Transformers

[TIMBO]

Hi guys, I can buy these transformers locally, but when its comes to the technical info i'm a bit lost. They talk about a VA rating and other stuff. These are the trannys and are either useful. On the spec of the auto trans it does not say but they also have 220v and 240v ac tap. Thanks

[stratele52]

let show you ;

100VA / 120 volts AC = 0.83 amps

100VA / 240 volts AC = 0.41 amps

How many amps do you need ? What for ?

[panhead]

If you're thinking about using the autotransfomer as a power transformer, keep in mind that an autotransformer has one continuous winding with taps, so it doesn't offer any electrical isolation, which can be hazardous to your health.

[kagliostro]

+ 1 for Panhead

other hand you can use safely isolating transformers which purpose is in fact safety

you can also put together the two things and use an isolating transformer (the primary) connected to the line and the secondary connected to the autotransformer to obtain different voltages

but this will be a suboptimal solution due to the weight and space, however if the transformers were for free ...............

Kagliostro

[PRR]

The autotransformer is not useful for amplifier supply.

The range of  isolation transformers is vast but only goes to 55-0-55 or 110V. If you want a 150V DC supply this is cool. A 300V DC supply can be done with a doubler. Higher voltages are possible but unconventional.

VA is just volts times amps. Look-up the tube(s) you will use. Since none of these are 6V winding, you will need a separate heater transformer (you can design for 12V heaters and use the 12V job). So just look at plate circuit demand.

Two 6V6 can be run at about 300V and (including screens, driver, preamps) about 100mA which is 0.1A. Multiply: 300V*0.1A is 30VA. Since there is an AC/DC conversion, and probably a multiplier, round way up. I prefer "double" which suggests 60VA. The real factor is 1.6 to 1.8, which suggests 50VA is pretty close. The 100VA is little more weight and may be similar cost for less sag, worth considering.

[TIMBO]

Thanks guys, Their list of transformers (isolating) say that they have a 50VA, 100VA and 200VA with the 230v output. Why these ones only have the single output and the others are centre tapped i don't know, but i was hoping that i could use these (230v x 1.4 = 322v)as a good supply for a single ender.100VA is about $60 and a heater trans about $30 and if i was to get one (hammond with taps)it would cost about $130 including shipping.So if these are able to be used in future builds that would be great.Thanks

[PRR]

> supply for a single ender.100VA is about $60

100VA on the AC side is about 60 Watts of available DC.

In a basic single-ended amp, you might waste 5%-10% in filtering and 5%-10% in bias, leaving 85% to cook the tube.

85% of 60W is 51 Watts available to cook the tube.

That is a BIG single-end amp.

No common single tube will stand 51W plate dissipation reliably.

51W in the bottle suggests 20W output, and a 20W SE OT alone weighs and costs like all the iron (OT and PT) for a push-pull 20W DeLuxe.

I'd ponder the 50VA. 25W tube dissipation suits 6L6 EL34 and friends, can make 10W output, and that's still a plenty big SE project.

Have you chatted on the Autralian Guitar Gear forum? Those guys may know the best local deals. Darryl has posted both here and there with some clever PT ideas.

[TIMBO]

Thanks PRR 

[TIMBO]

Hi Guys,I tried to absorb all that info and thought i'd better get out the calculator and crunch some numbers  I also found some info in a local mag that was very helpful.

As said, i can buy some isolating transformers locally but the only info was that they were rated @ 50VA,100VA and 200VA with an output of 230v AC.

Because they do not have a centre tap i am able to use a bridge rectifier and now i understand that 50VA can mean a few things so i've added a schem to show my calculations for the 50VA tranny.

Please check my calculations.

Tubenit has posted another great amp build (So-Low Watt ECL84)that i think this TRANSFORMER would be great, as he was not wanting to exceed 65mA current draw. Thanks and Happy New Year to all