AC * 1.4 = DC ???

[rdjones]

Actually that sounds about right to me. The 1.414 figure is for unloaded. Under load 1.2 is usually ballpark. So 293*1.2 = 352.

I haven't worked on this amp in a few days, but I put it back on teh bench last night, pulled all tubes, and measured the DC just after the diodes - 370V. Should have been closer to 420V.

Tried new diodes with no improvement.

If I disconnect the diodes from the rest of the amp (filters and beyond), and measure again at the diodes, what should I expect? Is it valid to measure with nothing connected like that?

You need some capacitance to charge up and hold the DC at the peak level of the AC voltage, otherwise you are just measuring a rectified version of the AC waveform, which is the AC rms voltage times 1.000.

Measure your AC voltage and your rectified and filtered DC voltages under the same condition whether loaded or unloaded and subtract the drop across the rectifier. It's as simple as that, there's no where else for the voltage to go unless there's some other unidentified problem dropping the voltage.
Either way you should be able to identify where the voltage is being lost by careful measurements.
You can also measure the ripple on the capacitor to verify that it's actually holding a charge, which it may not be.

rd

[Bruciep07]

I feel your pain, I've built 4 express amps so far, all of them with Pacific iron. 120V measured at the wall, 600V measured across the secondaries. My first one was the traditional stack-o-caps layout, the last 3 were multi-caps with the last 2 using a VVR. I have had lower than expected voltages on each build so far, I'm convinced it's the batch of 1,000 " Micro Commercial Components brand" diodes that I bought. My unloaded B+1 on every build with the diodes connected to just the first caps, nothing else, no bleeder resistors, etc, sits around 390V. I've swapped caps, diodes (from the same brand/batch), re-soldered, everything I could think of, always the same results. My current express is going to get a new quad of diodes of a different brand, can't think of anything else! I'm at 382V on my current build, otherwise everything else is an expected percentage less. With nothing but 80uF of filter connected to B+ using the 300-0-300 taps and the pre-rectified voltages are correct, you should see around 420V, is that a valid assumption?