Do pedals that feature a 12AX7/ECC83 tube really create preamp tube...

[overdrive city]

overdrive and distortion? Some people have said most of them are a gimmick.

[Taylor]

It has more to do with the actual circuit. Sometimes people use a tube as a clipping diode in an otherwise-solid state circuit. Other times a tube is used at low voltage. It's a matter of taste whether these uses will sound like what you're expecting from a tube preamp. But there are some pedal circuits which are just tube preamps, with high voltage and all the same stuff as a tube preamp in an amp, and these work and sound the same as the preamps in tube amps.

[caspercody]

Taylor,
Question for you on tube preamp made into a pedal form. Lets say one was to build the preamp portion of say a Mesa Boogie dual rectifier using the 12AX7 tubes but only using low voltages. The "12" in the name I believe means the heaters work on 12volts, but most amps have high voltage going into the plate of the tube. If one was to use this same 12 volts on the plate (instead of high voltage) would it sound comparable to the high voltage version? i am sure I know the answer, but thought I would ask.

Thanks
Rob

[Paul Marossy]

overdrive and distortion? Some people have said most of them are a gimmick.

Of the ones I am most familiar with (Shaka Tube, Chandler Tube Driver), the distortion is being created mostly by a solid state device with the tube adding the "starved plate" sound. But the Matchless "Hot Box" and "Real McTube", which are 100% tubes, is a different story. Real McTube uses semi-starved plate design and Hot Box is like a full blown amplifier preamp.

[Derringer]

a 12ax7  with 12 volts on the plate will not sound the same as one with HV (100+) volts on the plate

so in general, to get a true "preamp" tube sound you would need to use typical preamp voltages

it will work at lower voltages and the quality of the sound is subjective

[Renegadrian]

My personal experiments with the AX7 took me to think that it's very difficult to get a decent sound with only 12V.
But I have to get on the breadboard again and get some new tube circuits to share (else only Rock is gonna make new ones!!!) and see if I come up with a way to use the AX7 at 12V.

Let me say that some "hybrid" pedals can sound quite good, say Radial Tonebone, but I personally don't like to mix things - trannies or ICs are good, tubes are good too, the two mixed...mmm, a big NO
I didn't study a lot the PCB of the Tonebone I had to repair to a friend, but you have a 4558 in front of an AX7...nononono
So yes if tubes are to be in a pedal, let's go ALL tubes!!!

[george]

can't really agree with the "it has to be all tubes" sentiment

I built a shaka tube and it sounds very valve-ey to me (we call tubes valves in australia)

I built it in a night, it has a maximum of 40v anywhere internally so it is safe for DIYers.

OK it might not be "100% tube" but it's at LEAST 95% there - not worth risking getting killed trying to get to 100% IMHO.

just my $0.02

[Earthscum]

what's the difference between running a solid state booster before a tube pre and having the SS booster and tube in the same box? NOTHING... except less a battery, pot or two, and a stomp switch. Really, there is no difference at all. Ya want all tubes? Run a tube distortion into a tube amp... but, careful! That tube amp may have some silicon in it. Better check to make sure 

I wish I could find the picture again... was a tube amp that some guy decided to replace every SS component in with a tube. It looked ridiculous! Which is why he did it.

I like tube sound, too... but seriously, there's a point when you are just making a waste of power, and an extreme expense of parts that wear out without any improvement in quality or sound, but a degradation of stability. I've dropped SS alot, no problems... drop a tube, and you are out $15-20... and hope ya got a spare! So why not have the minimum amount of tubes possible? Is there really any noticeable difference in boost if you aren't clipping? A mosfet booster to get things up to a line level (when filtering is implemented properly) really isn't going to change the sound of the tubes gain and clipping enough for anyone but a sky-sniffing stickler guru to notice, IMHO.

[petemoore]

  And the data sheet says !
  What is says, worht a look if you want performance from tubes, RCA manual excellent resource.
  Reports and experience show that they 'start lighting up' around 90 or so volts, see what 'preamp' is defined as [check your favorite preamps schematic] in terms of voltage/distortion etc. 
  By strict definition 'preamp', is before the amplifier and increases voltage potential of the signals [boost], 12v 12ax7 plate can 'work' and fit the literal definition, starting with say 36v/regulated [printers often have a 36vdc WW] or more substantial power supply will afford more headroom/working room for the 12a_7 [substitute the last letter for lower Mu tube, 12au7 for example.
  Or try a different type of tube, something for historic automotive 12v supply seems to be a popular approach for LV tube operations.

[edvard]

I must concur with Pete.
Starved Plate has a sound it's own and deserves a place in the stompbox world, but to say it gives some sort of (tube amp brand name here) mojo is really stretching things.
Once I built a plain vanilla 2-tube 12AX7 preamp running off a Real McTube-style 150V supply and I gotta say it sounded MUCH livelier than the Craig Anderton SIAB that runs on ~42V.
Headroom has it's own mojo, no mistake.

