Solid State Rectifiers

It is possible (though not necessarily recommended) to replace the vacuum tube rectifiers in tube equipment with solid-state silicon diodes.

Pluses:

• Solid-state replacements should last "forever".
• Reduces load on the filament supply, so power transformer may run cooler (but see below).

Minuses:

• Voltage drop across diodes is much less than across tube rectifiers, so B+ will run higher unless series resistance or a regulator is included. Increased B+ might be desirable in a few cases but will certainly place more stress on other components, particularly electrolytics, and may screw up bias points.
• Fast turn-on of B+ supply leads to ?CathodeStripping. This problem can be avoided by adding a time delay circuit.
• The lower dynamic impedance of a silicon rectifier if used without series resistors results in reduced rectifier conduction angle, which can sneakily increase dissipation in the power transformer for a given B+ current.

General Precautions for Siliconizing (not specific to Hammonds):

• Unless you are fully aware of the various effects of modified power supplies on the rest of the circuit, stick to a tube rectifier, or use guaranteed proprietary substitutes.
• If you do not want to increase the B+, insert series resistance equal to the dynamic impedance of the tube at the nominal operating current.
• Before first switching on, connect the power line through a variable transformer and unplug all tubes. Then crank the volts up over a few hours to raise the B+ slowly from its old value to full. This forms up the electrolytics to better withstand the high voltage which will occur at switch-on.
• Be prepared for electrolytics to fail anyway; include a fuse in the B+ output, as the rectifier won't limit the fault current as a tube would.
• Once up and running, recheck the B+, and if it has changed, readjust biasing.
• NTC thermistors can be used to reduce inrush and startup voltage; make sure they are correctly matched to the B+ current, so that they run at their proper temperature.

In fact, there are so many caveats that I always advise keeping the tube rectifier unless there is an absolutely compelling reason to change it. Whatever you do, build it on the base salvaged from a dead tube, so that it just plugs in. Then, when you have pangs of guilt over depriving your machine of its natural complement of tubes, you can just swap back, without having hacked up the chassis!

Replacements

• TedWeber at WeberVintageSoundTechnology makes an interesting looking plug-in replacement called the "Copper Top". It is supposed to mimic the I-V curve of the original tube, as well as have some sort of inrush current limiting (or slow turn-on).
• HalfmoonElectronics also sells TedWeber solid-state rectifiers, including a 6X4 replacement with an integral B+ tap (to power your Leslie 122 hookup kit.)

TopicModification