McIntosh MC 250 Mono Mode

[52 Bill]

I have a customer that wants to run his MC 250 in mono mode with a 4 ohm cabinet. The online owner's manual (for a later version) sez set the mode switch to mono, jumper the two 8 ohm output taps and use the common and the 8 ohm tap as the output to the 4 ohm load.

The problem comes in that his amp is one of a series that has a circuit where the mode switch internally connects the two 4 ohm autotransformer taps when in mono mode. Does anybody have the correct mono mode speaker load connection for this version of the amp?

The owner tried using the jumper from 8 ohm to 8 ohm taps, but the amp starts to overheat and has reduced power output.

[g1]

I believe this should be the schematic on pg.5
Specified for ser.# above 23L48 if that helps find the correct owners manual.

[The Dude]

Maybe just remove the internal transformer jumper and use external- or move the internal jumper to the appropriate impedance taps?

[g1]

Would it work if you throw the switch, then jumper 4ohm to ground? Tie the 8's together for use with 8 ohm load?
All that so you can use the name of the tap your are actually going to use? (except 2 ohm load?)
Or is that just too wacky.

[J M Fahey]

Would it work if you throw the switch, then jumper 4ohm to ground? Tie the 8's together for use with 8 ohm load?
All that so you can use the name of the tap your are actually going to use? (except 2 ohm load?)
Or is that just too wacky.

That will short the output transformers.

I have a customer that wants to run his MC 250 in mono mode with a 4 ohm cabinet. The online owner's manual (for a later version) sez set the mode switch to mono, jumper the two 8 ohm output taps and use the common and the 8 ohm tap as the output to the 4 ohm load.

The problem comes in that his amp is one of a series that has a circuit where the mode switch internally connects the two 4 ohm autotransformer taps when in mono mode. Does anybody have the correct mono mode speaker load connection for this version of the amp?

The owner tried using the jumper from 8 ohm to 8 ohm taps, but the amp starts to overheat and has reduced power output.

Ok, but DID he also set Mode switch to Mono? Somehow I doubt it.

IF 4 ohm taps can be safely paralleled in that mode THEN 8 ohm taps can also AS LONG AS SWITCH IS SET TO MONO

EDIT: just thinking aloud: why does your customer try to use such an old boat anchor?

It was revolutionary in 1966 .... now is completely outdated and outclassed, any 70īs on amplifier, including any Peavey for example, matches or beats it and to boot can drive 4 ohm loads without any problem ... nor need for an autotransformer.

Of course, you can brag about the label and the exotic looks.

[52 Bill]

Ok, but DID he also set Mode switch to Mono? Somehow I doubt it.

Yes, but with the switch connecting the two 4 ohm taps together internally and the outside jumper connecting of the two 8 ohm taps, I assume that the shorted turns of the transformers caused the amp to overheat.

Apparently this is the type of amp that Jerry Garcia used for a time driven by an Alembic modified Fender Twin preamp.

I will try removing the internal switch connection and just use the jumper on the 8 ohm taps. I just figured that it came from the factory with the internal switching, so somebody might know what the recommended way to connect the speaker was.

[J M Fahey]

Yes, but with the switch connecting the two 4 ohm taps together internally and the outside jumper connecting of the two 8 ohm taps, I assume that the shorted turns of the transformers caused the amp to overheat.

I will try removing the internal switch connection and just use the jumper on the 8 ohm taps. I just figured that it came from the factory with the internal switching, so somebody might know what the recommended way to connect the speaker was.

No short if joining 4 ohm taps together or 8 ohm ones together, not even using 2 bridges to short (separately) 2 output pairs (4 to 4 and 8 to 8) since on each case you are joining exact same voltage points.

The amp puts out 50W per channel so you have (at just clipping) 14.2V on each 4 ohm tap; if both amps receive same signal, have perfectly matched gain (they spec 1% resistors at the NFB net) and transformers are wound exact same (easy peasy) then itīs safe to connect both 4 ohm taps, both 14.2V windings.

By the same token then itīs safe to interconnect 2 20V points, both 8 ohm taps together.

Now, if one of the amps does not receive drive signal, one tap is 20VAC, the other one is 0VAC so if switch is NOT set to Mono (or it fails) then both channels are shorted and overheat a lot.

Thatīs why I insist on checking that mono switch, starting on whether it was engaged or not.

[52 Bill]

No short if joining 4 ohm taps together or 8 ohm ones together, not even using 2 bridges to short (separately) 2 output pairs (4 to 4 and 8 to 8) since on each case you are joining exact same voltage points.

I guess I just don't understand these things. I would have thought that with the internal Commons and the internal 4 ohm taps and the external 8 ohm taps all connected at the same time there would basically be shorted (or nearly) turns.

[Helmholtz]

I guess I just don't understand these things. I would have thought that with the internal Commons and the internal 4 ohm taps and the external 8 ohm taps all connected at the same time there would basically be shorted (or nearly) turns.

No, connecting corresponding taps carrying equal voltages won't produce any negative effects and doesn't mean shorting turns.

Anyway, the most logical and safe measure is to rewire the internal stereo/mono switch to connect the 8 Ohm taps instead of the 4 Ohm ones.
The 8 Ohm taps provide the increased voltage and the paralleling the necessary current to produce 100W into a 4 Ohm load.

When rewiring the mono switch, please make sure that it actually bridges the inputs as well.

[Dave H]

I guess I just don't understand these things. I would have thought that with the internal Commons and the internal 4 ohm taps and the external 8 ohm taps all connected at the same time there would basically be shorted (or nearly) turns.

You should be able to jumper all the like named taps together and have it work OK (i.e. com to com, 4 to 4, 8 to 8, 16 to 16) as long as the two amps are driven by the same signal (switch set to mono). It just puts the transformers in parallel. It doesn't short any turns.

Edit: Simulpost