
MSHilarious, could you give us a little inside of the MSH-4? :) I don't think it's considering "advertising", since it's discontinued and not being sold anymore, and it's for technical reasons only...

Did it had the same principle of the AT3060?
I read in the faq that the only phantom powered tube mikes were the MSH-4, AT3060 and a really expensive one... How updated is that? Hasn't another brand made another one yet?

Well, now it comes to my mind an existencial question...
What's the difference between a Tube mike with it's own power supply, and a common mike with a Tube Pre-Amp?
 
dementedchord said:
anybody got a link to a data sheet for that tube???? understand that it's designed for lower voltage but not sure that disqualifies it as starved... hard to get those electrons boilling off at 20V it seems to me... and does it use a heater???
Click to expand...

No, it's really easy to get electrons to boil off at 20V!

But first, a few points: first, it isn't voltage that gets electrons to boil off, it's heat, or in electrical terms, watts. Thus a 12A_7 tube can have the heaters run in series or parallel; the voltage changes, the current changes, but the watts remain the same.

Also we are talking about electrons boiling off the cathode, which warmed by the heater (in 6418 though, the heater and cathode are not separate parts, which I imagine increases the efficiency of the tube at the cost of some performance parameter). They flow towards the plate because the plate is positively charged!

Yes, electrons actually flow the opposite direction that some might conceive. If you have a positive DC voltage, it attracts electrons, it doesn't emit them (of course, AC flows both ways). This is also why for a FET, the "output" is called the source and the "input" is called the drain. That sounds backwards relative to where voltage is applied, but it's not from to point of view of the flow of electrons.

OK then, since we know that electrons flow from cathode to plate, we know that in any tube, you just have to get the cathode hot enough for electrons to jump to the plate. That is a function of temperature, as well as distance from cathode to plate, and plate charge, and probably some things I don't understand like materials.

"Starved" is not a term found in any datasheet I've ever read. Loosely, it is used to describe a tube operating below its specifications. Thus, 12AX7, for example, would be starved plate at less than 100V plate, or starved filament at less than 1.9W. You will see some people say tubes need the maximum rated voltages to not be starved (30V for 6418, and 300V for 12AX7), but I see no basis for that in the datasheets, nor in my admittedly limited experience designing tube circuits.

As for the AT3060 and the MSH-4, actually although they use the same tube, the design is somewhat different. AT uses an output transformer, and I used another FET to buffer the output. 6418 can actually only supply a pitiful amount of plate current, so something is required. AT also set the 3060 to run in a much more linear range of the 6418 than I used. That is partially because I wanted the harmonic distortion, but also because a byproduct of a more linear operating point is much more gain from the tube. That's exactly what you would want feeding an output transformer, but I was feeding a FET which would simply overload much earlier with a hotter output from the tube. And before it overloaded, the mic would have had a sensitivity of around -30dBV/Pa or more, which I felt was excessive.

I don't have any MSH-4s left, even for a picture.

Why use a tube in the mic and not the preamp? Well, condenser mics need a high-impedance buffer, either FET or tube. If you decide the tube is OK in the preamp, then you'll need a FET as the first stage in the mic. Lots of people feel that first-stage buffer is critical to the sound of the mic, due to the very high impedance at that point of the circuit, and its interaction with the tube. It would be somewhat analogous to a tube amp for guitar; running straight in vs. using a Tube Screamer in front of it.

Also, the tubes used in mics and preamps are often different. Preamps seem to use triodes quite a lot; you see those in mics sometimes, but more with inexpensive models. Many tube mics use pentodes instead. You'd have to ask someone more knowledgeable about tube design why that is; I used 6418 (a pentode, but quite different than the usual externally-powered mic tube) because it's the only tube I know of that can be phantom-powered and is readily available. It has a similar twin, 6419, but I haven't found any of those. Another tube that may be interesting is 6088; that could be run off of phantom power for the plate + an AA battery for the filament.

Hmm, 6418 datasheet has gotten hard to find for some reason . . . I'll host it on my site later tonight.

http://frank.pocnet.net/sheets/138/6/6088.pdf
 
mshilarious said:
Also, the tubes used in mics and preamps are often different. Preamps seem to use triodes quite a lot; you see those in mics sometimes, but more with inexpensive models. Many tube mics use pentodes instead.
Click to expand...

Many tube mics use pentode tubes, but most higher quality mics (U47, U67, C800G etc.) wire the pentode tubes in triode mode. That is, they try to get the tube to behave as a triode.
 
PhilGood said:
Many tube mics use pentode tubes, but most higher quality mics (U47, U67, C800G etc.) wire the pentode tubes in triode mode. That is, they try to get the tube to behave as a triode.
Click to expand...

Yeah that is right. I did that too, I just wasn't thinking about it! Although one of the grids in 6418 is internally wired to the filament, so I suppose that means it can't exactly work like a triode, a tetrode then I guess. Really, tube physics are a little beyond me.
 
Hey, nice discussion! I'm no tube expert either, so I'm kinda shooting from the hip here. I do know that Pentodes wired in triode mode are more linear than a triode tube, but in wiring G2 and G3 to the plate, you lose half the power. No problem for a tube mic application, but why not use them in a preamp? Is it that a single ended Class A circuit with a Pentode in triode mode can't pull the bigger swing that a preamp output requires? Another reason might be that, while pretty much everyone agrees that pentodes in triode mode sound sweeter in mids and highs, a lot of folks find the bass response of a true triode to be better. This may be more af a factor in a preamp circuit than in a mic.
 
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