Some more 12U7 tube preamps @ 12V

[Fender56]

Hi!!!

Following the "Tube-boost with a 9V-cell" and the "Sopht amp", here are some new (at least to me!!!) preamps that use 12AU7/12U7 tubes @ 12V.


Vanilla Overdrive:

http://www.tube-town.net/diy/lov/lov01-vanillaoverdrive-eng.html


Pepper Shredder (this one has not been translate):

http://www.tube-town.net/diy/lov/lov02-peppershredder.html

[tranceracer]

Those look like pretty cool tubie projects.  Curious if here are any sound clips.

[Renegadrian]

Man, I am so glad you post those...I made some searches inthe last days but found almost nothing, apart from sopht site...
and Satan  knows I'm into tubes these days!!!

[Fender56]

If you need more ideas, you can read the thread named "Team low voltage - What to build?" in the AX84 forum, in the "Effect building section" :http://www.ax84.com/bbs/index.php

There are also a lot of info on this site. My next 12V projects will be to try the 12AJ6 (more gain than the 12U7, 50 vs 20) and try to put some triode in parallel, like the Marshall 18 Watts preamp!

[Renegadrian]

Pepper Shredder (this one has not been translate):
http://www.tube-town.net/diy/lov/lov02-peppershredder.html

Damn, I studied some little German back at school...See if I got it correct...

Design:
Based on the VanillaOverdrive from the LoV series the PepperShredder is heavier. The structure is again completely on 12V basis and is appropriate for 12U7 and 12AU7.

Sound:
Classical Heavy sound, but no NuMetal. The differences in the direct comparison are rather small and the user can use a 12U7 or 12AU7. Both types function perfectly.

Application:
Similarly as the VanillaOverdrive as pure Preamp or as boosters.

Structure:
The structure does not turn out differently than the VanillaOverdrive. All resistances are ¼ Watt of metallic film, which are best suitable surely for this case. Beware, short signal paths and a clean mass guidance are elementary. If the 12V voltage supply is clean and stable, then the Amp is extremely calm and also with full Gain no problems are expected.

[Renegadrian]

obviously you are more than welcome to let me/us know about your 12v tube projects!!!! 

[brett]

Hi
these are very cool.  Your construction style and quality is terrific.
But is there any chance of doing a 1 tube circuit, just to keep things simple.  (The "Triode triode" on the "tube at 9V" thread had quite a lot of gain with just one 12AU7).

RE:

and try to put some triode in parallel, like the Marshall 18 Watts preamp!

Running triodes in parallel results in barely any change in tone and gain (ie 2 x gain).  It was an odd, throwaway kind of thing to do with a spare triode stage in that amp.
cheers

[Fender56]

Oops! It is not my site nor my projects!!! I just discover it by browsing!!! 

Parallel triode is "supposed" to fatten the tone a bit since the two triode are not exactly the same. A way to increase the difference is to bias them a little bit differently (different cathode resistors). But it is kinda waste of triode if it does nothing to the tone! I will have to experiment a bit!

You can check the "Blues Preamp" on the Core preamp project section of the AX84 site. It use a parallel triode gain stage, bias with two different LED at their cathodes! Clever design!!!

[drewl]

RE:

and try to put some triode in parallel, like the Marshall 18 Watts preamp!

Running triodes in parallel results in barely any change in tone and gain (ie 2 x gain).  It was an odd, throwaway kind of thing to do with a spare triode stage in that amp.
cheers

Maybe at 12v but at high voltages paralleling preamp tubes yeilds a nice sweet overdrive and greatly increases touch response.

[brett]

Hi

Maybe at 12v but at high voltages paralleling preamp tubes yeilds a nice sweet overdrive and greatly increases touch response.

Hmmmm..... my Marshall 18W has sweet overdrive and touch response on both channels.  It starts breaking up at only 3 or 4, even with average single coils.
Of course, gain IS a little higher on the parallel channel, and it's a fairly low gain amp, so most people prefer the parallel better.  But the parallel channel is no different to the single channel with a little boost (3 dB) or turning up the volume a little (5 on parallel sounds about the same as 6 on single on my amp).

Depending on how an amp is wired, the input resistance might be halved if using two triodes if two gate-to-ground resistors are used in parallel.  This may roll off some treble and give a fatter tone.  If this is the case, though, it is due to changed input impedance, which is really a separate issue to connecting triodes in parallel.

cheers

[drewl]

I built an 18w with the parallel V1....it does add to the crunchiness!
Have you played any Matchless amps?
I've built a few Chieftains, and almost all the 12A-7's are paralleled.....pain in the arse, but sounds good.

