Rectifer tube mod

[althrax]

I have a question for you about tube rectifiers.Have you ever heard about this mod? You need two 1N5399 diodes. Looking at the rectifier socket, you'll see two red wires that go to pin 4 and pin 6 on the rectifier socket.

Remove the red wires. Install the banded end of the 1N5399 diode to pin 4 and the banded end of the other 1N5399 to pin 6. Solder these to the socket. Now attach one red wire to each unbanded end of each diode. Basically, this is a solid state rectifier in series with the plates of the tube rectifier.

It is said that this will not affect the tone at all of the amp, but will improve reliability by keeping the negitive voltage off the plates, thus cutting the peak inverse voltage that the tube sees in half.
Also in the event the rectifier tube shorts cathode to plate, the amp will continue to work and not go down on stage. Waddaya think?

[mhuss]

IME, rectifier tubes seldom fail due to reverse overvoltage. More often it is surge current, or an internal structural failure. Old recto tubes will lose emission eventually, and this won't be helped by the extra SS diodes.

I can see the SS diodes helping in the case of a cathode-anode short, but not in the case of any other internal short. The latter case will actually fry the SS diodes as well.

Unless you add a terminal strip to mount the diodes, they'll be hanging one end in mid-air, which seems less reliable mechanically.

It seems to me like extra work without a lot of extra benefit.

--mark

[mooreamps]

I think the benefit would having the voltage generation from using a full-wave bridge circuit verses a full wave circuit, with the sag of tube rectifiers, assuming it's a push/pull power amp stage. As for the rest, I've never seen a cathode-to-plate short, but I suppose it could happen.
-g