First transformer rating in a back to back arrangement

[Blair]

I got this Marsh in on trade for a preamp, so I decided I should put a tube in it somewhere. You can see that there is very little room in there. The tube is a 12AU7, and I'm running the filaments on 12VDC off a rectified 9VAC @ 400mA transformer (the smaller one). Then I am feeding a 12V transformer backwards off that for B+. The first transformer, after a few hours is pretty warm. Not ridiculous, but still pretty warm.

So, the 12VDC is generated by the 400mA transformer using 150mA. What kind of current is required to operate the second transformer backwards? I have it fused with a .5A fuse on the primary side of the small transformer. It has run all day, and going strong. The 12AU7 is a simple cathode follower.

Think it is OK?

Thanks!

Blair

[Blair]

[Geek]

It's easier to think it all out by using overall draw compared to first transformer VA rating. Always overrate the first transformer by 50%, especially if you're enclosing it all in a poorly ventillated cab.

Cheers!

[Blair]

Thanks, Gregg!

I'm at over 50% being a 400mA transformer and the filaments are using 150mA. The 400mA transformer seems to need to be a touch larger though. It gets hot. Not ST-70 hot, but hotter than I prefer when tucked into a chassis.

Blair

[Geek]

If you have a spec sheet for the transformer (or can find one), what degree C. over ambient is it rated? Most are at least 30C. It might be hot, but OK.

Cheers!