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Thread: Teisco Checkmate 20 blown power transformer

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    Teisco Checkmate 20 blown power transformer

    Hi, a friend just brought his trusty Teisco Checkmate 20 with a blown power transformer. He had the power tube sockets rewired to use EL84Ms in place of 7189As two years ago.What power transformer is the best replacement, he needs 240Volts primary? I think the power supply has also to be changed, no cheesy voltage doubler design anymore.Here's the schematic: http://www.larryjohnmcnally.com/Teisco/ckmt20_sch.pdf
    Thanks for suggestions and help,
    Zouto

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    Bent Member Chuck H's Avatar
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    I suppose it depends on how much retrofitting your willing to do. I'll assume you plan to do whatever is needed since you didn't give any mounting info and you plan to change the rectifier design anyway. Also, you only mention the primary voltage requirements. Of course, with the PT not working you don't really know what the seconday HV is.?. I've read that the HV on those amps is really too high for el84's. So I might suggest this is a good time to change that. Even though it will mean a small change in tone. Lot's of el84 amps out there doing fine at around 400Vp so that's what I'd aim for. I'm a Hammond guy myself. But Hammond doesn't make a good PT for this. Their offering would be the 300DX which means you'll have a couple of unused taps and secondaries. Which means extra size and weight. Since Hammonds are often larger than other transformers anyway it could be a cumbersome retrofit. Also, the HV is a little low at 550V CT. But if you look around I'm sure you can find something from another MFG. You probably want about 570V to 580V CT to hit the 400Vp spot with a full wave rectifier. Probably good for better than 100mA on the HV secondary and at least 4A for the 6.3V secondary.

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    "Take two placebos, works twice as well." Enzo

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    Chuck, thanks,
    I do whatever is needed to get this amp working again, fullwave rectifier, better filtering, new OT and speaker. For mounting I'll have to find a solution to fit different transformer sizes, it's drilling time, i suppose. I found this site RPM Developments information page, probably I'll use N1 or N2 for PT and U1 for OT.
    Welter Audio Electronic Seit 1975
    So I got to redesign the power supply for optimum operation/voltages with EL84M and preamp tubes.

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    Bent Member Chuck H's Avatar
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    They seem to have several models that will work at 360-0-360.

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    "Take two placebos, works twice as well." Enzo

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    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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    You could use a Fender Blues Jr. transformer, but you will need to switch from the existing voltage-doubler rectifier (a neat way to cut the transformer price on a budget amp, because you only need half the copper) to a fullwave bridge, which is EASY, because there are plenty of chassis-mount bridge rectifiers available.

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