back to back transformers

[jimmy]

hi

with the back to back transformers for heater and B+ voltages, do you need to use two of the same transformer, or can you use different transformers, as long as they both have the 6.3v windings (that are joined together) ??

thanks
Jim

[lightningfingers]

you can use different transformers, just make sure the 1st one (the one connected to the mains) can handle the valve heaters and the 2nd transformer.[/list]

[Ansil]

in doing this too make sure that the second one can handle the current from the first too..  or you can fry the second one with too much current.

i have had better luck too with using two small ma ones for the b+ and one larger one for the heaters. i also use 12v heaters too as it is a bit easier. and i like them better.

depending on what you need mouser makes a nice flat pack transformer that has 230v @ 20ma  for like 12 bucks each price drops as you buy more.

[jimmy]

if i were to use a 2A transformer for the heaters back to back with a 500ma one for the b+ would it be alright? how much current can the second one handle before it gets fried?

cheers
Jim

[puretube]

...the 2nd one doesn`t "handle" current, but draws...

[jimmy]

ooooooooh right.  so most power transformers have a 1A fuse on them, so they dont draw to much current? so as long as theres less then that getting drawn by the second transformer itll be alright.

so if i was to use it to power a champ with a SS rectifier, id need 0.45A for the 6V6 and 0.3A for the 12AX7, total 0.75A, and used a 1A transformer all would be well? or should i use a 2A transformer in case i want to add some more tubes later?

thanks
Jim

[Regan]

Jimmy,
Your only taking into consideration the 6.3v heater current- your not taking into account the current load from the step up transformer supplying the plates. you have to figure a few mA (say 5mA) at high voltage for the 12ax7, then maybe 50mA max for the 6v6 to be safe. So thats 55mA draw at high voltage, now multiply that by 20 and you have approximately the current being drawn from the first transformer for the high voltage. I'm multiplying by 20 because current and voltage are inversely proportional and its appr. 20x6.3=120v.
Adding them together and your needing near 2A for your first transformer, and the 20x55mA=1.1A for your second transformer-I overrated the draw so you would probably be safe with a 1A transformer.
You would be much better off to use a standard transformer- it would take much less room.
Regan

[jimmy]

ok, i didnt quiiiiiite follow all of that. i got the bit where you said ill need 1.1A and 2A for the transformers. i agree, a standard transformer would be WAY better, but i just cant afford them. this way, i can spend 30 bucks and get all the transformer i need! and im folding the chassis myself, so ill account for the extra space. thanks alot for your help

thanks
Jim

[lightningfingers]

You know whats easier? Parallel transformers. One for the heaters, one for B+. That way you can buy smaller transformers, and needn't worry as much about the effect they have on each other.

[jimmy]

hey now thats a good idea! except ive never seen a step up transformer in my local area. what a pity.

thanks
Jim