Warning! Vacuum tubes, transformers, capacitors, and amplification circuits
described on these pages operate at high voltages that
can cause permanent injury, disability or death. Vacuum tubes operate at
high temperatures that can cause severe burns. Never attempt to repair,
construct, alter, test, work on or touch electronic equipment unless
you are **trained or otherwise qualified** to do so. Likewise,
never open or remove a protective cover from electronic equipment
unless you are trained or otherwise qualified to do so.
This site is for reference only and is not meant for instruction,
training, or to provide guidance to someone who desires to build,
repair, or otherwise touch electronic equipment.

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A **Line Out** jack on an amplifier provides a versatile way to send
the amp's output to a recording console, a PA system, or another
amplifier.
Some amps put the **Line Out** between the pre-amp stage and the
power section, but the magic tone of some amps come from the
phase inverter, power tubes, or output transformer, and such
a **Line Out** bypasses those. Plus, adding a **Line Out** in the pre-amp
often requires a dozen or more components, which may possibly
include another triode.

The simple way to do add a **Line Out** is with two resistors and
a output jack:

**R1** and **R2** need to be high enough values to not change the overall
load on the speaker. R1 and R2 in serial connection form a voltage
divider at their junction (the line-out jack), thus attenuating the
output signal down to a voltage that is acceptable to use as an input
to another amp, a recording console, or a PA.

Choose the values of R1 and R2 such that the sum total is 2K or higher,
and that the voltage divider attenuates the signal down to a voltage
that is acceptable to send to a mic input, or another amp. A 5W amp's
speaker voltage is much lower than what you'd have with a 100W amp.
If the desired **Line Out** voltage is 1V for either 5W or 100W amps, you'll
need different voltage dividers for either one. If you aren't sure,
to can always make it *variable* (see below).

The **output impedance** of the **Line Out** will more or less be set by
the value of R2. 100ohm is a good versatile lower bounds of impedance,
but up to 300ohm should be okay for recording devices or PA's.

A Variable output is an simple as making R2 be a Potentiometer, with
the center terminal being the **Line Out** signal (below). R1 + P1 values
should still be 2K or greater. You might decide what your lowest
attenuation might be, and put a *lower bounds* resistor under the
pot, such that at the lowest setting, the voltage divider ratio isn't
100:0.

If this looks like voodoo, or somehow would tamper with the speaker, consider that any Negative Feedback (NFB) loop or presence control of an amplifier is this exact same circuit, except, instead of an output jack, it is connected to the cathode of a pre-amp stage. Below is a common power section of an amplifier with a NFB circuit. To help illustrate, all but the NFB and speaker circuits are greyed-out. The resistor R11 serves two purposes: it is the cathode resistor for the triode, and it is the "R2" resistor in the R1/R2 voltage divider pair of the NFB circuit.

Below, I've moved the voltage divider resistor pair to the other side
of the schematic to show how it is exactly the same as the **Line Out**
circuit at the top of the page.

It is most certainly the case that adding a **Line Out**, or an NFB circuit
has some effect on the load of the output transformer, but if the overall
resistance of R1+R2 is more than 2K, the effect is so tiny, its not really
worth doing all the math. "load impact = insignificant".

*Still not satisfied?*

The **Line Out** R1+R2 pair is a impedance device parallel to the speaker.
for resistors, **DC resistance = AC impedance**. So the impedance of
the **Line Out** pair is R1+R2. Simple parallel impedance math is the
same as parallel resistance:

Z= (Z1 * Z2) / (Z1 + Z2 )

Give it some real world values, like the ones in the example amplifier NFB circuit above:

Z1=16

Z2=22K + 5.6K

Z=( 16 * 22000 ) / 22016

Z=15.98

So adding the NFB changed the impedance that the transformer sees from
16 to 15.98. The same calculation for a **Line Out** R1+R2 pair of 2K
changes that impedance to 15.87.

The resistors at left are way over-sized, from a power handling perspective.

If your speaker is rated at 25 watts, the **Line Out** resistors can
be selected with power rating to match that.
Use Ohm's law to calculate
a suitable wattage rating:

*Ohm's Law:*
V=I×R

P=V×I

*Where:*
I = current (amps)

P = power (watts)

R = resistance

V = voltage

I barely remember enough high school algebra to take
*V=I×R* and
*P=V×I* and make this leap:

** I=V÷R**
* and *
** P=V×I**

So if you plug some numbers into that forumla, like 25W (P) and 16ohm (R) for the speaker, you'll come up with 20 volts (V).

Plug that 20v and the
**Line Out** 2K resistance into **P=V×V÷R**
and you've got 0.2watts.
This shows that ¼watt resistors are minimum. Plug in your
numbers, but for a 100W amp, **Line Out** resistors rated
at ½watt should be fine.

The following images show a small pedal size universal **Line Out**
device, that allows you to tack on the **Line Out** resistors to any
speaker connection that uses a 1/4" mono plug. Although physical
device is placed in between the amp and the speaker, we know that
the circuit "hangs off the side" and places the speaker
and the the **Line Out** *in parallel,* not *in serial*.
The pot provides a variable attenuation, with 2.2K:150ohm being
the minimum voltage divider, and 27K:150ohm being the maximum
voltage divider.

Vintage 47 has a nice example of this type of box that you can purchase.

If you have questions, or you've found an error contact me

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