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Thread: Tube Works Tube Driver TD-742 schematic needed

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    Tube Works Tube Driver TD-742 schematic needed

    I have one of these with blown output, anyone have a schematic? Also, does anyone know what the power rating is? I found one place that says 40w, but can't find anything definitive, even the user manual says nothing about it.

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    Supporting Member Jazz P Bass's Avatar
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    I have never worked on a TD-742.
    I believe they are rated at 60 watts into 4 ohms.

    Now the TD-752 I Have worked on.
    Rated at 100 watts into 4 ohms.
    The 752 uses J50 & K135 mosfets for the outputs

    Not too sure what BK Butler used for the outputs in the 742.

    Here is the 752 schematic.
    Maybe it will be of use.
    Attached Files Attached Files

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    Quote Originally Posted by glebert View Post
    I have one of these with blown output, anyone have a schematic? Also, does anyone know what the power rating is? I found one place that says 40w, but can't find anything definitive, even the user manual says nothing about it.
    On unknown SS amps, you can estimate power :

    1) measure main voltage rails, as an example , say,+/-40V DC.

    2) multiply that value by 0.6 if loaded with 8 ohms, by 0.5 if loaded 4 ohms, that will be very close to output RMS voltage, then do the Math, say:

    * 8 ohm load: 40*0.6=24V RMS , meaning some 72 W RMS
    * 4 ohm load: 40*0.5=20V RMS , meaning some 100W RMS

    redo using your actual measured voltages.

    Based just on PSU specs, but like Enzo once said, "the amplifier is just that thingie between power supply and speakers".

    EDIT: thatīs some minimalistic power amp, just 1 Op Amp and 2 MosFets .

    Beware it is an (unacknowledged, hidden in plain sight) flying rails amplifier.
    MosFets are grounded , NO ground symbol is drawn anywhere (naughty Mr Butler tsk tsk), ground is just somewhat implied by speaker out sleeve connection , and power supply center tap is NOT ground but speaker out.

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    Last edited by J M Fahey; 12-23-2017 at 05:35 PM.
    Juan Manuel Fahey

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    The 742 is definitely a different output circuit than the 752, the output transistors are TIP31C and TIP32C transistors (two each, appear to be full parallel). For the voltage rails I disconnected the transformer secondary wires (thought the transformer might have been blown) but am getting 35V RMS on each leg (with center tap), which should get rails of 48ish V right?. So for an 8 ohm load using Juan's approximation it would be close to 100 watts. That would explain why people say these amps are loud.

    Can't figure out why they spec'd a 4 amp mains fuse and no fuses on secondary side. I am still wondering if the transformer is going to hold up. Unfortunately the speaker is blown too.

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    Got the schematic from Fender.
    TD 742.pdf

    Mr. Butler was liberal with the diodes and resistors.

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    Just bought a TD-752 (BK Butler, no less) for cheap. Great to see the schematic - makes it much easier to keep these oldsters running. Thanks mucho!!

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    TD-742

    Quote Originally Posted by glebert View Post
    I have one of these with blown output, anyone have a schematic? Also, does anyone know what the power rating is? I found one place that says 40w, but can't find anything definitive, even the user manual says nothing about it.
    Did you ever get your amp working? I have one that had blown output transistors.... it's a long story... replaced those.. fired it up and the power transformer got smoked.. been sitting in my closet ever since.

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    Quote Originally Posted by adroit5177 View Post
    Did you ever get your amp working? I have one that had blown output transistors.... it's a long story... replaced those.. fired it up and the power transformer got smoked.. been sitting in my closet ever since.
    No, ended up giving it away to someone who was looking for projects, along with a bunch of other stuff.

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    Old Timer J M Fahey's Avatar
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    Just noticed (slow mind) that there is NO NFB from the output stage.

    Probably contributes some "Tube sound".

    Now using puny TO92 2N5401/5551 as power drivers is *murder* and should certainly contribute to the design being abandoned as unreliable ... for good reason.

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    Juan Manuel Fahey

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