[sdiy] slightly OT: current demands for 12ax7a & 12au7?

[Antti Huovilainen]

On Thu, 27 Apr 2006, anthony wrote:

> I'm building a small el84 based tube amp with reverb. It's the high 
> octane AX84 design actually. The power supply calls for a 6AC4 or an 
> EZ80/81. I have a 6X4 and a 6AX5GT. Now I know the 6AX5GT can handle way

I went with solid state rectifier. The only effect a tube rectifier has 
for a class A amp is some constant voltage drop. There will be no sag.

> more current than I need but I want to use it in a bigger amp. So the 
> preamp uses 2 12AX7A's, 1 12UA7 and 1 12AT7. I was wondering if the 70mA 
> capability of the 6X4 would be able to handle all of the tubes in the 
> preamp. I know the el84 uses about 48mA, but I can't remember if that's

The 12ax7s are biased at about 1mA. EL84 can handle max 50mA at 250V V+, 
so you have almost 20mA left for 12au7 and 12at7. They're probably biased 
around mid-way, so current is then roughly 120/Rp.

> use the overkill 6AX5GT or should I be able to use the 6X4?

Use solid state diodes for the rectifier and save yourself trouble and the 
tubes :)

Antti

"No boom today. Boom tomorrow. There's always a boom tomorrow"
   -- Lt. Cmdr. Ivanova