Real McTube: Increasing Headroom/Output?

[MikeH]

I built a Real McTube (http://www.tonepad.com/getFile.asp?id=43), which I like the sound of a lot.  With a 12AU7 I think it would make a good tube DI for recording, but it breaks up a little too early.  I was thinking about raising the plate voltage, until I notice the resistor network/volume control.  The 100k\500k pot make up about 83k.  That in series with R8 which is 1M, make that whole thing behave like a 1.83M pot that can only be turned up about 1/10 of the way (although, in logarithmic terms, it's probably much higher than 1/10 of the output) Right?. 

So I figured I'd start by dropping R8, problem is: how much output can I give this thing safely before it has any ill effects on the inputs of my recording interface?  If I dropped R8 to 500K or so, it would double the ratio to 1/5 (but again not necessarily the output volume, due to the logarithmic nature of volume and the human ear).  Too much? Too little?  What say you?

[MikeH]

I dropped R8 to 330k for now, seems more in the range I'm looking for.  I don't think the 12AU7 has enough gain though, seems to work best with a 12AX7.

[iccaros]

if you want to add gain, reduce the plate resistors (I think you know this, as it increases voltage on the plates).. Those are really high, say half them..  the cathode resistor will adjust headroom, to keep it from breaking up early, you would have to do the math on the load line, but if this was setup for a 12ax7 its load line is different. I put 50K pots at the cathode and 1meg on the anode and turn until you like it, measure and go with the closest. If you want to swap tubes, then put a switch on the cathode to swap the resistor between what you like on each tube. I keep the 50K in the mix as some makes of tube need adjustment to sound their best.

[tubelectron]

MikeH,

Do not loose much your time with the Real Mc Tube : the design is already near to its max possibilities...



You don't add gain by doing that, you increase the plate output, because of the higher plate voltage. As the gain is G=SR (simplified view, supposing that Ri is very high, which is nearly true for a pentode, not for a triode), if you lower R, then G decreases... (G=gain, S=transconductance, R=plate load).

A+!

[iccaros]

sorry, your correct, reducing the cathode resistor should increasing gain, but may effect headroom.
sorry..

[MikeH]

MikeH,

Do not loose much your time with the Real Mc Tube : the design is already near to its max possibilities...

I think as far as gain on the tubes, yeah (although I am still going to try bringing up the voltage on the plates a little) but there is a serious attenuation nework tacked on to the end of the thing, probably to make it more friendly as a stompbox.  But that 1M resistor is robbing a lot of the output.

[tubelectron]

but there is a serious attenuation nework tacked on to the end of the thing, probably to make it more friendly as a stompbox.  But that 1M resistor is robbing a lot of the output.

Ah, yes ! the final network 100K+1M is exactly intended for that. If you remove it, adjusting the output level will be touchy, but conversely you will have a huge booster, if it's what you're searching for, in fact...

A+!

[PRR]

> R8, problem is: how much output can I give this thing safely before it has any ill effects on the inputs of my recording interface?

Power supply is 150V. A good tube plate can swing peak voltage 20% of supply, 30V peak. This plan is not optimized for MAXimum plate output, because that would just mean more padding for a "safe" output. Perhaps 20V peak at the plate.

> a 1.83M pot

Dropped a zero. 1.083Meg.

> that can only be turned up about 1/10 of the way

83K/(1Meg+83K) = 0.077 or 1/13 or 1/10th close-enuff, yes.

So 20V at plate comes out 1.5V peak at your interface.

Or less, if RV2 is not maxed-up.

But futzing with the post-tube pad does NOT change the intrinsic overload, just the strength of the overloaded signal.

> with a 12AU7 ... it breaks up a little too early

What actually happens?

Assume 0.1V-0.4V guitar.

12AU7 gain is about 15. So after V1 we have 1.5V-4V. Since V1 can swing 20V, V1 is _NOT_ going to break-up except with heroic string-banging. May as well not have it. That leaves only V2 for breakup. And a one-tube distortion is not as rich as two tubes _rigged_ to break-up at similar playiing level.

12AX7 gain is about 50. So after V1 we have 5V-20V. Since V1 can swing 20V, V1 will strain and break-up a bit on the hardest plucks. Now it is doing something. Looking ahead, we want to bring V2 plate to 20V peaks so it will break-up. V2 gain is near 50, so we need 20/50= 0.4V at its grid. V1 is giving 5V-20V. We need to _drop_ level between V1 and V2 by the ratio of 20/0.4 or about 50:1. That is what RV1 is for. With an "Audio taper" pot we have to turn to "1" (on 0-10 dial) to get 50:1 loss. In fact we want V2 to overload first, 2X-4X sooner, so we anticipate 25:1 to 12:1 pot loss, "1" to "2" on an audio taper pot.

In Fred's words:

What sound you get from your guitar or other instrument via The Real McTube will depend greatly on the settings of the various controls; specifically, the volume and tone controls on your axe, and the GAIN control. The OUTPUT control has little or no effect on the sound, and is used primarily to balance the volume of the sound with or without the effect engaged.
Note that both stages are inverting amplifiers; the overall phase of your signal will therefore not change. However, how you adjust your controls will determine what portion of your signal gets clipped. The first stage will tend to clip more on negative half-cycles as the tube reaches cutoff.... The second stage, however, will tend to clip more on positive half-cycles, since it sees an inverted signal at its input. By carefully balancing your instrument controls and GAIN, virtually any distortion sound from a mild, "warm fuzzy feeling" sound, to a hard "monster metal shred- head" sound, rife with odd-harmonic distortion, can be attained.

Use 12AX7.

Set RV2 quite high.

Set RV1 _WAY_LOW_, like "1". (Or copy the R8 R9 pad in from RV2 to RV1 so you can use most of the dial.)

[MikeH]

Thanks Paul - appreciate the info.

I guess as long as I'm never clipping the track in ProTools, I'll never be feeding it so much voltage as to do any damage.  I'll just have to make sure I don't do that, hahaha.  I'm going to salvage a couple of input/output transformers from an old mixer I have here and see how it sounds running a balanced mic signal through it.  Might be good for warming some things up here and there... I don't know, we'll see how it goes.  But I played around using it as a Bass DI the other night and it actually sounded pretty good.  Of course, it really shines when slamming it into a tube amp input, naturally.