100V line amplifier output transformer query.

[philicorda[_9_]]

Would it be true to say that a valve amplifier with a 100v line output
only differs from a normal one by the amount of turns on the secondary of
the output transformer?

I have one that has multiple unlabelled taps on the output, which appear
to be wired in series at the moment, and I wondered if I could rewire it
rather than getting a new transformer.

[Scott Dorsey]

philicorda > wrote:
>Would it be true to say that a valve amplifier with a 100v line output
>only differs from a normal one by the amount of turns on the secondary of
>the output transformer?

Pretty much, yes. The feedback arrangement might be different also,
though. With a direct speaker output with one side tied to ground, you
can pull feedback off the speaker output and bring it back into the input
to linearize the transformer. With a floating output you can't do that,
so you wind up using an additional winding.

Consequently most amplifiers that have floating balanced windings for
constant-power distributed systems also have at least one secondary for
regular speakers, since it comes for free.

>I have one that has multiple unlabelled taps on the output, which appear
>to be wired in series at the moment, and I wondered if I could rewire it
>rather than getting a new transformer.

What is it? Get the documentation and see. Or connect a load across the
lowest tap, put signal through it, then measure how much it drops as you
increase the load and calculate the output Z that way.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Mark]

*Or connect a load across the
> lowest tap, put signal through it, then measure how much it drops as you
> increase the load and calculate the output Z that way.
> --scott
>
> --
> "C'est un Nagra. C'est suisse, et tres, tres precis."

the negative feedback will lower the apparent small signal output Z.
i.e. the neg feedback will try to hold the output voltage constant
against load changes.

Mark

[Scott Dorsey]

Mark > wrote:
> =A0Or connect a load across the
>> lowest tap, put signal through it, then measure how much it drops as you
>> increase the load and calculate the output Z that way.
>
>the negative feedback will lower the apparent small signal output Z.
>i.e. the neg feedback will try to hold the output voltage constant
>against load changes.

Yup. Even at close to rated power, you'll see the effective output Z
for most of the frequency range dropped by as much as three times. But
it's enough to let you know you're in the ballpark or not.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Engineer[_2_]]

On Mar 5, 12:44*pm, philicorda > wrote:
> Would it be true to say that a valve amplifier with a 100v line output
> only differs from a normal one by the amount of turns on the secondary of
> the output transformer?
>
> I have one that has multiple unlabelled taps on the output, which appear
> to be wired in series at the moment, and I wondered if I could rewire it
> rather than getting a new transformer.

Measure the transformer ratios. With amp off (of course!), put a
reference 100 VAC across the p-p primary and measure all voltages on
the secondary. Calculate all the ratios. Find the 4, 8 and 16 ohm
ones based on reflecting a credible p-p impedance to the tubes, e.g.
4K to 8K ohms. Impedances are proportional to ratio squared. You
should be able to find the 70 volt and 100 volt secondary windings
this way.
Hope this helps.
Cheers,
Roger