Help in choosing a PT

[Joseph Meditz]

I'd like to make a PP guitar amp of around 30W using a pair of EL34s.
The range of current is estimated to be from 175 to 225 mA. The
rectifier will be silicon. I have two candidates from Hammond:

1) 263CX 116VA 180-0-180 250 mA using a full wave bridge rectifier.
2) 273BX 182VA 350-0-350 175 mA using two half wave rectifiers.

The 263CX seems like a good choice. However, it does not have a 6.3
vac heater winding.

The 273BX has a 6.3 vac heater winding but would have to run above its
175 mA rating. But it has the higher VA rating. And when it is
putting out 175 mA, the average current on each side is 175/2 and
running cool, right? So, could the PT be safely pushed higher? If so,
how high? Could it output a total of 175sqrt(2) = 248 mA? Would it
saturate? In this case the current peaks on each side would exceed the
PT's rating, but the I^2R rating would not be exceeded.

TIA,

Joe

[John Byrns]

In article om>,
"Joseph Meditz" > wrote:

> I'd like to make a PP guitar amp of around 30W using a pair of EL34s.
> The range of current is estimated to be from 175 to 225 mA. The
> rectifier will be silicon. I have two candidates from Hammond:
>
> 1) 263CX 116VA 180-0-180 250 mA using a full wave bridge rectifier.
> 2) 273BX 182VA 350-0-350 175 mA using two half wave rectifiers.
>
> The 263CX seems like a good choice. However, it does not have a 6.3
> vac heater winding.
>
> The 273BX has a 6.3 vac heater winding but would have to run above its
> 175 mA rating. But it has the higher VA rating. And when it is
> putting out 175 mA, the average current on each side is 175/2 and
> running cool, right? So, could the PT be safely pushed higher? If so,
> how high? Could it output a total of 175sqrt(2) = 248 mA? Would it
> saturate? In this case the current peaks on each side would exceed the
> PT's rating, but the I^2R rating would not be exceeded.

My understanding is that transformers like these, those that are
designed for use with center tapped full wave rectifiers using two
diodes, are rated so that when the entire secondary winding is used with
with a full wave bridge rectifier, the current rating must be reduced so
that a 250 mA winding becomes something more like 177 mA when using a
full wave bridge rectifier. Also the current rating is generally higher
if a choke input filter is used.

Also it is my understanding that transformer saturation is controlled by
the primary voltage, not the secondary load current.

However I'm not a transformer guy so you should probably wait for one to
confirm all this.


Regards,

John Byrns

[Eeyore]

John Byrns wrote:

> My understanding is that transformers like these, those that are
> designed for use with center tapped full wave rectifiers using two
> diodes, are rated so that when the entire secondary winding is used with
> with a full wave bridge rectifier, the current rating must be reduced so
> that a 250 mA winding becomes something more like 177 mA when using a
> full wave bridge rectifier.

Actually, a full wave bridge uses the transformer more efficiently.


> Also the current rating is generally higher
> if a choke input filter is used.

Yes, slightly, by extending the conduction angle ( period ) which reduces the
rms value of the load current.


> Also it is my understanding that transformer saturation is controlled by
> the primary voltage, not the secondary load current.

It is indeed. Entirely so in fact.

Graham

[Ian Iveson]

John Byrns wrote

> ...Also it is my understanding that transformer saturation is
> controlled by
> the primary voltage, not the secondary load current.

Not directly.

It is useful to consider the primary winding as its inductance in
series with its winding resistance. In parallel with that inductance
is the reflected secondary load resistance in series with the
secondary winding resistance.

Saturation occurs when the *current* through the primary inductance
exceeds a certain value. It is true that for most transformers we
should be using, the peak current depends to a good approximation upon
the voltage across the primary, and the frequency.

However, as current is drawn from the secondary, it causes a voltage
drop across the primary inductance. For a given frequency, this
results in a drop in peak current.

Consequently, as more current is drawn from the secondary, saturation
becomes *less* likely.

cheers, Ian

[Ian Iveson]

Eeyore wrote

>> Also it is my understanding that transformer saturation is
>> controlled by
>> the primary voltage, not the secondary load current.
>
> It is indeed. Entirely so in fact.

Rubbish

Ian

[Eeyore]

Ian Iveson wrote:

> Eeyore wrote
>
> >> Also it is my understanding that transformer saturation is
> >> controlled by
> >> the primary voltage, not the secondary load current.
> >
> > It is indeed. Entirely so in fact.
>
> Rubbish

For all practical purposes its purely the volt-seconds that count.

