MC-240 preamp octal power socket info needed

[spkrman57]

I have a MC-240(1960 vintage) and would like to know if anyone can send me the pinout of the socket with the different voltages available.

I am thinking of building a preamp that could take advantage of the power supply in the MC-240.

I'm thinking of using 12AU7's for the preamp. One gain stage w/cathode follower. So a total of 2 tubes would be used(4 triode sections for stereo preamp). Basically a clone of the Bottlehead preamp.

Regards, Ron

 

[spkrman57]

Ping Terry DeWickt for assistance on this.

Thanks, Ron

 

[jdandy]

spkrman57.......I am not positive if they are identical or not, but I have a schematic for the original MC275. The 600 ohm/70.7 volt output receptacle connections are as follows:

Pin 1 = ground, 70.7v & 115v return and 600 ohm C.T.
Pins 1 & 7 = Left 70.7v output
Pins 1 & 2 = left 115v output
Pins 2 & 3 = left 600 ohm output, left 230v output
Pins 1 & 8 = right 70.7v output
Pins 1 & 4 = right 115v output
Pins 4 & 5 = right 600 ohm output, right 230v output.

Hope this helps.

 

[dshoaf]

Just checked my MC240 schematic and there's no provision for external preamp power supply. The octal plug onm the 240s are only for 70.7v and 600 ohm output taps only.

Cheers,

David

 

[jdandy]

Just checked my MC240 schematic and there's no provision for external preamp power supply. The octal plug onm the 240s are only for 70.7v and 600 ohm output taps only.

Cheers,

David

David.......Are the Pin designations for the MC240 the same as the ones I posted for the original MC275?

 

[dewickt]

First version MC240 had power for the C8/C8S preamp, this is the version that has 2 octal sockets and the speaker terminals on the left.
PDF of the service manual for this model, note the power transformer has a winding just for the preamp filament supply 25V center tapped.

 

[spkrman57]

Thanks Terry!

This is the one you refurbed back in July 2008!

Serial nr #1P143

And yes! this is the early model with 2 octal sockets on the chassis.

I still would use the regular inputs on the amp, just want to use the power supply to run the preamp section I would build!

Regards, Ron

 

[dewickt]

Use the 25V for the 12AU7 filaments, rectify, filter, and about 4.7 ohm resistor an series and you have a good clean 24V for two 12V tube filaments in series.

 

[spkrman57]

Thanks Terry!

The plan is to use one 12AU7 per chnl, so a total of 2 tubes total.

I'm planning on cloning the original Bottlehead Foreplay amp.

Regards, Ron

 

[dshoaf]

Cool! I've never run into the earlier 240s. One more little detail to file away....

Cheers,

David

 

[spkrman57]

1960 vintage is my guess

Cool! I've never run into the earlier 240s. One more little detail to file away....

Cheers,

David

So mine if from 1st year production if my guess is correct per the serial nr!

By the way Terry, the amp sounds great!!!

Regards, Ron