From claire_trikes_aust@iprimus.com.au Mon Dec 29 19:02:06 2003
Subject:Re: Tapping Into Leslie 860 with Hi Fi Tube Preamp?

Mark (?),
Your idea sounds quite feasible. I am not familiar with the Eico tube
preamp but I can shed some light on the Leslie 860.

The 860 *is* bi-amped as you say, but the active crossover network is
before the two Power Amplifiers (as you would expect). No doubt Leslie has
specifically matched the cross-over frequency with the drivers and other
idiosyncrasies of the amps, rotors and boxes. Unless you're deep into this,
you generally wouldn't want to make changes in this area.

Consider the 860 as a mono device. It has a single mono input it's best to
treat it that way. It would be imprudent to consider the separate
amplifiers and bass/treble speakers/rotors as being akin to stereo.

The simple answer to your question is to take the 1/4" output from the VK7,
feed it into *one channel* of your valve preamp and feed the valve preamp
signal into the Leslie. You may need to do some fiddling with signal levels
to get the level of gain and warmth (distortion) you desire. Choose to use
one channel of the VK7, or connect them together (may need resistors or
small mixer) to form a mono feed based on taste/sound.

The two gain controls on the back panel of the 860 (one for the treble rotor
and one for the bass) are internally between the active crossover and the
power-amps so you may need to be judicious with the output level of the
valve pre-amp. i.e. if the valve preamp output level is too high you may
overdrive the active crossover input stage in the 860 even though you have
the gain controls quite low with subsequent low sound level.....

Note that Leslie's are typically designed to be fed from a 'speaker size'
source impedance (eg 8 or 16ohms). You may or may not need to 'load' the
valve preamp output to a suitable impedance. 600 ohms would be a good place
to start. (Not too high, not too low, and an industry standard for balanced
and transmission line feeds.)

Have you figured out an elegant way to get the output signal from the valve
preamp into the 11 pin connector for the Leslie (while leaving the earth
switched motor control leads intact ?).

As an aside, if you really want to take advantage of the stereo output of
the VK7 and the unused channel of the valve preamp you will need another
power amp and speaker (box). I have seen this idea (- one 'side' leslie,
one 'side' straight) used to great effect with the fixed speaker physically
facing 'into' or 'through' the leslie. The intermodulation effects
(amplitude and frequency) can really tend to enhance the leslie effect if
the levels are managed appropriately. And the fixed speaker helps to
retains some 'bite' for the top end, and percussion......

(P.S. If you are in Oz I'd consider coming and wiring it up for you.....)

Claire
Australia