output transformers

[jestaudio]

Can some explain in laymans terms without getting hugely technical the limiting factors with output traffos, ie, if i use a OT rated at 15 watt with kt66,s will it still limit to 15 watts asuming that the amp is cathode biased and the voltages are standard for say a SE build or would i need to step the transformer up to say a 30 watt unit,
The reason for asking is i,ve all ready blown one OT( silly mistake i won,t repeat) and need to get a idea of wht i can do or not. Indepth tecnical descriptions don,t make a huge amount of sense hence me asking the experts

[Tillydog]

OK I'll try:

All transformers work because they can store the power coming in as magnetic energy in the core.

A given weight of any core material can only store so much magnetic energy before it becomes "full" (i.e. it saturates).

The power rating of a transformer is a reflection on how much magnetic energy it can safely store (taking into account all sorts of factors). To get more power through it, you need a bigger magnetic store i.e. a heavier core, or a different core material (but mostly a heavier core).

So what happens if you try and push more power through a transformer than the core can handle (I hear you ask )?

The transformer winding stops acting like an inductor and starts acting like a low value resistor.

So your (say) 4000 ohm impedance load on the power valve(tube) drops to a few tens or hundreds of ohms and big currents can flow, overloading the circuit and/or burning out windings.

You get various non-linear effects as the core material approaches its magnetic capacity which vary by material (hence the mojo factor), but once the core reaches saturation the change is very abrupt.

Transformers aren't self limiting - if you try to push to much through them they just give up.

Any use?

Andy

[martin manning]

Ok, in a nutshell, the transformer converts voltage and current in the primary into a magnetic field which is transferred through the core to the secondary where it is converted back into voltage and current. Each of these three elements has its limitations: The wire in the windings is sized to handle an amount of voltage and current consistent with the overall power rating of the transformer, and similarly, the core is sized to handle a certain level of magnetic flux density. If that level of flux density is exceeded the core is said to saturate, and it will no longer be capable of linear (undistorted) transfer of signal from the primary to the secondary. Worse, when the core saturates the primary inductance will drop, making it look more like a short circuit than a proper load. This will result in very high current flow, causing resistive heating of the winding and eventual destruction of the transformer. So in short, you need to get a transformer that is capable of handling the output power that the KT66's are expected to produce.

Andy, you were a bit quicker than me, but here's my version anyway.

[jestaudio]

Ok, in a nutshell, the transformer converts voltage and current in the primary into a magnetic field which is transferred through the core to the secondary where it is converted back into voltage and current. Each of these three elements has its limitations: The wire in the windings is sized to handle an amount of voltage and current consistent with the overall power rating of the transformer, and similarly, the core is sized to handle a certain level of magnetic flux density. If that level of flux density is exceeded the core is said to saturate, and it will no longer be capable of linear (undistorted) transfer of signal from the primary to the secondary. Worse, when the core saturates the primary inductance will drop, making it look more like a short circuit than a proper load. This will result in very high current flow, causing resistive heating of the winding and eventual destruction of the transformer. So in short, you need to get a transformer that is capable of handling the output power that the KT66's are expected to produce.

Andy, you were a bit quicker than me, but here's my version anyway.

So in short, know the kt66 is similar in output to a 6l6 i can expect 30 watt with a 300 plate volatge in comparison to 16 watt (in pp) for the 6v6 therfore the transfromer must match the rateing of the tube

[martin manning]

It's not clear to me whether you are doing a SE or PP amp, or contemplating using a PP transformer for SE. SE transformers are built differently, having a small gap in the magnetic circuit (the core), so you don't want to use a PP transformer for SE. But yes, you need to get the right type and the appropriate power rating based on the tube type and operating voltage. Fixed or cathode bias will make a small difference, since with cathode bias the power output at a given HT is a little less.

[Andy Le Blanc]

the power transformer will only give you so much juice and the output transformer will only handle so much juice.

if you used a 15w opt with a 50w pt and tried to run 50w thru it you'd melt the wax out of the opt,
if you had a 50 w opt and a 15w pt you wont get more watts than the pt can supply and you'd cook the pt trying.

but its really the power tranny that limits the watts, and usually you can tell the power handling of a transformer by the physical size and weight,
if you cooking the opt get a bigger heavier one next time, but at the same time do the math so you know how many watts your asking the opt to handle

look at the power supply rating in milliamps at the rated voltage

say a power tranny is supposed to handle 200ma at 450v, assume a bias around half the milliamps.... .100 x 450v = 45w or around 50w in round figures, you need to convert ma. to amps,
move the decimal over to do the math, so you want to chose a opt in the neighborhood of at least 50w to go with the pt


if you can put a 1 ohm resistor in the cathode circuit to measure ma. over and it will give you enough info to figure the watts in your power side