A SAP15 based power amp

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The SAP15/16 had a SOA of around 2A at 50V, the OPs supply rail, so this places severe limits on the ability of the design to drive real speakers safely. The STD01 is a little better, but this is just not a part for 50V rails.
The Cambridge Azur 640AV2 used the SAP15 for 80W and has some rather intrusive SOA protection
 
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The schematic and parts list do not clearly specify the transformer ratings. Perhaps Laney would advise this if you sent them an email. I think it is important not to use a larger transformer rating than the original as the power rails are probably designed to fall under load to protect the output transistors. The size or mass of the transformer iron core is a good indicator of its total rating, but you must also include appropriate ratings for the other windings to arrive at a rating for the power amplifier windings.

The other windings' actual load currents can all be measured by powering separately with bench power supplies and measuring accurately across a known series resistance with a Voltmeter. Anyone you engage to make or rewind the transformer will be aware of how to estimate the transformer rating. If not, get someone else to wind the transformer who does know what they are doing.
 
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The bridge rating is not indicative, it just tells you it exceeds the maximum RMS current demand. You can always test the bridge diodes to measure whether they are shorted or not. Otherwise if you don't have at least basic test equipment and know how to use it, a repair or rebuild is going to be extremely difficult.

I understand from similar designs, that the transformer is likely only quite small - probably no more than 120VA. Fender 100W/4R rated amps are even smaller. More to the point, have you weighed or measured the transformer yet? Is this a toroidal or conventional E-I transformer? Perhaps post a clear pic if you don't know what we are talking about.
 
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