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From: Mark Beckles Willson
Date: Fri, 27 Dec 1996 11:42:30 +0000
Subject: Amp line out

I have 3 basic techniques for "stealing" a line out from the speaker output
of valve amps, none of them original thinking. I have never been entirely
happy with this as a way of achieving small amp distortion at higher volume.
As far as I am concerned all this does is add another set of knobs to
twiddle but I am sure that it may suit others better=85=85

WARNING

Valve (tube) amps run at potentially lethal voltages, some of which can be
present internally for a considerable time after the amp has been switched off.

The simplest method, borrowed from Gerald Weber, is easiest to install on a
Fender with an extension speaker jack:-

Replace the lead from the main speaker jack to the extension with a 2.2K
ohm, =BD watt resistor.

Bridge the hot and ground terminals of the extension speaker jack with a 100
ohm, =BD watt resistor if you are running your amp into 8 ohm (Deluxe,
Princeton etc). Use a 400 ohm resistor if you are running the amp into 4
ohm (Tweed Champ etc).

If there is a feedback loop leading from the extension jack, back to the
front end of your power amp, move this from here to the same connection on
the main speaker output jack.

The extension speaker jack is now a line out.

If you don't have an extension speaker jack you can add another jack socket
to the amp if you don't mind drilling the chassis. Alternatively you can
add the circuit across the terminals of the speaker with a line jack socket.
This is easily removable if you don't like the sound or want to sell your
vintage amp in unmodified condition.

It may be worth trying this circuit with hot and ground reversed if you
aren't happy with it - it is important that the speakers of the two amps are
all moving together rather than one set moving forward and the other back.

Bruce Zinky of Fender's circuit is similar but has a 560 ohm resistor in
the
jack's hot connection, 100 ohm across hot and ground and 22 ohm between
the
ground terminal and chassis ground. I have never tried this and can't
quite
work out how it works but BZ certainly knows what he is doing.


I also have a couple of circuits for line out boxes/circuits which are
similar to MC's but slightly fancier.

Feel free to contact me if you need any further information.

Mark