Miniature Tube DI Idea

Guest · Jul 21, 2004

With recent discussions of Di boxes I thought I may try to come up with a tube DI that would run off of phantom power.

There are some low voltage tubes around so I did a quick search and decided to look at the 6088 miniature pentode. It is both cheap and available. It has a maximum voltage of 67.5V and normally runs at 45V. It has low (less than 1mA) current draw and a low voltage (1.25VDC) heater that could be run from a battery. Here is my draft circuit.
http://users.adelphia.net/~thomasholley/Guitar/Mini Tube DI.gif

It may sound like pure crap but I am going to buy one to test out. Does any one know of some other candidates to test?

Sleeper · Jul 21, 2004

you might want to check out the 5048 tube used in the royer mic mod,
http://www.prosoundweb.com/recording/tapeop/tube_mic_25_3.shtml
you might get some ideas from this circuit
even though it runs on 105 volts, maybe you could add a little voltage doubler in there
sleeper

moosapotamus · Jul 21, 2004

Old car radios used to use tubes tha ran on 12V, and I think there might be some similar 24V versions, too. Someone has done a bit of work making amps with them.
http://www.diystompboxes.com/sboxindex.php?topic=22194&highlight=

... interesting.

Might work well in a phantom powered tube DI circuit, too.

~ Charlie

Guest · Jul 21, 2004

Thanks for the suggestions. The "problem" with most of the automobile radio tubes is their current draw. Phantom power is usually a fairly wimpy supply as far as it's ability to deliver much current because mikes don't require it.

Heater supply is another consideration. It needs to be low power so it can use a battery in the box or be derrived from the phantom power supply without over taxing it. That is why I looked first at the miniature types. Car batteries have lots of current resurves and arent affected much by the radio tube draw.

I limited myself also to tubes I could get easily and CHEAP.

moosapotamus · Jul 21, 2004

Good points. The 6088 does seem like a natural candidate for 48V phantom powering. I'll be interested to know how it works out.

Thanks
~ Charlie

Gus · Jul 21, 2004

It is my understanding the AT 3040? phantom power tube microphone and the Microtech gefell um900 use hearing aid tubes. I would guess they use a dc to dc converter for the heaters and plate voltage. Think in terms of watts. for DC Voltage X current =watts. Find the max watts you ca get out of a phantom circuit and still have a usefull voltage at the DI. I would look at NS and TI DC to Dc converter chip app notes.

dale116dot7 · Jul 21, 2004

Yea, you could use a 40some volt to 1.2 volt DC-DC, assuming 50% efficiency (typical for a low voltage output - the rectifier losses are 35% right away unless you use synchronous rectification), some tubes with a 1.25V filament also take 10mA to light - so 12.5 mW. That's around 3/4 of a mA at 35 volts.

Guest · Jul 21, 2004

I didn't think about DC-DC converters. I hav not used them before. That may be a good way to get heater voltage. I'll look up the app notes.

I will order a couple of 6088 tubes to try. Should have a breadboard circuit next week.

Any other tubes to try?

kevinc · Jul 22, 2004

There's another tube to consider called the 6GM8/ECC86. It runs off of a 24v plate supply and 6 volt filament. It's a 9 pin miniature, so not so small as a subminiature.

sismofyt · Jul 22, 2004

But do they sound good those tubes? Phantompowered tube mics is a neat idea, but will it sound nice?

The miniature tubes used in the Neumann M147/149 are pretty crappy.

sismofyt · Jul 22, 2004

A 6DJ8/ECC88 will run on 48V, but it needs a regular heater supply.

Guest · Jul 22, 2004

I know it has beed discussed before, but I couldn't find it. What is the typical current capability of a phantom power supply? I thought it was in the neighborhood of a few mA, 5mA-10mA, but that is a SWAG.

If the current capacity is as low is my guess, it severely restricts available candidates for this expiriment. I think the 6GM8/ECC86 or some of the car radio tubes would be better to use, if the phantom supply can accomodate them.

So, anyone have the general specs?

kevinc · Jul 22, 2004

Phantom power is typically supplied on Pins 2 and 3 through 6K81 resistors. A 10mA current will drop the supply voltage from 48v nominal to nothing. Up to 4mA is typical current draw.

Gus · Jul 22, 2004

I don't think the tubes are crappy in the m147/149 I think they are used in a "not good design" Oliver had a hand drawn front end of the m147 at Klaus's forum.

There is also two pentodes 1e5? 1 something 4, one has a diode inside I just can't get an image head right now of the numbers.

sismofyt · Jul 22, 2004

Didn't the original tube, 6111(?) crap out in the M147 after a short time? I did send two back some years ago for a studio. I think those are marketing tubes. Doesn't the M149 only use the tube in one of it's patterns and opamps for the rest?

I've owned the M147 and while it's a nice mic and I'd luv to get rid of the 'tube' and all the other complicated parts and PSU. Whole lotta components for nothing, imo.

I think it's going to be difficult to get something nice sounding out of a 1/4 W Phantom PSU with a tube.

Guest · Jul 22, 2004

Thanks for the information on phantom power capabilities. It looks like the 6088 is still the only tube I can find with parameters that put it in the ball park for this experiment. It draws less than 1mA plate current.

It may be difficult (impossible) to get a decent sounding flee power DI running on phantom power. Just looking for acceptable candidates has severely restricted the field to one so far. It will still be fun to find out though.

I ordered two 6088's to try out. I guess I will have to use a D cell battery for the heater supply as derivation from the phantom supply for 20mA would be too much of a strain.

Gus · Jul 22, 2004

Go here http://recforums.prosoundweb.com/index.php/f/33/0 for the Oliver posts. Look for theunderheating posts and all Oliver posts. In one of the posts he tell what to do to raise the fil voltage in the tubes. neumann used the 6111 as an improvement over the first tube used if I understand Olivers posts. The 6111 does not like to be underheated.

Guest · Jul 22, 2004

Gus, I went to the link, looked at the underheating thread. It is interesting, but specs for the 6111 place it way out of the realm of possibility for a phantom power use. Heaters take 300mA, Ia is more than 8mA. Not a chance I can see for this one.

Gus · Jul 22, 2004

sorry I ment the post about the 6111 for sismofyt

jrmintz · Jul 24, 2004

Microtech gefell um900 use hearing aid tubes.

I have a UM900. I was told by the former importer that the tube was not a hearing aid tube, but something the former East German secret police designed for miniature eavesdropping devices. I don't know how to open the mic, but if I ever figure it out I'll take some pics of the tube. It's a great mic and I'm reluctant to monkey around with it. Gus, do you know how they open?

:shock:

I'm on vacation and I promised my wife I'd stay away from the internet, but I couldn't help myself. :green:

Miniature Tube DI Idea

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