Miniature Tube DI Idea

[Gus]

I have not seen a UM900 up close: However the Gefell UM70 head I have uses two pushed in plastic pins. I drilled then and tapped them for a 4/40 screw. After I pulled them it was screws to take the head apart.

I like the body and grill design of the 900 microphone. It looks like art and makes alot of sense to me.

 

[Guest]

I did a breadboard of the 6088 DI circuit and worked out some initial component values that give reasonable looking sinewave traces on my scope. Here is the revised schematic: http://users.adelphia.net/~thomasholley/Guitar/Mini Tube DI.gifWith these values, a one volt input gives a 10 volt output at the plate.

The next step for me is to take it to my friends studio so someone can listen to it.

 

[zebra50]

Good stuff - I'm thinking about trying one.

Let us know how it sounds.

What is the battery life like?

:thumb:

 

[Guest]

The heater draws about 20mA so it is not a hugh draw but it would drain small batteries fairly quickly I think. I got a D cell that is good for a couple of thousand mAH so it ought to last a while.

 

[Gus]

A LM2575HV adjustable should work well as a DC to DC converter.

 

[Gus]

When you get a chance could you hook it up like a triode? I am think phantom powered tube microphone.

 

[Guest]

Gus, I will try it as a triode as well. The cathode current is less than 900uA with a 1V input run as a pentode (really a direct heated tetrode). Triode may sound nice but it is pretty flea power. Pentode only has a gain of 10 (20dB). Triode will be small but I guess if it is clean, it may work OK. Worth a try at least.

I think a DC-DC converter would place too big a draw on the phantom supply. A 20mA heater current drawn through the 6.81K phantom resistors would drop the voltage to zip. I am inexperienced with phantom power so if there is something I am not doing right please let me know.

 

[Gus]

thomas

lets say the DI "pulls" 4ma.

4ma thru 3.41K is close to a 14volt drop

48V - 14V =34V at pins 2 and 3

34V at 4ma is about .136 watts this is the watts at a 4ma pull with phantom power

now we know the heater is 20ma (.020A) X 1.25 volts is .025watts

.025watts is less than .136watts

so even with 50% eff DC to DC converter you should have enought watts

you can kind of think of a Dc to DC converter as an transformer the energy in equals the energy out with some loss.

 

[Guest]

Gus,
OK, I am trying to grasp DC-DC converters. As I understand your explanation, the converter takes the phantom voltage (after associated drops) and converts it to lower voltage/higher current for heater so that V*I (phantom) = V*I (heater) + some minimal loss.

Now, if the cathode current is say 1ma, the voltage drop is 1E-3*3.41k=3.41v. This gives energy consumption of (1ma)^2*3.41=45mW. The heater draws 25mW as you previously stated. Can I extrapolate that the total energy consumpyion is going to be around 70mW? If so then I can estimate the total draw on the phantom power supply to be:

W=I^2*R so I=(70mW/3.41k)^1/2=4.5mA.

This will give a voltage of about 48-(3.41k*4.5mA)=32.6v for B+.

Is this sort of how it works?

 

[CJ]

6092 is in the same ballpark, more heater, 50 ma, less Rp but twice the pwr out.
still looking...
5854 (hearing aid! when I get old, Is a gonna diy a hearing aid! a tube hearing aid of course!)
5676
5875
5971
6050
6051
6121
looks like you found the lowest fil. draw.
cj

 

[Guest]

CJ,
I looked at most of these and was scared off by current draw taking down the phantom power voltage too low. I didn't look at 6092 which would be a reasonable candidate I think. Also 5971 may be good for a real triode.

I lost a good deal of my hearing due to noise (mostly big guns on Navy ships). My hearing aids help some but are a pain in a lot of ways, plus one just quit and I have to try to get the VA to repair it (what a joy). I should try to DIY a set with these tubes. Anyway, my hearing loss is why I need to go to my friend's studio to get someone to listen to the circuit to see if it sounds OK. I can't tell that from my test gear.

 

[Brent Casey]

Keep in mind that the 6088 has a directly heated cathode, so whatever you use for filament power must be Very Quiet.
Maxim has DC-DC chips that will output 1.25V at up to 100mA. They make SOIC's look like Wyoming though. You'll need some small traces.

Brent Casey
PMI Audio Group
877-563-6335

 

[Flatpicker]

Hiya Brent! Good to see you here - Stay longer and post more! :green:

 

[Guest]

I don't think I can do anything smaller than a dip package. My idea of miniaturization is the 6088 it's self.

Are DC-DC converters noisey? I don't want to use a lot of other parts to tame noise as well as loose voltage.

