What is wrong with valve rectifiers?

Nothing wrong, either will work fine if the designer/bilder does a good job.

I will take a shot, smaller size, lighter weight, lower heat production, and improved reliability.

A rectifier valve is basically a diode. A 800 volt 1 amp solid state diode costs less than 0.1 GBP. 3 amp versions may cost some more (I haven't looked at the price of those for years).

Valves don't directly drive speakers - there's a transformer to up the current. A valve is a high voltage device and therefore supply current is conceivably low enough so even a 1 amp solid state diode could be used.

Full wave bridge rectification would require 4 of these solid state devices - very cheap.

In actual fact, the diode valves I've seen are two diodes: They have two anodes and one cathode. One anode is connected to one "end" of the mains transformer secondary, the other anode to the other end, and therefore the cathode is the rectified output.

In such a case, the rectifier is a half wave bridge and the output current is considerably less than that of a full wave bridge. Therefore two rectifier valves would be required? No, it doesn't work. You cannot turn a rectifier valve round to have two cathodes and one anode. We had to wait for transistors to do that sort of thing - valves can only do electron "flow", whereas transistors were developed to do electron flow (NPN) and "hole" flow (PNP). But you don't need to go that far in solid-state rectification - you just turn a diode round to do the negative half cycle.

So, the two cases for solid-state being: 1] It is considerably cheaper to use solid state rectifiers, an 2] a full wave bridge is only available using solid state (unless some Russian/American espionage department invented the "impossible" during the cold war like it seems one of them developed the low voltage valve... )

Oh, forgot to mention the "real-estate": I can hold 4 x 1N4006 (800 volt 1 amp) diodes between my finger and thumb...

I only ever got as far as understanding conventional valve amp circuits (I used to have to repair them - I never built a valve amp), but I've seen some very complicated dual rail supply valve circuits on the web, seemingly trying to emulate transistor circuits - very strange. At this level of complexity there would be no way to utilise a rectifier valve.

One further observation: until a relatively few enthusiasts started making valve amplifiers again some 15-20 years ago, and using reclaimed valves at that, the valve was just about obsolete. It is only really since China became part of the "west" that ingenious valve manufacturers sprung up again. Maybe they are not too interested in manufacturing rectifier valves? It's at this point I become "out of my depth", and I am sure there are a lot of valve experts who may like to join us, and tell us more on the subject?

Edited by Graham Slee - 23 Aug 2009 at 5:51am