Help deciphering Tube MP schematic

[PBE6]

I just got a second hand ART Tube MP Studio V3 preamp from a friend for about half price. Of course, I had to try and get a schematic for it too

ART was nice enough to send me this as part of the service manual:



It's a bit more dense than I was anticipating 😳

What I was wondering is, will the tube will have any real impact on the sound the way this circuit is laid out? It looks like it has to go through a few opamp stages before it gets to the tube, although there are a bunch of circuit structures I'm unfamiliar with in the path as well (a few current mirrors? I think?). But it appears the input gain stage happens after all that, so as long as it doesn't clip on the way in the distortion should come from driving the tube, right? I dunno, I know zilch about tubes.

Also wondering if the voltage (33V or 35V) is high enough to make the tube act tube-y.

Really asking just because a number of random internet trolls say the only thing the tube does is light up to let you know the unit is on. This could be entirely true, but I'd rather have someone here who knows something make the diagnosis.

Thanks!

[anchovie]

Is the tube in a socket? If so, take it out and jumper pin 2 to pin 8 with a piece of wire, and see if it sounds any different!

[PBE6]

Ok I will give it a try.

I won't kill myself by doing that, will I? Voltages are too low, right?

[Johan]

It will give you a slight tube compression before it goes into ugly clipping. . Keep it in any of the opl modes and a fast fet limiter catches the peaks before it clips
The other modes are basicly a different eq on the signal and in some modes defeating the feedback around the tube. For fast project/demo recordings it's a great little produc. Especially considering price
J

[anchovie]

Ok I will give it a try.

I won't kill myself by doing that, will I? Voltages are too low, right?

2 and 8 aren't carrying any high voltage.

[teemuk]

...jumper pin 2 to pin 8 with a piece of wire, and see if it sounds any different!

There's a directly coupled emitter follower in the cathode circuit. Short circuiting pins 2 and 8 will most likely mess up its bias entirely, which will undoubtedly lead into sub optimal  performance.

Yes, the tube is in use. No need for stupid test arrangments to verify it. Yes, it introduces "tube-yness" because it is a tube. Inherent issue is that "tube-yness" was never a consistent characteristic to begin with. "Tube-yness" of a single 12AX7 preamp stage is different to "tube-yness" of a complete push-pull tube power amp and tube-yness of a Valve Junior is different to tube-yness of a Diezel Einstein. Get the picture? Things with tubes inside don't sound the same. Tube circuits don't sound the same. Here you have the tube working like it would work in this type of circuit.

Yes, 12AX7 will work with 45VDC plate voltage, heck it even works with 9VDC. The operation, however, takes place in the area of loadlines, which is not represented in great accuracy by datasheets. But it works.

If we strip it down, the tube stage of this thing is actually pretty damn simple: Common cathode amp directly coupled to a cathode follower. The cathode follower is furthermore buffered by another follower, being solid-state now much more effective buffer. There is a global feedback loop around the entire stage. A good estimation is that the stage will clip hard an nasty, weren't there a FET limiter preceding the tube stage.

Also, do note that this is a mic preamp. There's not much point to treat the signal with silk gloves and run it through all that low-noise discrete stuff only to distort the hell out of it with the tube stage. Any impact to tone from that stage will be subtle. This is a mic preamp not a distortion effect. The limiter FET will probably have far greater effect on tone than the tube, which's main function is to amplify some but mostly lure customers with the euphonic glow.

But yes, the tube does some amplification in this circuit. Probably distorts too if pushed enough. Since all this is practically very, very subtle it's another story whether you actually needed a tube for that. But now it's a "tube preamp", right?

[Thecomedian]

Ok I will give it a try.

I won't kill myself by doing that, will I? Voltages are too low, right?

If you're ever in doubt, remember the rules of working with high voltage. Unplug, put one hand behind your back, and work on the other part.

I've heard that unplugging a computer while the power is on can drain and dangerous capacitor storage, not sure if that works for amplifiers as well. Can always make sure to short caps out by crossing a screwdriver over the metal tops of two or more of them to be absolutely sure. At least from what I've read.

[PBE6]

@teemuk thanks for the circuit analysis. So by the sounds of it, pushing the gain will indeed overdrive the tube, but it's very likely that any tube clipping will be hashy as opposed to nicely behaved with predominantly second harmonic content. I'll check it out with the scope and see what output spectrum looks like for normal operation, and also do a few tests with the overdrive up high.

[PRR]

This is a less elaborated ART Tube Channel, which I hacked on a lot.

The ART TC has an LED meter which purports to indicate tube action. Heavy slamming did cause clipping, not like opamp or fuzz clipping.

For HIGH-fidelity recordings, stock, it was very good but always had a softness, even when the tube-LEDs hardly ever flickered the bottom LED.

I did some hacks and it got a LOT crisper and cleaner. It also came-on in 1 second instead of 12 seconds... I had bypassed the tube!

I believe these products are excellent *clean* tools after you bypass the tube. The tube stage is well intentioned, but "dry". (However bypassing the tube stage is a lot of work and too complicated to describe through the interweb. You can't just jumper the tube.)

> do note that this is a mic preamp

FWIW, the MP seems to have a fine Instrument Input (marked "line input").

[GibsonGM]

Ok I will give it a try.

I won't kill myself by doing that, will I? Voltages are too low, right?

If you're ever in doubt, remember the rules of working with high voltage. Unplug, put one hand behind your back, and work on the other part.

I've heard that unplugging a computer while the power is on can drain and dangerous capacitor storage, not sure if that works for amplifiers as well. Can always make sure to short caps out by crossing a screwdriver over the metal tops of two or more of them to be absolutely sure. At least from what I've read.

Good tips, Comedian.  A better way to work on tube stuff is:

1) get a piece of dowel, as big a diameter as a pencil.  You can use this to "chopstick" - move wires a bit to see if there's a loose connection, BEFORE the need to put your hand in there.    I wrapped mine in electrical tape, just to be 100% that the wood wouldn't conduct.  Not that it will, but it also looks like a cool tool that way

2) when draining caps, it's a good idea to make a 'shorting stick'...a non-conductive stick with say, a 100k 1/2W resistors on the end.  One end of the R goes to a jumper you connect to ground, THEN you touch the other end to the caps, holding until they've bled off, which may take a couple minutes.   This is safer, and better for the caps - the sudden arc made with a screwdriver isn't the best way to do it   It does work, tho, lol.

I wouldn't count on unplugging something when "on" to drain caps. Some things do, some things don't.  Well-designed items have bleeders, but not everything does...even the modern Fender Hot Rod Deluxe keeps the caps charged, switch on or not!    Any capacitance that operates at HV in a circuit must be verified to be discharged prior to handling it...even caps down stream - just do them all.    And hope you got them all - some things, like oscilloscopes etc., may have them inside the power "cage", and you may not be able to see them....schematics are great for this....