Crazy question for you tube experts...

[Renegadrian]

Since I built the Valvecaster, I fell in love with valves...But I don't feel confident on building hi voltage devices...
I just love the sopht site, using "space change" valves to build 12V amps...
Also I read somewhere some info about those 12V tubes (not so much information, unfortunately... )
Actually these valves could be used in a lot of ways...My nighttime thought was
IF SOMEONE TRANSLATES/ADAPTS TUBE PREAMPS CIRCUITS FROM BIG AMPS (MARSHALL, FENDER, MESA BOOGIE...) JUST SWAPPING TUBES WITH FETs (FETZER, DR. BOOGIE...), WOULD IT BE POSSIBLE TO ADAPT SOME OF THOSE CIRCUITS TO USE THOSE 12V TUBES INSTEAD OF THE CLASSIC TUBES RUNNING ON HI VOLTAGE AND MAKE A 12V STOMPBOX???

Like to say, a Dr. Boogie with tubes, stompboxed...

Is that doable or just a crazy fantasy???

[aron]

You can, but it will not sound like the "real" tube versions.

I really tried :-(

[Renegadrian]

So you say it's not worth trying?!

[d95err]

I guess an interesting question in this context would be - how does a 12V tube circuit compare to a JFET (ROG-style) tube emulation?

[theundeadelvis]

Do your homework on high voltage circuitry. If you can learn to build pedals you can learn how to respect high voltage. Then build regular tube amps. You'll love doing it and the payoff is great. Read the tube amp safety at geofx.

[Renegadrian]

I know there is a lot of documentation about hi voltage tube circuits, I read something and it's not an impossible task...
But you know, everyone thinks TUBES=HI VOLTAGE and those 12V valves make you want to be different and go for a less used path...

[brett]

Hi
Mostly, this is true:

You can, but it will not sound like the "real" tube versions.

Why?  Because classic pre-amp tubes like the 12AX7 have high internal resistances and have low current capacity, and so are hopeless unless you get at least about 80V for a B+.  (With a variable B+ it is interesting to see how 12AX7s go at different voltages.  I was surprised to find that at about 80V they are probably workable.  But if you are going to rig 80V, why not go for 220V?).

HOWEVER, it is not always true.  The circuit below sounds remarkably like an early (eg JTM) Marshall preamp (I have an 18 watter and a JTM60, so I't not wishful thinking).  IIRC, the "valvulator" is similar to two of these put in series, and can provide a high gain sound.

The secret to these designs is that the tubes (12AU7 or 12U7) provide reasonable gain at low voltage.  In the past, people have used 12AX7s, and it just seems to be that they are no good for this.

So I reckon there's no harm in experimenting.  If you have reasonable expectations, you probably won't be disappointed.  The components to tweak will be the plate resistors (no too high and not too low.  100k to 220k? Not 470k.), cathode resistors (probably need to be lower than in the standard designs.  Try 1k and less.) and bypass caps (probably need to be bigger than usual.  100uF or so for full bass, less for tight bass). 
cheers 

[MarcoMike]

well, high voltage is present only -during and after- the first time you turn it on.
you can build it with no voltage concerns at all, install an external bias thinghie, double-triple check everything and then close it and power it on!
if it works, you are done and you haven't seen any high voltage. the high voltage concerns start if it doens't work... be confident! 

[Renegadrian]

Brett, I appreciated your words so much!!! Well, i know that something can come out of this tubes, and that they work so well at that 12V...Also, it seems that not so much people went experimenting, and I believe those space charge could be more useful...So surely I'll try something, starting just for the fun of it, may I'll come across a decent project you'll surely know...

Marco, I already read something about hi voltages, and yes you're right, but as I wrote before, I like to go through a different path...(BTW my italian comrade, where do you live? Which is your football team?  )

[MarcoMike]

I am from Modena, what about you?

regarding the football team, I'm not even sure we still have one... 

[aron]

No one ever said don't experiment. All I am saying is that from my experience the 12 volt tubes I tried were not as good as an equivalent jfet circuit. Just a crappy exaggerated overdrive. But I am picky.
Sometime you ask and get an answer :-)

[fredy]

if you want to play with valves, then a valve amp would be the best option. I made one from a kit, it was suprisingly simple to do and worked first time. if you have checked out my little gem thread, you will know that is saying something.

[Renegadrian]

I got the "experiment" part, as some good pedals were born that way...And my "Frankenstein" part of my mind is already at work!!! 

Marco, I am from Rome and obviously a ROMA fan. And I believe even some of our foreign friends here know the name of our captain...