12au7 on 30v???

[Alpha579]

Hey all,
Ive seen 12ax7 Overdrives using 12v power supplies, so could you run a 12au7 on 30v as in a self split power stage ala firefly? Just coz im wondering if i could build a tiny lil amp like the firefly, but with out using to high voltages.
Thanks for input,
Alex

[The Tone God]

There are two voltages to be aware of when working with tubes, the power supply voltage and the filimants. The power supply voltage is not the reason for the common use of 12v. The reason is that it takes 12v to fire up the filimants. Using the 12v for the power supply just simplifies the circuit.

So to answer your question you can use 30v for the power supply but you'll have to do something to get your filimant voltage.

Andrew

[Alpha579]

Do you suggest funning the filaments off 12v or 6.3?

[Gnome]

There has to be a reason why all amp schematics you can find uses 6.3v filament supply. But remember, to use a tube, you need the plate-voltage to be saround 300v or more. Less and it will sound like soup...

[puretube]

the reason for 6 instead 12V in amps is: most powertubes can not be wired for 12, but only (single filament) for 6V.

[Alpha579]

:oops: Soz to sound newbieish, but whats the plate voltage?

[Phorhas]

http://www.svetlana.com/docs/tubeworks.html

Try this... it should give you some info

[Paul Marossy]

Running the filaments on 12.6 volts in a tube amp can also be a lot noisier as far as hum is concerned. I found that out with my Firefly build.

You could also run a preamp tube on 6.3 volts and use a voltage doubler, tripler of quadrupler circuit for the plate voltage and solid state devices, if used. I thought about that the other day as I was changing the heater filament wiring on my Firefly. I came to the conclusion that using 12.6 volts is kind of the best compromise between simplicity and complexity.

[Mike Burgundy]

But remember, to use a tube, you need the plate-voltage to be saround 300v or more. Less and it will sound like soup...

Whoa there pardner!
This would mean any preamp I know of sounds like soup. Anything from bouillabaisse through kentucky onion to Granma's chicken. Problem is a lot of tubes (all currently used preamp tubes I know of for one) don't take 300VDC or more at all.  A 12AX7 maxes out at 300. This qualifies for the "It sounded *great* just before it crapped out" award.
High-voltage is the way to go, wether it's a preamp or poweramp, but take the tubes and circuit used into account.

[Peter Snowberg]

I was running a 12AX7 at about 420 volts for a while and it sounded wondeful. I kept wondering when the amp was going to explode. Swan song anybody?

I think as long as you have more than 100 volts in there it can sound great. Less than 100 starts to sound "starved" which isn't good or bad, just different. I don't like the starved sound as much, but that's a metter of personal taste.

A Fender Super Reverb runs the preamp at a little over 270V and it sounds wonderful, even if it is over the 250V rated max, it seems to work and be solid. Lots of those amps out there.

Take care,
-Peter

[puretube]

Mike: don`t mix up " Ua-max" (plate to ground), which is 300V for 12AX7,
with " Ub ", or "B+", which may well go near the 500V region, as long
as the total dissipation is taken care of (1W per triode-section):
from a data-sheet (!):
B+ = 400V, Ra = 220k, Rk = 1k2, Ia = 1.02mA, gain=76.5 @ 1.1% dist.

(be sure, btw., to use plate-resistors specified for 500V).

[Paul Marossy]

Good point, puretube. The Fender AA270 Twin Reverb schematic I have shows +385 volts on the plates of some of the preamp tubes. I thought of that when I first read this post, but I didn't say anything because I don't quite know enough about tubes to accurately say why that works OK in this amp. I do know that Fender pushed it with plate voltages on all the tubes in those older circuits, though.