Can you please explain my meter reading ?

[Colas LeGrippa]

Hi everybody, I'll be short and sweet. I started to bake my own bread and my wife is anxious to eat my baguette.
Ok to the point now.
I have made another nice sounding 50w guitar amplifier, el34 cathode bias.
The HT is fused on the center tap of the power tranny and to test the beast b4 it goes on the road I have put a Am meter in series with the HT fuse holder to check the current changes over a long period of time, more or less a stability test. What I don' t understand is this: ac current of 175 mA
 is surfing over a dc currentof 100mA ( more or less the sum of the power tubes K dc current.


Now, what size of fuse should I use ?  The sum of both currents plus a security margin ?


Omg my bread starts to smell


Thnx


Colas

[sluckey]

I'd use a 1/2A Fast fuse.

[Colas LeGrippa]

So the 500mA fuse corresponds to the sum of both currents x 2 more or less is that how we must calculate ?
You are more an expert than me in current calculationI am pretty sure  (but not in the bread baking though..


Thanx buddy

[sluckey]

I thought you needed a fast answer.

1/2A Fast is a typical value I've seen used in 50W EL34 amps.

[Colas LeGrippa]

Yeah LOL
I TRIED A 250 mA it blew after a couple of sb switch throws
My meter showed a 130mA dC surge on powerup but we must add the ac current surge wich totalize over 300m A .  So a 1/2 A fuse is good as a matter of fact works fine on powerup and sb operation.
I was just surprised to see both ac and dc currents appearing on the pt center tap and to realize that the rectifier has an influence on the previous stage !!!!! After all a power tranny has both it s outputs working on ac not dc.


Colas Coca

[Colas LeGrippa]

Correction : a pt has both it s input and ouput working on ac.

[PRR]

If it is working OK the load is much less than 0.5A.

If something shorts out the load will be MUCH more than 0.5A.

[Colas LeGrippa]

Yeah I know. What I was surprised about is to find both ac and dc current at the pt center tap

[sluckey]

What you have at the PT center tap is all dc. But it's pulsating dc that has not been filtered very well. Most meters think that is ac.

[Colas LeGrippa]

Gee, I oughta write a letter to Mr Fluke about his shitty multimeter




Ok I understand your point sluckey but why my meter shows 175mA ac and 100 mA dc ?

[vampwizzard]

the different values likely has to do more with how the meter is measuring the current. youre almost a perfect square root of 3 different.. tell tale sign.

Fuse wise, 125% overcurrent protection is standard for transformers and i usually round up if its inbetween normal values. The term for that DC spike is "inrush" current. It sucks but is a necessary accommodation.

[PRR]

> why my meter shows 175mA ac and 100 mA dc ?

Go down by the river. Where it meets the sea.

The river runs to the sea. Steady (direct) current.

The sea rises and falls with the tide. Alternating current.

They can both be in the same piece of water, or circuit.

[shooter]

Go down by the river. Where it meets the sea.

DO NOT SWIM there !! 

[Colas LeGrippa]

Exactly like the guitar ac signal is taken from the plate dc voltage by the mean of coupling caps