If you REALLY want to mix soup with nuts, build a mu-amp with a high-voltage MOSFET working the plate load.
Headroom AND full gain without the resistor tax is a beautiful thing.

Damn, I just gave away the secret to my next pedal design... 

[teemuk]

Do pedals that feature a 12AX7/ECC83 tube really create preamp tube... overdrive and distortion? Some people have said most of them are a gimmick.

In my experience: it depends on the circuit. Never saw that coming, huh?

Some I've seen use the tube as cathode followers (e.g. Radial's Tonebone series). These will hardly clip at stompbox signal levels so I rank them in the category of putting an impressive lamp to the design to boost sales with impressions of "tube sound". Naturally all clipping within those has been achieved with solid-state devices. Some say the cathode follower has a "tube sound" but I've never experienced that myself.

Some I've seen use the tube as clean amplifier, later clipping the amplified signal with diodes (e.g. Westbury W-20). That particular pedal was a high voltage design so the tube in it actually did something because it introduced a significant voltage swing (about 90Vpp) that you would have had great difficulties achieving with everyday solid-state technology like OpAmps. This signal was then clipped with the silicon diodes.

Most pedals I've seen do in fact use the tube as a typical common cathode voltage amp, sometimes as a cascade with a directly coupled cathode follower. Sometimes they are "starved plate" designs (e.g. Vox Cooltron). And yes, they do they create preamp tube overdrive but basically that is pretty much similar to clipping the signal with LEDs, possibly with a little bit of asymmetry. The starved plate design of Vox Cooltrons, on the other hand, already operated on such low voltages that with the level of overdrive used within that pedal the clipping went to the same realm as clipping plain OpAmps. Possibly the tube may also introduce a bit of grid conduction ("grid clipping"), but you can still easily replicate that with harder clipping of a plain diode. So I don't view the preamp tube distortion as such a big deal. You can achieve similar effects with solid-state technology if you just put your mind into it. Also, most of those "tube" pedals still have solid-state means for signal clipping as well (e.g. Blackstar HT series, Tonebones etc.).

Then there's naturally all-tube, high-voltage designs. A good example is the Faustone Valve Klipper that sounds just as horribly farty and fuzzy as the Laney Klipp preamp it cloned. Tubes aren't neccessarily a guarantee of anything.

[Renegadrian]

Guys, don't get me wrong - I have nothing against trannies or ICs, I do love to use them - also I love to fiddle with valves.
My personal PoV is that I feel too easy pushing a tube with a 4558 as I have seen in some pedals - not that they are horrible sounding pedals, they are good, but I personally do wonder how much the tube sound is involved. I love the sound a single tube circuit can give. And I couldn't think to experiment with an hybrid circuit in mind.

[anchovie]

I've found that a circuit that works for a single 12AX7 at 12V B+ won't necessarily work if you swap the tube. There's a big variation between manufacturers at that end of the transfer curve, with some providing attenuation rather than gain.

[tubelectron]

Well...

If you only want clean gain boost, a single stage or single+cathode follower or even a SRPP circuit powered on 250VDC will give you nearly insaturable input operation, and more than 20VAC output facility. It is possible to enter it in a relatively small pedal, I have done it and it works well. But, in fact, we don't need this insaturable input and hi-voltage output : a simple FET or bipolar (like the LPB-1) is more than enough, and way practical and simpler !

Now if you want distortion coming from the pedal, a tube can do it, but if you only have 2 stages (like you would have with a 12AX7) you won't be able to achieve smooth distortion and decay. It works quite correctly with subtle overdriving but if you go further, it sounds like a fuzz with blunt decay. I speak for 100-250VDC powered units, never tried lo-voltage. The best results I obtained is a diode + tube combination, in the style of my 1982 Westbury W-20, which is a 250VDC tube overdrive (and maybe the 1st one ?). Here are some of my HV tube overdrive :



the W-20 schematic can be found at the freeinfosociety website :

http://www.freeinfosociety.com/electronics/schemview.php?id=985

A+!

[zambo]

I dont know about you guys but I made a pedal that runs a 12au7 , 12at7,12au7 ( in that order ) running on 9 volts and it distorts everything as much as I ever needed. I gig with it almost every week and get nothing but compliments on my tone. Its a big pedal and I am not saying its the best answer but it gets very nice distortion and enough of it. 6 gain stages do a lot to the signal and it has enough boost to push any amps input into submission. I guess if you look at a preamp that makes truly impressive distortion you will see it usualy has more than one tube, not just high voltage.      http://www.youtube.com/watch?v=qbDJYMAp6Mo     sorry about the lack of audio quality. its just a camera mic getting very beat up....anyway. No solid state distortion here. Just three valvecasters in series with a few little mods like input caps etc.

