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Thread: New amp build transformer suggestions

  1. #1
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    New amp build transformer suggestions

    Dear colleagues,

    I am looking to build a Bad Cat Hot Cat amp. The schematic is included. Instead of making a dual EL34 version I want to downsize this to a single EL34 tube. I am looking for how to calculate the PT and OT and choke for this configuration. I have what I think are the original specs for the full version. Any help would be appreciated.

    *575 volt ac ct 600 ma************285-0-285 250MA
    * * *6.3 volt 6 amps***************5.0 Volt 3 Amps
    * * *5.0 volt 5 amps***************6.3 Volt 6 Amps

    30 watt Choke: T541690
    * * * Inductance 36 Henries

    30 Watt Output: T541691 Cutout 1 x 2
    * * *Primary Impedance 4000 ohms
    * * *Secondary Impedance 4,18,16 ohms


    Thanks in advance,

    Daniel
    Attached Files Attached Files

  2. #2
    Supporting Member eschertron's Avatar
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    Welcome to the forum, Daniel!

    Quote Originally Posted by Bonsaimaster View Post
    * * *6.3 volt 6 amps***************5.0 Volt 3 Amps
    * * *5.0 volt 5 amps***************6.3 Volt 6 Amps
    Not sure what this means. How many filament windings are there on the PT? Also, the schem linked does not come up. There was a server crash a few years ago that corrupted the links.

    But in general answer to your question, turning a push-pull amp into a single-ended amp will require some design changes to the power amp section - mostly the phase splitter circuit - and a downgrade to the power supply. Then the gain throughout the amp will need to be checked and massaged to see how it compares to the original design. Why not find a SE EL34 circuit out there to build, try it and then see what could make it more like the bad cat. I'm not familiar with the bad cat, but I'll hazard a guess and say the preamp is similar to many Fender and early Marshall designs.
    Quote Originally Posted by Steve Conner View Post
    If it still won't get loud enough, it's probably broken.

  3. #3
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    This is the schematic: http://schems.com/bmampscom/badcat/badcat_hotcat_30.pdf
    I'd go from V1b to V3b with another 470k la Marshall 1959.
    Then from F to the Power tube.

    As for the PT, please specify what do you have actually, as per the information shown on my link, voltages are different.

    Thanks.

  4. #4
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    Quote Originally Posted by Roberto View Post
    This is the schematic: http://schems.com/bmampscom/badcat/badcat_hotcat_30.pdf
    I'd go from V1b to V3b with another 470k la Marshall 1959.
    Then from F to the Power tube.

    As for the PT, please specify what do you have actually, as per the information shown on my link, voltages are different.

    Thanks.
    Ok I feel like an idiot. I Thought this was a SE design but realize it is a cathode bias PP amp. Ok I am using this transformer (320-0-320) at 150mA center tapped, 6.3V (3.15-0-3.15) at 4A center tapped and 5V at 2A.

    Ok I think best to keep the PP design. So the question is which output transformer to use? I have two Classic Tone transformers
    40-18049
    40-18006
    40-18080 Hi Fi version

    The first two have the same specs but different in size. So there must be something else going on.

    Also do I need a 36H choke? I would like to use a 30H one.

    Thanks
    Bonsaimaster

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    Hi Bonsaimaster,

    Do tou have the direct links or specs of the cited OTs?
    Just to tell you which one fits better the actual configuration.

    You'd like to use one single SE to reduce the power?
    you could choose a EL84 PP instead.
    Around 8k Raa.

  6. #6
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    Quote Originally Posted by Roberto View Post
    Hi Bonsaimaster,

    Do tou have the direct links or specs of the cited OTs?
    Just to tell you which one fits better the actual configuration.

    You'd like to use one single SE to reduce the power?
    you could choose a EL84 PP instead.
    Around 8k Raa.

    I think I will stay with the EL34 PP
    40-18080
    Vox AC 30 Vintage Style Output Transformer, 4/8/16 Ohm
    Fender Output Transformer, Bandmaster, Pro, Vibrolux, Tremolux, Vibroverb, 4/8/16 Ohm

    Thanks!

  7. #7
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    If you want to do some trials on the sound and not an exact replica, you can chose the first OT: it has 4.4 and 6.6k taps plus the UL ones. Many switchable options.

    Otherwise the other two seems quite similar (but I don't have direct experience on them).

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