Too much heater current?

[tipetu]

Hi, I`m building a stereo SE tube amp. I`ve got one 12AX7 and two EL84.

I noticed my transformer (Hammond 370X) only puts out 1.5A on the 6.3V winding.

Just enough for the pair of EL84 heaters! (2*0,76A)

What about the 12AX7? It draws 0.3A.

It`s only 300mA. Not that bad right? I was hoping to connect a pilot light too.



The tranny is not hooked up yet. Nor the OPT`s

[WaltC]

depends on what "fudge factor" is built in to the PT. I'd get one with 2.0A minimum on the 6.3V tap (I'd probably for me get 3A just to be safe and figuring I might want to add something later on <G>).

[tipetu]

True. Too bad I`ve already bought it and it`s on the way..

[SatelliteAmps]

Usually Hammonds filaments run high, so this might not be a bad thing. You want the voltage to hit 6.3v on the nose (or as close as possible). Hammond PT's I've can run as high as around 6.7v to 7v.

Hook it up without the pilot light, but with tubes, and see where the filament voltage is. If it's spot on, then you might need to run the pilot light off the 120v AC. If it is a tad high still, then run the pilot off the 6.3v. If it is low, Then you need to either return the power transformer and get another one, or run a separate filament supply. Most transformers are returnable if you don't cut the leads. Check with the supplier to find out ahead of time.

[tipetu]

Thanks for the reply. I got some room in the chassis so I`ll go for a tiny spare filament tranny! Hammond has got some neat tiny ones!

[gldtp99]

Here's a nice 105V-125V neon lamp assembly---available in red, amber, or green:
http://www.mouser.com/ProductDetail/...zJLZKUdA%3d%3d

[tipetu]

Thats neat. Only problem is we got 240V mains and my transformer has no 110V tap. Good idea though. Anyway, I'll go for a tiny fil-tranny. It would be ok to connect in parallel with the other filament winding on the power transformer right? As long as it is in phase and dont cancel eachother? I want to distribute the load on both transformers

[Keyser Soze]

Here you go.

http://www.mouser.com/ProductDetail/...311uoW97d7A%3d

But it might be cheaper to find something more local...

Edit: oops, that one is out of stock, try this

http://www.mouser.com/ProductDetail/...Dy6yrsF9zL4%3d

[SatelliteAmps]

Or you could just wire up an LED and a resistor. They only need about 10-20 mA of current.

[WaltC]

yeah, there used to be a big debate going around about Hammonds because they were all spec'd at 115 volts AC instead of the more common 120 (+) VAC. Because of that they all tended to run high on all the secondaries (of course <G>) and you had to deliberately under-buy or make other allowances for the different actual values.

I've not heard any more lately, so maybe they're starting to spec them at 120VAC. 'Course it sound like the OP may be starting from 220VAC on the primaries so everything I just said is NA, .... wouldn't be the first time <BG>.

[tipetu]

I`ll probably put a 5W resistor in there if it creeps up over 6.3V.

I have already pre-drilled the chassis and therefore I`ll have to stick with the 6.3V bulb.

I bought this little thing :
http://no.mouser.com/Search/ProductD...lkey546-266JB6

It fits excactly in the holes drilled for the OPTs`

The original 370X hammond will power one 12AX7+EL84 = 1,06A OK
The new tiny tranny will power one EL84 + Pilot = 0,91A OK

And btw I will not bother connecting the secondaries in parallel. It`s just more hazzle.

Thanks for the input!