pasarski
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No, it's a DI-box. But you could study it for inspiration. You need higher input impendance and also a polarization voltage for the capsule.
I have a question about these micro tube mic ?
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Help Support GroupDIY Audio Forum:
Try building it and report back here?
I'm trying to find a way with TinkerCAD where we can simulate everything. Let's see if we can make 1 out of the 3 switching circuits that runs with large diaphragm capsules. I found this tube and you can also buy it 1AD4 Uf 1.25 Volt / If 0.1 amps
pasarski
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0.1A is 100mA. There's no way you can power that tube with regular phantom power.
oh yes, I see now that there are only these
https://www.laboratoriomusicale.net/en/6088-raytheon-jan-nos-power-pentode-subminiature-tube
https://www.laboratoriomusicale.net/en/6088-raytheon-jan-nos-power-pentode-subminiature-tube
Wavelength
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Gang,
The 6088,6418 and other tubes like this are directly heated. I use these all the time and have like 2K of the 6418 here. The 6088 draws 20ma at 1.25V and the smaller 6418 draws 10ma on the filament. BUT here is where it becomes tricky. Unlike an indirectly heated cathode, a directly heated bias point is in the center and it also needs to float bias and for these tubes best to use a current source for that. I use a LM317L with a 121 ohm resistor into pin 5 with a capacitor to ground and bias off pin 3 with a pot. The big problem with these tubes is that they are microphonic if mounted wrong. Put them on a 1/2" of foam. Digikey sells 1/2" foam double tape that works great.
So in triode mode the max gain I have ever gotten is 8. In pentode mode you can get up to 30. The Rp is really high either way so I use typically a 0.33ma current source then use an LSK189 as a source follower to drive the signal.
Here's is one of my clean boost pedals using the 6418 or the 6088. Both do triode strapping of pins 1 & 2 together. These tubes get a little funky in pentode mode. I am working on a 500 series DI and tried to use pentode mode and it's not being consistent across a lot of tubes.
The 6088,6418 and other tubes like this are directly heated. I use these all the time and have like 2K of the 6418 here. The 6088 draws 20ma at 1.25V and the smaller 6418 draws 10ma on the filament. BUT here is where it becomes tricky. Unlike an indirectly heated cathode, a directly heated bias point is in the center and it also needs to float bias and for these tubes best to use a current source for that. I use a LM317L with a 121 ohm resistor into pin 5 with a capacitor to ground and bias off pin 3 with a pot. The big problem with these tubes is that they are microphonic if mounted wrong. Put them on a 1/2" of foam. Digikey sells 1/2" foam double tape that works great.
So in triode mode the max gain I have ever gotten is 8. In pentode mode you can get up to 30. The Rp is really high either way so I use typically a 0.33ma current source then use an LSK189 as a source follower to drive the signal.
Here's is one of my clean boost pedals using the 6418 or the 6088. Both do triode strapping of pins 1 & 2 together. These tubes get a little funky in pentode mode. I am working on a 500 series DI and tried to use pentode mode and it's not being consistent across a lot of tubes.
What sort of life time do you see on these lowpower subminis?
Wavelength
Well-known member
This depends on a number of things. If you have a good set current source for the filament at the set amount, you don't draw much grid current, which requires good biasing, lowish grid resistor to ground (under 500K best), reasonable plate voltage then they last a real long time. I bet my pedal has been powered up for over a year 24/7 (8760 hours a year). I don't turn off my pedal board often. Most of these tubes made later were good to 5000 or more hours.
Thanks WL, this makes sense - but dosent the lowish grid resistor requirement kinda defeat this circuits reason for existing?
/Jakob E.
/Jakob E.
..and with decent lifetime?
Reason for asking is that many of these hearing aid tubes were made for 200-500 hours expected service iirc..
Reason for asking is that many of these hearing aid tubes were made for 200-500 hours expected service iirc..
No idea about this.
It's not impossible to do a 3-pin tube mic. Though not optimal, it would work. But that does still require a power supply for the heater voltage. Even using a weird tube with a 50V heater (they did exist), you're looking at about 19mA for the same 0.95W as a typical 12A_7 triode, and that's not going to make the phantom supply very happy. AND - Unless there is additional circuitry in the power supply to 'fix' this, the overall signal output from the mic would be unbalanced.
1 - Gnd
2 - (+ signal) + (tube B)
3 - heater voltage
I built a mic that uses phantom power on the triodes of a 6021, and it sounds great (at least I think so), so yeah, tubes will work with a 48V B+. (my mic does use a 7-pin connector)
ETA:
Looking at the pictures of the mic in question, I'm wondering how they get the tube to work while having zero wires connected to the tube socket.
1 - Gnd
2 - (+ signal) + (tube B)
3 - heater voltage
I built a mic that uses phantom power on the triodes of a 6021, and it sounds great (at least I think so), so yeah, tubes will work with a 48V B+. (my mic does use a 7-pin connector)
ETA:
Looking at the pictures of the mic in question, I'm wondering how they get the tube to work while having zero wires connected to the tube socket.
Last edited:
Wavelength
Well-known member
Both the 6088 and the smaller 6418 were audio tubes used for headphone output.
I kind of agree, the 3 pin example is not powering the tube, it's just there for looks and advertising. If you want to do a true tube unit there are a number of DIY sites that have kits. Or take a design and run with it. Do the same thing I was told when I started. "It's not rocket science so it can't be that hard." Then Chuck passed me a bunch of DIY magazines and that was it.
Who says they do?
The difficulty in driving an electronic tube microphone from a standard 3-pin XLR interface lies not in voltage, but in current. The phantom power supply of most microphone amplifiers only provides a current of about a few mA, which will never exceed 20mA, while the filament current of common electronic tubes exceeds 500mA! In contrast, the high voltage required for the anode seems insignificant. Perhaps some modified power supply systems? But more current means greater heat generation, and the voltage doubling circuit is also quite dangerous. I'm not sure if it's worth it......Isn't it possible to build a tube mic with only 3 pin XLR? with one
pasarski
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I've tried to point this out multiple times in this thread and the op is now looking for a hearing aid tube with low enough filament current.
Also, @kingkorg has succeeded in the task. I would appreciate some details btw, like is the tube underheated or something because the current draw still exceeds P48 specs?
Usually, the heat generation of an electronic tube mainly comes from the energy released by its filament (like an incandescent light bulb). Compared to this, the heat generated by the anode voltage is still negligible. Therefore, if a voltage conversion component is not placed inside the microphone, a lower filament power means lower heat generation. Considering that many commercial products nowadays excel in placing electronic tubes such as ECC88 in such small microphone housings, I believe this will not be a problem
pasarski
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I think we are talking about powering the whole tube with P48 and I believe that's what done with Kinkgorgs mic.
