pasarski
Well-known member
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- Jan 29, 2010
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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
Well-known member
- Joined
- Jan 29, 2010
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- 436
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
Well-known member
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.
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