Help with 710 amplifier output speaker impedence

[PeterK]

I am in the middle of a 710 conversion. So far I have installed an active crossover from HammondWorks to eliminate all but the 16 ohm V21 and the 4ohm 15" bass driver. The crossover has two inputs which are interally summed by the crossover which separates the freqs to the "proper" amp (treble or bass). I also built a fancy foot switch controller (another post with photos coming).

And now I will be removing the lower shelf and installing a 122 style shelf from Valhalla Woodworking.

But my questions is about the rotary (treble) and stationary (bass) amplifiers.

Does anyone have a spec to state the expected speaker impedence of each amp?

I am wondering if I should add a 4ohm load resistor to the 15" to bring it to 8 ohms. I am afraid of overdriving/heating the bass amp if I leave it at 4 ohms. The original design had two 6x9s and rotosonic which I have removed. And is a 4ohm wire wrapped ceramic resistor, is this the correct type and which wattage resistor (20, 30watt?) should I chose?



btw- my fancy footswitch project came out really well. Latching footswitch as well as an input for a modified 1/2 moon or a momentary fast footswitch.

[Momboc]

Scans of the Service Manual for a 710 can be downloaded from here:

http://www.captain-foldback.com/Lesl...ie_manuals.htm


Speaker impedances are in the Specs on page 8.


Schematics are here:

http://www.captain-foldback.com/Lesl...schematics.htm

[PeterK]

Got that thanks. But it wasn't my question.

What I am looking for is the AMPLIFIER side expected speaker impedence. Most amps can drive various loads and are designed as such. But too low an impedence , say 2 ohms, would burn out most amplifiers from overdriving and overheating.

I would like to find the design spec of the 710/760 bass amp that states the expected speaker impedence load. I am connecting directly to the 4 ohm 15" and bypassing the original crossover completely. So my 710 now has the same setup as a 760 only with a upgraded crossover.

Does anyone have a spec to state the expected speaker impedence of each amp? Maybe a power spec stating 50 wpc RMS in to x.x ohms ?

[sfp1954]

Most of the 15" speakers used in the Leslies were 16 ohms.

[PeterK]

Especially true for tube amps but solid-state amps in 710s and 760s use a 4 ohm 15" speaker as well as a 16 ohm treble driver. Others too.

[Momboc]

I understand that you are converting a 710 into a 770 by (1) removing the rotosonic unit and replacing it with a 122/147 type bottom rotor, (2) removing the 6x9 stationary speakers, (3) adding an active crossover before the two amps and converting it from 2 channels into a bi-amped arrangement by (4) using the 'stationary' channel amp as the 'bass' amp and (5) using the 'rotary' channel amp as the 'treble' amp.

The relevant service manuals don't refer to minimum load impedance for the amplifiers. However if you compare the factory configured 760/770 amplifier & speaker configurations with your modified 710 you will see that they are the same.

The image below is the amplifier schematic from the 760/770.



In the 760/770 the 4ohm 15" bass speaker is connected directly to the bass amp's output, and the rotary horn is connected directly to the treble amp. Also you will see that the 760/770 amplifiers are identical to those used in the 710 as per the schematic below.



If you download the respective service manuals you can see that the rotary horn and bass speakers in the 710 have the same part numbers as those in the 770.

All you need to do is to reconfigure the 710 speaker wiring to match that of the 760/770. The only speaker side wiring/component differences is that the 710 has a 3.2 milliHenry choke/inductor (acting as a low pass filter) in series with the bass speaker which becomes redundant and should be bypassed when the amp is only fed bass from the crossover and the stationary 6x9s are removed, and the treble horn's series resistor (R111) is 2.2ohms at 10watts in the 710 but is 6.8 ohms in the 770, and the series capacitor C80 (acting as a high pass filter) can be by bypassed.

So if directly driving the 4 ohm bass speaker and the treble horn in a bi-amped configuration works for every 760 or 770 without damaging the amps, then you can emulate this arrangement and it will work for your converted 710.

Hope this helps.

[PeterK]

Thank you! I've been on the same track, reading service manuals and schematics and came to the same conclusion, that being the same as the 760/770 internally except for the crossover, I'll be OK with the 4ohm bass driver and 16 ohm upper.

So next is removing the speaker shelf and installing a new one from Walhalla.

I'll add photos when I'm done if it could help anyone else.