[Paul T]

I built a few tube amps in the past, and on one of them (2 channels, spring reverb, 40 Watts RMS), I decided to put two 12AX7 (in parallel) for the phase inverter/driver stage.

The result was quite astonishing; the dynamics and "touch response" was so vibrant and alive !

The reason is this: any preamp tube can give lots of output voltage but a very small output current. Therefore, by putting two triodes in parallel (or a single 12A-7 tube with both separate sections wired in parallel), you get twice the output current, which translates into better dynamics and headroom (or "touch response").

It's a very interesting way to go, but also means twice the cost if you want to wire the entire preamp section this way !!!

Think of this: on 100 Watts amplifiers, you will see 4 power tubes (EL34, 6L6...). This is the way you get more power; by putting parallel pairs of output tubes. The same goes for preamp tubes.

[brett]

Hi
this is getting way off topic, and it seems that all sorts of spurious arguments are being put forward....

The reason is this: any preamp tube can give lots of output voltage but a very small output current.

And the input stage sees a very high impedance.  The very small current that most pre-amp triode can put out is many times more than is required. 

And a little unusually, the 12AU7 can actually produce enough wattage to drive an output transformer and speaker.  In the Firefly amp, the 12AU7 is driving a load of about 10k, whereas in the input stage of most amps, the load is usually about 0.5M (specifically designed that way so as not to load the tubes!  Those high-value pots add noise, so designers would have got rid of them if they could).  And please don't believe me (I wouldn't), go do the math yourself.

I think of it this way: If my car had an engine that produced 10 times as much power as it needs, what would be the effect of using two of those engines? 
If I built such a monster, would I be likely to proclaim it as superior to other cars?
cheers

PS technically, you *can* get lots of current out of a preamp tube - by using it as a source-follower.  But I understand there was a difference between what you meant to communicate and what you actually typed.

[Paul T]

Dear Brett,

thank you so much for pointing out I was talking about the preamp circuit, because I obviously was.

Even the PI/driver circuit is wired as a "preamp" circuit. Running a triode as a follower will not give you that same "punch" as putting two triodes sections in parallel. A 12Au7 will give the most current, but almost no voltage gain and less "tube signature grit" - if that's what you are looking for. That's the reason a 12Au7 (most of the time, both sections) is used as a reverb driver stage, or even a headphone driver sage, like a mini tube power amp; lots of current capability but no voltage gain, in this PARTICULAR USE.

On the downside, using a 12A-7 as a voltage follower (or source follower) will degrade the sound : you get a loss of voltage (the output is usually about 0.9 than what you put in) . Oh well, no big whoop.

Running 12A-7 at low voltage will NEVER be the same as running 12A-7 at high voltage (150V and up, for example), so comparing PREAMP tubes running at 12V and PREAMP tubes running at 300V... well, there is a difference. The performances might not be the same and yet, the basic laws still apply. Putting two halves of a valve hand in hand will give you more current and therefore, more "punch", "touch response", "dynamic", "headroom", "grit", "definition"... etc... why do you think that most spring reverb driver circuits are made with a 12Au7 with both halves of the triode wired in parallel ? More current. Even a small triode (12A-7) can drive a speaker load (8 ohms, for example).

However, there are no small voltages or circuits.

... only small designers.

But Brett, I've rambled long enough, and at this hour I'm sure you must tend to your pet kangooroo, as I must tend to my pet polar bear.

G'day mate !

[Renegadrian]

...at this hour I'm sure you must tend to your pet kangooroo, as I must tend to my pet polar bear...

Strange pets you have my friends...

[Renegadrian]

http://www.montagar.com/~patj/tubedist.gif


One more for the records...
Altough it's not on 12v, it could be modified easily, I believe...

[ambulancevoice]

http://www.montagar.com/~patj/tubedist.gif


One more for the records...
Altough it's not on 12v, it could be modified easily, I believe...

it is 12v

[Renegadrian]

Well, yes it does run on 12v but has that 120v ac plug...I believe it'd be just taken off the circuit, right!?

[MikeH]

The whole upper part of the schem with the 120V and the rectification etc, is basically (in that it can be replaced by) a 12V wallwart.  And a simple voltage divider will give you the 6V ref.

[ambulancevoice]

yeah, it can be run from a 12v wall wart (regulated, must be regulated), and a simple voltage divider can be added  to get the 6v