Minimum ratings: <b>When using silicon rectifiers, they must have a peak inverse voltage (PIV) rating of 2.8 times (2 * root 2) the RMS voltage being delivered by the transformer.</b> (This is a higher PIV rating than necessary with a bridge rectifier.)
For example; with a transformer rated at 300-0-300Vrms the diodes must be rated at:
2.8 * 300 = 849V PIV
<b>For this reason, it is quite usual to place two or more diodes in series</b>; their PIV ratings will sum, increasing the PIV of the rectifier as a whole.
(On modern data sheets, PIV may be listed as 'Reverse Repetitive Maximum' (Vrrm) instead). I<b>f diodes are used in series, a capacitor chould be placed in parallel with each one to ensure equal voltage sharing between each diode. Values of 10nF to 47nF (1kV or better) are common, and they will also act as snubbing capacitors to supress the voltage transients produced by the diodes switching.</b>
The most popular silicon diode used is the 1N4007 (PIV = 1000V, 1A)
The diodes should have a current rating greater than the peak current you expect the amp to draw (ignoring ripple current). Thankfully, valve amps usually have fairly meager HT current demands, and 1A diodes are usually sufficient even for 100W amps. The maximum current value quoted on the data sheet already takes ripple current into account so this is not a major issue.
When not under load, the voltage after rectification will be close to the peak AC voltage, which is equal to the 1.4 times (root 2) the RMS voltage.
For example, with a transformer rated at 300-0-300Vrms, the DC voltage after rectification will be close to:
1.4 * 300 = 420Vdc
(There will be a couple of volts lost across the diodes, but we normally ignore this.)
We can expect this DC voltage to fall by 10% to 15% when we start drawing current, due to voltage being dropped across the transformer winding. So, we can realistically expect to achieve about 365Vdc of HT under load using this transformer.