Graham

[John Byrns]

In article >,
Eeyore > wrote:

> John Byrns wrote:
>
> > My understanding is that transformers like these, those that are
> > designed for use with center tapped full wave rectifiers using two
> > diodes, are rated so that when the entire secondary winding is used with
> > with a full wave bridge rectifier, the current rating must be reduced so
> > that a 250 mA winding becomes something more like 177 mA when using a
> > full wave bridge rectifier.
>
> Actually, a full wave bridge uses the transformer more efficiently.

Exactly, however that does not contradict what I said, the maximum
current rating is still reduced. The total power output capability of
the winding increases however because the output voltage approximately
doubles, while the current rating is reduced by less than a factor of
two.


Regards,

John Byrns

[Ian Iveson]

Eeyore wrote

>> >> Also it is my understanding that transformer saturation is
>> >> controlled by
>> >> the primary voltage, not the secondary load current.
>> >
>> > It is indeed. Entirely so in fact.
>>
>> Rubbish
>
> For all practical purposes its purely the volt-seconds that count.

"For all practical purposes" and "Entirely" are rather different.

Further, "for all practical purposes" is still wrong.

Some mains transformers saturate with no load.

Further, mains frequency varies from country to country. This is an
international group.

You may argue that your introduction of the "volt-second" covers this
second point. You are squirming and confusing the issue. What you
originally said was wrong and silly. Own up.

Ian

[Eeyore]

Ian Iveson wrote:

> Eeyore wrote
>
> >> >> Also it is my understanding that transformer saturation is
> >> >> controlled by
> >> >> the primary voltage, not the secondary load current.
> >> >
> >> > It is indeed. Entirely so in fact.
> >>
> >> Rubbish
> >
> > For all practical purposes its purely the volt-seconds that count.
>
> "For all practical purposes" and "Entirely" are rather different.
>
> Further, "for all practical purposes" is still wrong.
>
> Some mains transformers saturate with no load.

Such as ?


> Further, mains frequency varies from country to country. This is an
> international group.
>
> You may argue that your introduction of the "volt-second" covers this
> second point. You are squirming and confusing the issue. What you
> originally said was wrong and silly. Own up.

For all practical purposes it's right. There's no *need* to introduce a
more complex explanation and I doubt he wanted one.

I would hope that the OP is smart enough not to buy a 60Hz transformer
for use on 50Hz, this is covered by the volt-seconds issue btw.

Graham

[Ian Iveson]

You are not worth educating.

Your original statement:

>> Also it is my understanding that transformer saturation is
>> controlled by
>> the primary voltage, not the secondary load current.

***> It is indeed. Entirely so in fact.***

Is nonsense, as I pointed out.

It is not true that it is entirely controlled by primary voltage.
It is not true that it is entirely not controlled by the load current.

Anyone who believed you could make serious mistakes in circuit design
or transformer specification.

Now shut up and go away.

Ian

"Eeyore" > wrote in message
...
>
>
> Ian Iveson wrote:
>
>> Eeyore wrote
>>
>> >>
>> >> Rubbish
>> >
>> > For all practical purposes its purely the volt-seconds that
>> > count.
>>
>> "For all practical purposes" and "Entirely" are rather different.
>>
>> Further, "for all practical purposes" is still wrong.
>>
>> Some mains transformers saturate with no load.
>
> Such as ?
>
>
>> Further, mains frequency varies from country to country. This is an
>> international group.
>>
>> You may argue that your introduction of the "volt-second" covers
>> this
>> second point. You are squirming and confusing the issue. What you
>> originally said was wrong and silly. Own up.
>
> For all practical purposes it's right. There's no *need* to
> introduce a
> more complex explanation and I doubt he wanted one.
>
> I would hope that the OP is smart enough not to buy a 60Hz
> transformer
> for use on 50Hz, this is covered by the volt-seconds issue btw.
>
> Graham
>

[Eeyore]

Ian Iveson wrote:

> You are not worth educating.
>
> Your original statement:
>
> >> Also it is my understanding that transformer saturation is
> >> controlled by
> >> the primary voltage, not the secondary load current.
>
> ***> It is indeed. Entirely so in fact.***
>
> Is nonsense, as I pointed out.
>
> It is not true that it is entirely controlled by primary voltage.
> It is not true that it is entirely not controlled by the load current.
>
> Anyone who believed you could make serious mistakes in circuit design
> or transformer specification.

Utter nonsense.

It was a simplification but a valid one for the purposes of the discussion
i.e. secondary current is *not* the issue wrt saturation.

Graham