real mctube 2 with line level ?

[DiggyFresh]

Hi,

I'm almost done building a Real Mctube II. My first tube project, simple enough.

I want to use it as a tube sound/distortion box "preamp" to record directly in my computer. I am not using an am

Is it possible to record the output directly in a soundcard?

I read the output is high impedance, will it work with line level input?  Or will it work through a D.I input ?(for guitar etc).

Also the input of the Mctube, can it be driven by low impedance line level?

Basicaly, if i could use it as some kind of "insert efx" in my recording system would be cool.   

Thanks

[GibsonGM]

Hey Diggy,

Neat idea, and you COULD do it if you took into account the factors you just brought up.  I don't know tons about McTube other than having built and messed around with one.   Nor am I any expert on soundcards.

I can tell you, tho, that you do need to account for the output level vs. what your soundcard wants to see.   You could do this with a simple resistive voltage divider.  Not at least gaining some insight into what it outputs could fry your card pretty easily!  They tend to want very low level input....might have to 'do some stuff' to make it sound good, too, due to mismatches (think "sansamp" here).     There is more that needs to happen here than meets the eye, as it probably sounds kinda lame on its own with no speaker sim (same with any overdrive/distortion).

You CAN try driving it with a low impedance source, as a guitar is such a thing.  Maybe if you have specs for the soundcard, that would make it easier for a REAL pro to advise you?  I'm sure someone on here has DONE just this with the same kind of effect....I'd tread lightly/slowly, myself, but I WOULD try it, ha ha.     Just make sure it a VERY low output to start with, and be very careful not to get excited and crank it up into your computer!    Think before you act...

[DiggyFresh]

Thanks for the reply GibsonGm!

The soundcard is a Digi002r with some some inputs at 2k ohm and some at 10 megaohm.   The outputs are 50ohm.

The real mctube has a resistor attenuator at the output (10:1) and also the output volume pot (i used 1 meg).
So i'm guessing i can attenuate already enough.

I'm guessing it would probably work maybe just the frequency response will suffer? I read the mctube has a very high input impedance and an output impedance of around 68k (not sure). (I think its a common cathode??)


Maybe a tube pro can shed some light.

[GibsonGM]

Your Z values etc. on McTube seem pretty close, after a quick look.  I would personally try turning the output way way down, and see what I get when connecting to a 10M input.   I don't see the value in using a 2k input, unless you like your guitar tone turned down sound, he he.    If possible, try to find what combination of resistors would equal your 'maximum input' dialed on the pot and hard-wire them to an input jack as a divider for this, so you can never exceed that value and destroy the card.   

Granted, I am being WAY overconservative...the divider on the McT's output gives you Vin (.09),  so it's whatever gain we achieved on the signal * .09, I'm thinking not a very large voltage.  The 200VDC before the 1M resistor is cap-blocked, of course. 

Remember, it COULD sound better and better as you turn up and ...  oops, fried the card!   You have to know when to stop, preferably by actually measuring the McT output vs. what the card can take for an input.  I like nothing more than empirical evidence over theory just in case I got it wrong (ha ha).   You should be able to look that up...somewhere I THINK I heard sound cards really don't like more than 5V, but don't take that as anything factual.   I take no responsibility if you zap it, ha ha ha!   Why not fire it up and just see what AC (and DC, just in case) you get when playing hard?    I'd like to know as well.

Your thinking on this seems to be correct, Diggy.     If it sounds dark and odd, see what a cab sim following it sounds like.  Lots of people like the Sansamp.   It helps reproduce the sounds you only get from a speaker, gets you away from the "Strange Brew" overdriven console sound

What you're REALLY sort of re-creating here is a tube preamp to warm up your sound, right?  Others have done it with good results.

[DiggyFresh]

Yes exactly, what i would like idealy is just a "tube sound"/distortion preamp.  Would be realy great if i could also patch in a line signal from the computer, like a drum mix or wtv. I'm not realy building this for guitar but on occasion, also bass, rhodes piano etc, mainly for recording.

Good idea on the fixed divider at the output. I don't have a scope or a way to measure ac right now unfortunately.

That made me think what if i add a zener or a bunch of diodes to clamp the ac signal voltage below the max my line input can take, distortion stompbox style, would probably sound super nasty but at least my line input wouldn't be harmed. ( and it's another sound).

I'm also curious about the input tho,  is it possible to drive the mctube with a line level signal? Pro level or consumer?

Anyone did it?

[GibsonGM]

Diodes WOULD work that way; old HAM radio guys used that trick with a bias voltage (can use 4148 etc, as long as the bias voltage is there) to set the clipping threshold...maybe why we USE diodes for clipping in guitar!   Lots of work for this, tho, and I wouldn't bother.

In my best estimation, line level will be higher than a guitar's output, so I wouldn't be afraid to try that aspect of this:  Line in >> McTube > Amp

Sorta like running your CD player into your guitar amp...you can do that    Turn the output AND the McTube down first, ha ha.   Pro vs. consumer line level isn't a big difference, just a few dB I believe.

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You don't have a DMM that can read AC for you?    I think the vol. pot into sound card (10M input) will be ok for you if you go easy, as the output of this IS meant to input to an amp, which doesn't expect to see 20VAC or anything.   

**********EDIT: Ok, I simmed this for you in LT Spice, and rough and ready I'm seeing you as having no more than 5V AC on the output.  So you should be safe using the vol. control from zero and up a bit, into the 10M input.     For extra surety, make a 1/2 voltage divider to run it thru first to limit to no more than ~3 v.  IF your sound card can take a 3 to 5V input.....max out for McTube looks like maybe 6VAC positive, and -2 or so negative (very rough estimate).   Keep your levels down and see what you get....

[DiggyFresh]

Thanks alot man for this, very helpfull.

But i didn't get whatyou are saying about the diodes. With the right biasing?   What i meant was just lets say two back to back 3v or 5 v zener from output to ground to 'clamp" the voltage below That ( or like four 1n4148 in series). Kinda like alot of distortion are doing.

I'm almost done, should be able to test soon.

[GibsonGM]

You CAN do that with diodes, but it may not work like you want.  However, you're right that if done properly it WILL send anything over the Vf to ground and not let a higher voltage into your equipment.  Your way will clamp and maybe pop (noise), but will do the job.

You can 'back bias' diodes - think of having opposed diodes on the output of something, like a Dist +.    If you don't connect their low end to ground, but instead to a VOLTAGE (usually a battery), you can change the threshold they'll start conducting at!   If you do it thru a pot, you can adjust the threshold on the fly!    They used to do this in radio to keep spikes from killing you when wearing headphones.   It's called a diode limiter.  Somewhere I have a schem for it.

The way you described doing it is more of a "clamper", it's all or nothing.  But certainly would be effective and would protect your equipment.

[GibsonGM]

Here's what I was talking about.  A limiter using diodes that are back-biased to adjust when they 'turn on' and clip.  Meant for use with headphone outputs from receivers, from ham radio.   It's implied that the following stage or whatever would be high impedance.   Maybe something to play with; I haven't messed with it, but it might have some potential in overdrives or what have you.   The drawback is providing the 1.5V batteries etc.     Diodes are 1N34A - might be ok with 914's, I don't know.