Subminiature Tubes

I would really like to see some sort of mic preamp, eq, compressor, or something cool using submini tubes! Anyone got any ideas?

Aaron

Have a look here:

http://amps.zugster.net/projects/sub-mini/

I did a project with mini tube for a direct box. The thread is here :http://www.groupdiy.com/index.php?topic=1195&highlight=direct+box

I've seen that website Marik, a couple of the projects look interesting.

I also saw your post thomasholley, how do es the DI sound?

It just seems you could ge some cool flavors in an eq or something with these.

Aaron

I've also been looking into miniatures... I got a hold of some Hungarian mics with subpar ac701's and really want to get them in good shape.

So far I've tried 5703's, which sound very good but the couple I've tried have been too noisy, but i'll keep going.

I dont want to convert the to FET but that might be my only hope...

Any ideas?

:guinness:

Michael

[quote author="mfilter"]I've also been looking into miniatures... I got a hold of some Hungarian mics with subpar ac701's and really want to get them in good shape.

So far I've tried 5703's, which sound very good but the couple I've tried have been too noisy, but i'll keep going.

I dont want to convert the to FET but that might be my only hope...

Any ideas?

:guinness:

Michael[/quote]

Hand selected AC701 is probably one the quietest tubes available. Try 5840 in triode connection. They are cheap, sound good, and you can find a pretty quiet one.

I've had good luck with the 5744, but with a bit of a feedback capacitor (maybe 15 to 25 pF). I like the 5840 in cathode-follower mode, though I find the sound in triode / plate loaded mode to be a bit fussy on the output loading. I've also used a 5703, I did find a quiet one, maybe one out of five or ten. I did like the sound of the 5703 better than any of the other tubes I tried in one of my mics. The 5840's have all been pretty quiet, but I usually use them in a follower configuration (check out the Altec M20 lipstick mic).

Try the 6205, you'll find it identical to the 5840. All of the ones I've run into are JAN USA Phillips. They're very nice, usually cheaper then the 5840's.

5718's are worth fiddling with too, close to the 701 tubes.

A stomp box guitar compressor w/sub mini's would be tres tres. Tubed ring modulator stomp box?

Thanks for the suggestions!

I'll try swapping out some more 5703's and see if I can get a quiet one, if not I'll move on.

One thing, I tried a 221b circuit (these were bad copies) and I notice in the schematic that there isn't a grid to cathode resistor like in all of the neumann mics... could this effect performance?

And sorry for hijacking your thread Aaron

Michael

Looks to me like '7' is the grid-to-cathode resistor, but there's a fixed bias applied to it via '4', '5', and '6' - actually a slight positive bias. That seems a bit strange, and that might be something to fiddle with?

Looks to me like '7' is the grid-to-cathode resistor

Click to expand...

There lies my confusion, it looks like "7" (250meg, which I thought was high to begin with for an ac701) is tied to the grounded end of the filament. Is there something obvious I'm missing :?

Michael

Yes, and I almost missed it too. Terminal 3 to the power supply is ground. Terminal 2 is filament voltage ... but you notice it also goes to the cathode. This gives the cathode a voltage of +4 volts. To set the bias of the tube, a voltage divider is used between the plate and the grid of the tube. A bit clever, actually. They want the grid to be somewhere around a volt or so below the cathode, so they set up the voltage divider thing to set the grid to that voltage - probably around 2 volts. If you look at the U47 schematic, a similar trick is done with that 29 ohm resistor in the bottom of the heater leg, but without the feedback arrangement. The thing here is that the heater power supply voltage is being used, but the basic principle is the same.

Hey, speaking of submini's, there was a (phono?) preamp using one of them 6021, I believe, on an issue of Glass Audio. If anyone wants to host it, I can dig it up, scan it up, and send it up your way.

Peace,
Al.

5899 Remote cutoff pentode. Am I just dreaming here?

Bear

The 6205 sharp cutoff pentode is used in the Grrove Tubes Vipre, which has variable input impedace from a custom Cinemag transformer:

http://www.barryrudolph.com/mix/vipre.html

-Jack

The 6205 and the 5840 are identical spec wise. The 5840 has grid 3 internally connected and the 6205 has it externally connected. The 5840 data sheet says it is similar to the 6AK5 electronicly.

http://www.mif.pg.gda.pl/homepages/frank/sheets/137/5/5840.pdf

http://www.mif.pg.gda.pl/homepages/frank/sheets/137/6/6205.pdf

The are many possibilities. The 6021, the 6111, and the 6112 to name a few. All dual triodes by the way. Neumann uses either the 6111 or the 6112 in the M147 or M149...I forget right now...

That phantom powered Audio Tecnica uses a 6814 or something.....

I got a box full of various types I plan on experimenting with.

Gerry

Klaus's forum has a sticky about ac701s. Also I have seen posts on the web that the heater voltage need to be 4.00V at the tube to high or low and the tube gets noisy.

The tubes cathode was desiged to run colder than a alot of tubes. It is kind of what happens with underheating some tubes in microphones.

Also do not bend the wire leads close to the body of wire lead subminis because you might break the seal.

From what I have read on the web the AC701 and the VF14 are the only tubes ever designed to be used in tube microphones.

good link on ac701s

http://recforums.prosoundweb.com/index.php/t/2927/0

Ok..now that we've got out ac701s under control and our vf14s in their right place... :green:

I still want to build something pro audio with sub-miniatures...maybe... :roll:

Aaron

> I still want to build something pro audio with sub-miniatures...maybe...

So do it.

But note that most of the "big" sub-minis are repackaged versions of tubes you can buy in standard 7-pin mini bottles. As Gerry says: "The 5840 data sheet says it is similar to the 6AK5". And the very-sub-mini tubes made for hearing aids are really too weak for "Pro" use above mike level.