Low voltage tube preamps

[Denio]

Greetings guys and gals!
So I'm writing this thread in need for some info regarding low voltage tube preamps/pedals.What's the catch?Less gain?Not as rich tone as their high voltage brothers?Something else?I am certain that they can't be as good as the traditional high voltage designs.I am specifically talking about circuits utilizing our best friend- the 12AX7.I've seen them run on as low as 12 volts(BTW is this known as a"starved plate" circuit?).

An idea I have in mind is to build a tube preamp using a single 12AX7(or 2 if needed,but not 4-5 as it would be a weee bit pricey as for maintenance)and run it in front of a solid-state amp or right after a dirt pedal in order to smooth out the sound and use the amp not so much for the actual clipping but more for just sound reproduction(kind of the same idea as like a Mesa Boogie Triaxis or the AXE fx,in that sense that it is self-contained,and the amp would be just to drive the speaker).
And what should I consider for power-a low voltage or some sort of switch mode supply?
Yes,I know a full blown tube amp is the way to go,but I'm a poor student  and I'm doing this in the spirit of learning and experimentation.
Wish you all love and happiness! 
Denislav

[GibsonGM]

Hi Denio, my take on this is that it really has more to do with safety & convenience.   Starved tubes actually have LESS headroom than if used at their 'proper' operating voltages, so tend to be more dirty.  They will clip more abruptly, and are a little bit LESS like tubes when operated this way.

I use a Fender hot rod deluxe for an amp, and do what you're thinking...I use my pedals, or a tube preamp, to get my sound and keep the amp pretty clean. It works very well for me.

What 'sort' of tube pre you want to try seems to depend on your experience and skill level, IMO.  I'd try some low-voltage stuff, then graduate to something more high-test.    I'm messing around with SMPS supplies myself now, and they can be hard to keep quiet...but are not as bulky as a transformer, of course.     But then you also have to consider powering the heaters...

2 12AX7s are 4 triodes, that should be MORE than enough to get a solid rock tone...maybe a third one if you want more metal sounds...just my 2 cents!    Do some research, there are many threads on this here, and talk to people about what sound you're trying to find...

[Denio]

Thank you for the fast response GibsonGM! 
I'am kind of looking for a heavier sound,but every now and then I like to play around with lower gain,so maybe I can bypass the third tube when needed,but this isn't my biggest worry for now.
Hmmmm I have 2 ideas in mind:
1)Wire up 2 identical transformers back to back(for example 220V-12V-12V-220V),but I am not sure if that is going to provide enough current for the plates,plus I need some other source to power the filaments(because with 3 12AX7's they would probably eat up a good 1.5A)
2)Use a torodial transformer with 50V(or more) and another beefy 6.3 for the filaments.
Thanks again!

[PRR]

> not sure if that is going to provide enough current for the plates,... filaments... 3 12AX7's ...probably eat up a good 1.5A

Get fresh pencil, re-check your math.

Tube plate and heater currents are on the datasheet.
http://www.mif.pg.gda.pl/homepages/frank/vs.html

[GibsonGM]

With a 12AX7, I'd expect each triode to draw maybe 3mA in use, Denio.  Now, the heaters...450mA if using 3 in series.    The heaters can be powered by the same transformer, or something else, altho one power source IS convenient! 

Look up "the Real McTube"...he did just what you're proposing with the transformers.   The circuit isn't really what I'd call "high gain", tho...no shred, more OD.

If you want a real B@LLSY project, you might search the net for a true dedicated tube preamp, something based on Soldano or something.  There are lots of tube forums (AX84 being one).   The GTFO project we've been playing with on here is another option....I use that design and it's dirty enough for what I do (think G'N'R...).   

McTube may be worth messing with just for the experience, as mains voltage can kill you pretty easily if you're not familiar with its rules for safe use....whatever you do, please take the time to learn how to do so safely!     

[Denio]

Thanks for the fast responses guys! 

Errrr,I kinda screwed the math because I always thought that the heater of a 12ax7 draws like 500ma(silly me),but after but I checked out the data sheet and It all sunk in.
As for a power source I had a look at a video by Uncle Doug,where he talks about voltage doublers( https://www.youtube.com/watch?v=nw4bYBnB_G4  )and he used the same method as I described above,using 2 transformers back to back,and a voltage doubler.He used the secondary of the first transformer(which was 12 volts with a center tap)to power up the filaments.

Thanks for the safety warning GibsonGM! 
I always play it safe with these things. 

[duck_arse]

there is also a mid-point between the 12V builds and the hi-volts builds. you can use a cmos inverter, or a 7/555 and diode string, and run the valve on something like 48V instead.

[GibsonGM]

Yes, if you look around, you'll find that there are a few different schemes out there for powering the heaters AND the plates.   Some are what you see there, 2 transformers with heater 'tap' between them, some are a 12V supply feeding SMPS for hundreds of volts, some are in between, maybe 50-100V... and so on.   

Once you get your feet wet, you'll have a lot of things to try 

[tubegeek]

Now, the heaters...450mA if using 3 in series.

Just to clarify that statement:

150 mA per tube with its heaters in series, i.e., 12.6V operation.

The three [series] tube heaters would then be placed in parallel with each other (the typical arrangement) and draw 450mA @12.6V.

If the three [series] tube heaters were placed in series with each other, you'd want 37.8V @ 150mA. Which is also a useful voltage to operate the plate circuit at for a small tube preamp, which is why I bring it up.

[Denio]

duck_arse:yeah the 48 volt thing was in my head but now the idea with the 2 transformers back to back can't leave my head.
I am still going to keep this idea in mind.

GibsonGM:The SMPS is another neat thing,although requires a ton of experience and knowledge.Plus I have heard that the noise when using an smps is a bit of a pain in the but to tackle.

[GibsonGM]

Tubegeek:  Yup! 

Denio....yeah, I'd recommend starting simple, build a Valvecaster just because you can, see what it sounds like if you change the plate resistance.

Then a McTube, using your transformers.....then you can modify a bit, get something going a lot more crunchy.  I'm all for buying the right iron for a nice big build, have done that with good results, so you don't need to worry about SMPS yet...

MerlinB, also a member on here, has/had a book for sale about how to design tube preamps, and I found it to be a GREAT way to learn because he's condensed 100 yrs. of tube lore and hard math/theory into one volume that's easy to understand.   I wouldn't recommend the book if I didn't find it extremely useful!!  If you are really interested, I'd go ahead and grab a copy.  It will keep you from some time-wasting dead ends, I think   A few hours spent with it, you'll be looking at tube circuits and right away will know what each part does, and what to change if you don't like something....
http://www.valvewizard.co.uk/