Making a Leslie 147 Valve Amplifier

I had been hoping to buy for a 147 amplifier in distressed condition so that I could renovate it. Two appeared on Ebay and I bid but they went for much more than I was prepared to pay, given the changes that would be needed to fit into my ex-710 Leslie so I made one. Here it is.

Passive Crossover for a Leslie Speaker

 

This article describes the process of designing and constructing a passive crossover for a Leslie speaker. Passive crossovers were installed in the classic Leslies: 147, 145, 122 etc. and in most other two-way and three-way loudspeakers. The alternative is an active crossover but this requires multiple power amplifiers. The drive units employed for this design are from a Leslie X77L and would seem to be similar to those used in the 710, 760 etc. which do use active crossovers. The impedance of the bass unit is a nominal 4 ohms and differs from the classic Leslies; those were 16 ohm but the horn drivers would appear identical. The differing bass driver means that the classic Leslie crossover cannot be used. Nevertheless this design is in most other respect similar to the original.

Transforming a Leslie 710 into a 770

This article describes the conversion of a 710 Rotosonic Leslie into a 770 model. The modification involves:

    • Making a new bass rotor drum
    • Making a bass rotor upper bearing support strap
    • Moving the bass rotor lower bearing position
    • Making and fitting a new shelf
    • modifying the lower motor to operate the other way up (to make space for a future valve amplifier)
    • Making an electronic crossover
    • Repairing and refinishing the case
 

 

The Leslie 710

Stripped Carcase

The 710 is a Rotosonic Leslie whereby the lower rotor houses a spinning elliptical speaker. The upper rotor is a conventional treble horn. Two static elliptical speakers and a 15 inch bass driver complete the line up. The amplification is provided by two 50 W transistor amplifiers. Rotosonic Leslies are not considered as desirable as those with a rotating drum beneath the 15 inch bass driver. I bought mine from Ebay as 'spares or repair' for about £70. Apart from it "beat-up" state, the only fault I have found is an open circuit voice coil on the rotating speaker. I suspect this is a common fault in these models; this speaker should not receive low frequencies but there is no internal filter to protect it and connecting it to the wrong organ will direct damaging low frequencies to the small rotating speaker.

 

The image shows the part-stripped carcase. The bottom shelf has yet to come out.

Active Crossover


completed_pcb

 

 

Here is the completed crossover PCB. It is constructed more or less to the original circuit diagram of the 760/770 crossover board. Before building,I redrew the circuit in LTspice and checked its behaviour. This also provided a means of generating the netlist. After making the board, I tested it by injecting a swept sine wave signal, 20 hz to 20 khz into the input, whilst monitoring the output using the freeware HOLMImpulse . This programme is designed for measuring the impulse response of a room but proved excellent in this role too.

The MIG II Preamplifier

This is an add-on for a Leslie 760 or similar speaker. It was designed and marketed by Keyboard Partner and is meant to make a transistor Leslie sound like a valve Leslie. It is no longer available because of the availability of the Russian miniature pentode valve. I have such a valve so I made my own MIG II preamplifier using the circuit diagram supplied by Keyboard Partner as a starting point. All credit goes to Keyboard Partner for an excellent design and I hope I have not caused offence by reproducing the design for my own use.