Akai M8 Monoblock Amps and Voltage Input-Help Needed

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by drbob1, Mar 11, 2015.

  1. drbob1

    drbob1 Silver Supporting Member

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    I found a tape recorder with a pair of tube amplifiers as both mic preamps and power amps, so I'm repurposing it for guitar. I don't have the schematics.

    Each mono block has an input transformer, and there's a master transformer. Unfortunately, the maximum voltage input on the mono blocks is 117v, so with 128v wall voltage I'm getting 6.7v on the heaters for the 12AX7s and 5.9 on the 5.3v heater for the rectifier. Are these safe voltages?

    The master transformer has a single output, but has inputs for 100v, 112v, 120v, 200v, 220v, 240v. Is there some way I can finesse that to get a 120v or 117v output from it?
     
    Last edited: Mar 11, 2015
  2. drbob1

    drbob1 Silver Supporting Member

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    Hmm, pretty quiet around here! Anyway, the power transformer had some kind of weird connector that I wasn't able to trace because of wires running here and there. When I just unwired it from the harness, I found that if I feed wall voltage (125-128v) into the 120v labelled winding, I get 108v from the 100v labelled winding, using the same ground wire for both. This works, but means that the ground from the transformer "secondary" is attached to the wall AC ground, which seems dangerous?
     
  3. drbob1

    drbob1 Silver Supporting Member

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    Ah well... It seems that each mono block has an individual input transformer, so I guess I don't need to worry about isolation of the main PT. I'll just wire it up AC to fuse to hot of 120, other AC to 0, and output from 0 and the 100v tap.

    Now if only I could figure out how to wire in a 3 prong cord, since the main PT is electrically isolated from the two chassis by the wooden cabinet. Can I run the overall ground to the two chassis, with no actual connection to the PT or should I run a wire from the frame of the PT to both chassis and ground that?
     

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