[zambo]

here is the one i use for gigs    http://www.youtube.com/watch?v=hT9KUAV5-7M&feature=related  . once again, no solid state anything. running into a peavey solid state amp though  . I have since switched to all fenders. Here it is in a bar live and pretty loud. at 3 minutes in you can hear it pretty good.   http://www.youtube.com/watch?v=d4aoH1STXUs&feature=related

[amptramp]

Well...

If you only want clean gain boost, a single stage or single+cathode follower or even a SRPP circuit powered on 250VDC will give you nearly insaturable input operation, and more than 20VAC output facility. It is possible to enter it in a relatively small pedal, I have done it and it works well. But, in fact, we don't need this insaturable input and hi-voltage output : a simple FET or bipolar (like the LPB-1) is more than enough, and way practical and simpler !

Now if you want distortion coming from the pedal, a tube can do it, but if you only have 2 stages (like you would have with a 12AX7) you won't be able to achieve smooth distortion and decay. It works quite correctly with subtle overdriving but if you go further, it sounds like a fuzz with blunt decay. I speak for 100-250VDC powered units, never tried lo-voltage. The best results I obtained is a diode + tube combination, in the style of my 1982 Westbury W-20, which is a 250VDC tube overdrive (and maybe the 1st one ?). Here are some of my HV tube overdrive :



the W-20 schematic can be found at the freeinfosociety website :

http://www.freeinfosociety.com/electronics/schemview.php?id=985

A+!

Apparently, they don't have a problem in the Westbury W-20 with reverse voltage on the electrolytic capacitor in series with the diodes.  But over time, it will become a problem.

[tubelectron]

Hi amptramp,

At the - side of the 10µF/50V diode coupling cap, I measured 1,24VDC at idle vs ground. As this voltage is reverse polarity, yes, it may be a source of failure in the long term, but I think that a 50V cap is able to withstand it, by experience.

Nonetheless, the unique drawback I find on the W-20 is its poor S/N ratio... Unless you remove the 120VAC primary transformer from the inside, or try to replace it with a 12 to 230VAC and use an AC mains to 12VAC wallwart, there will be no improvement. But I choose to left this unit as is, for "historic reasons". Despite that, it is a good sounding unit, very smooth, with regular sustain and decay... Thanks to the diode clipper and tube combination !

A+!

[brett]

Hi
my 2c....
while people are mostly talking about 12AX7s, which have low transconductance below about 40V (plate volts), the 12AU7 is more suited, and the 12U7 is designed specifically for 12V (old car radios etc).  There's not much difference between the AU7 and the U7 (which are much harder to find).  To make the 12AU7 work at 12 V you really only need a high impedance load, such a buffer (a JFET stage works well).

However, there are plenty of simple voltage multpliers around that are capable of turning your 12V DC into 100 to 200V DC at low current (a couple of mA, which is all you need).  If I were starting again, I'd look at that rather than a Real McTube type supply with 2 power transformers in back-to-back arrangement.

Cutting through the multitude of options, two seem dependable: stay simple and use a 12AU7 at 12V or dig in and spend the time and effort to set up a 12AX7 at 100V+.

It's up to you, really.
have fun!

[tubelectron]

Hi brett,

while people are mostly talking about 12AX7s, which have low transconductance below about 40V (plate volts), the 12AU7 is more suited, and the 12U7 is designed specifically for 12V (old car radios etc).  There's not much difference between the AU7 and the U7 (which are much harder to find).  To make the 12AU7 work at 12 V you really only need a high impedance load, such a buffer (a JFET stage works well).

Interesting information - your 12AU7 advice is a good entry. For me, working on tubes with high voltage is usual (as for others designing and etching PCBs), so I choose the "easiest way" for me, and I shamely never tested a 12VDC tube distortion device...

However, there are plenty of simple voltage multpliers around that are capable of turning your 12V DC into 100 to 200V DC at low current (a couple of mA, which is all you need).  If I were starting again, I'd look at that rather than a Real McTube type supply with 2 power transformers in back-to-back arrangement.

I know that exists compact integrated DC to DC 12/250 (or so) converters able to supply the plate voltage for a 12AX7 - I should search more info about that interesting alternative. Any of you had product references ?

If you speak about diode-capacitor voltage multiplier starting from 12VAC, I'd rather say that it is not more compact as a small 12/230VAC 3VA transformer, and less stable (nor to say less reliable) in voltage output. But yes, there is no hum induction possibility, as there is no Xfo inside the stompbox.

Nonetheless, a "reversed" transformer (P 12VAC to S 230VAC) radiates neatly less than a P 230VAC Xfo, so it needs a light shield only if it is close to the audio circuit, as shown below on my HCTO overdrive innards - which is noise and hum free at any setting, 250VDC plate supply and 12VDC heating, from an ACmains to 12VAC 0.5A wallwart adaptor.



Here is the detail of the shielding : a simple 0.5mm folded aluminium sheet.



A+!