 

[Guest]

OK, I got my friend to listen to the breadboard circuit and he gave it his stamp of approval. He listened to both tetrode hookup and triode hookup. He said triode had a bit better bass character and all highs were still there. We played guitar through it and listened to several CD's through it, both listening through speakers and watching the signal with a scope.

With tetrode and the LL1538 strapped as 5:1 I still have a gain of 2 (1v in and 2v out) which is OK for a direct box I guess. With the 6088 in triode mode I strap the LL1538 as 2.5:1 and the gain is about 1.7 or so.

I am really pleased with the outcome of this little experiment. Bassed on the sound form my breadboarded circuit, I think the 6088 would be a decent candidate for a phantom powered tube mike as well.

I still have a question about noise with DC-DC converters. If they are decent I think they could be used with success here instesd of a battery for heaters. If they are noise generators I can't see using them, but as I said before I only know what a few of you told me about them.

 

[Gus]

thomas


I think Brent has a very good point about the tube being direct heated. I guess one would have to build a circuit and find out if it causes to much noise. I was looking at the NS LM2527HV adjustable datasheet it loooks like it has a good input voltage range and goes down to 1.25 V.

I have not used a 6088 I have a question, can you just use a battery as the Heater and cathode bias? What I mean connect one side of the heater to ground and the - side of the battery to ground. Connect the + to the otherside of the heater. That would reduce some parts. I would think the tube was desided for low parts count circuits

Some HIFI people use batterys as a type of tube bias I think of it like a fixed cathode bias.

 

[Guest]

Gus, the 6088 data sheet actually has a + and - heater connection. I hooked the + lead to the battery and grounded the - lead initially. This works fine and actually gives the grid a good bias point.

One note about triode connecting the 6088. It seems to work best if the screen grid is tied directly to the plate.

 

[Brent Casey]

[quote author="thomasholley"]I don't think I can do anything smaller than a dip package. My idea of miniaturization is the 6088 it's self.

Are DC-DC converters noisey? I don't want to use a lot of other parts to tame noise as well as loose voltage.[/quote]


It stands to reason that whatever is used to power a directly heated tube in an audio circuit better be really quiet, since the noise is coupled straight into the cathode.
If a DC-DC converter is used, it should have clean power to run its circuitry, good filtering on the output, proper grounding (star), and possibly some shielding from the audio circuitry. If these conditions are met, than I think a switching converter will produce good results.
Or you can use a battery and avoid all the above hassle except for dealing with a power source that is constantly running down.

Brent Casey
PMI Audio Group
877-563-6335

Gus: Check your PM.

 

[PRR]

> There are some low voltage tubes around

ALL tubes will run at low plate voltage. Normal "100V-300V" tubes "kink" around 50V and work less-well at low voltage. Some low voltage tubes optimize away a little of this kink, but you can't cheat Mother Nature too far. Many of the "low-voltage" tubes work little or no better than a high voltage tube fed low voltage.

So if heater power is a non-issue, don't limit your choices. The TV Tuner tubes work quite well, for tubes, down to 24V or 12V. (But they all draw about 2 watts of heater power, far more than you can get from Phantom, and only a couple hours on a stack of D-cells.)

If you want to heat the cathode with Phantom, your only choice is battery filament tubes. Probably not the Beach Radio tubes, because these all draw 50mA at 1.4V. Better than wall-plug radio tubes, but still fairly greedy. (Big secret: battery companies promoted and designed battery radios, so there was not much urge to really reduce battery consumption.) Some other mini-tubes were made for Bomb Fuses, and battery size is not an issue in a 500-pound bomb. The Hearing Aid (and Secret Bug) tubes are your best bets.

> 40some volt to 1.2 volt DC-DC, assuming 50% efficiency (typical for a low voltage output - the rectifier losses

Just to blow your mind: there is no NEED to run these filaments on DC. They work fine on AC 50Hz and up, except you get a helofalot of hum. So use a 100KHz oscillator running on the 30 volts, wind a few turns on the core, and feed the filament from that. Stick a cap on the audio output and use a transformer, 100KHz leakage is manageable.

Small detail: many of these battery-filament tubes used DC filament drop as part of the grid-cathode bias, so you may have to add bias. With a 4-tube radio and a common filament battery, this was a pain. With a single tube and a floating filament winding, just center-tap the winding and return to ground via a few-K of resistor.

 

[Guest]

I got a sample of a MAX5033 DC to DC converter rated at 500mA with adjustable voltage as low as 1.25. I experimented with this in every way I could come up with but if I get more than 10mA out of it the output voltage has sagged to nothing. Input is nice and steady with a very low power consumption. I even set the output for 5 or 10 VDC and it drops to ziltch with 9-12mA. I need 20mA for the 6088 heater.

Does anyone know of another alternative that could work better here? The battery is still my best bet so far.