Theshold T800D throws circuit breaker.

Ya' me thinks it might be a faulty breaker.---Go to this breaker and get a feel for the spring action ----compared to the other breakers. The spring action gets old and weak.

Amps are often tested using inductive circuits, which limit the inrush current. The inrush current can actually be larger when they are plugged into a low-inductance high-voltage dedicated circuits. B&K's have been known to do this. They blow fuses a lot. It is nothing to be concerned about. You should probably put the amp on a 20-amp circuit. You probably have dedicated wiring for this don't you? Another solution is to put inrush limiters or a relay and a power resistor in the amp. This is more typical of high-power amps.

As mentioned, this is related to in-rush current ( the surge filling up the capacitors ) and / or the circuit breaker itself. Big amps with large power supply reserves can do this, especially if the amp is older and / or the breaker is older and / or the breaker / electric circuit is too small and shared with other devices.

The first thing to do is to make sure that the breaker itself is solid and in good working condition. It surely would not hurt to replace the breaker if it has more than a few years on it. Quite honestly, this amp needs and deserves a dedicated 20 amp line all to itself. That is, if you want to get the type of performance that it is truly capable of. A dedicated 15 amp line would probably also suffice but may provide some sag during long duration high impact peaks. Trying to run this ( or any other high powered, high bias amp ) off of an older 15 amp circuit that is shared with other components is most assuredly compromising system performance.

If the amp continues to trip the new breaker on a dedicated line, it might be time to replace the filter caps in the power supply. This can be relatively costly, so shop around for both the parts and the labor. Having said that, Jon Soderberg probably has more experience with these amps than anybody else in the country, but he's not cheap nor fast as far as turn-around times go. None the less, your amp will be fully inspected and up to snuff when he's done with it, so the cost may be worthwhile to you.

As a side note, that in-rush current has to be passed by the rectifiers, so if you're going to replace the caps, replace the rectifiers at the same time. No sense in fixing the problem and leaving in older parts that are probably just as stressed. If you did just the caps and left the old rectifiers, you would "probably" be okay, but while it is apart, it would be wise to take care of everything at one time. As you know, this amp is neither light in weight nor inexpensive to ship, so save some time and money and do it right the first time. Sean
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Thanks Avguygeorge,Audioengr and Sean for the excellent advice. The amp is on a dedicated 20 amp line all to itself with a new breaker. I spoke to Jon and he thinks its the relays and power resistor. He'll know more upon inspection. Once fixed I'll report the findings.
Once again, THANKS FOR THE ADVICE.