Okay, I give up...what is a rectifier?

[greatmagnet]

...and what does it do? I'm talking about in regards to tube guitar amplifier heads and such specifically.

As in: I've heard that the primary difference in circuitry between a Fender Bassman and a Marshall Plexi from the late 60's is that the Bassman has a tube rectifier and the Plexi has a solid-state one. But I don't have any idea what they do.

It must be important, though, since Boogie actually puts the term in the names for their friggin' amps!

[object88]

The short answer: the rectifier turns the AC power coming into the amp into the DC power used by the tubes.

The longer answer: power comes into the amp and hits the power transformer. That turns your 110V (or whatever's local) into something more like 400V (depending on the particular amp), but it's still AC. The rectifier takes that and spits out only positive signal. After that, you've got some caps and/or inductors which smooth out the power, so it's not all ripply.

The big difference between tube rectifiers and solid-state rectifiers is twofold:

1) Tube rectifiers eat up 10-50V right off the bat. If your power transformer is producing 400V, the tubes will get at most (say) 350V after hitting a tube rectifier. However, solid-state rectifiers (a network of diodes) drops only a couple of volts, so they're more efficient. You can either use a smaller transformer, or hit the tubes with more power.

2) Tube rectifiers sag under load. That is, when the amp needs a lot of current, the voltage out of the tube rectifiers drops. This is a pleasing, compression-like effect to some players. Solid-state rectifiers don't sag, so the power supplied is more constant. Now, in a class-A amp, the current requirements are constant, whether you're playing full-blast or at a whisper, so you won't see that sag effect from a tube rectifier.

2.5) Tube rectifiers are more expensive, rare, and delicate than solid-state rectifiers. They also tend to require a 5V heater supply, and 5V power transformer windings aren't as common as the 6V (6.3V?) heater that most other tubes require.

I hope this answers your question.

[greatmagnet]

You are the shit, Oaktown.

[jtienhaara]

Great thread!

Just one nit-pick though: diodes are not solid state.

A half-wave rectifier can be made using a single diode. This simply removes the negative portions of the signal.

A full-wave rectifier can also be made. This flips the negative portions of the signal to be positive. So a sine wave would be turned into bumpy hills, for example.

Full-wave rectifiers can be made many ways. A bridge rectifier can be made using 4 diodes.

However a solid state full-wave rectifier can be made using op amps. Solid state is, by definition, silicon chips. This means that the signal will be affected by things like slew rate and all that other fun stuff that I don't really understand. (Slew rate will limit the responsiveness to transients.) Also you have to power chips, though generally you're looking at only 3-5 V and not much current. And finally, chips can only handle up to somewhat less than the amount of voltage they require to be powered. So if you send a huge peak through an op amp, it will break up and possibly damage the op amp.

Cheers,

Johann

[greatmagnet]

So, the rectifier in, say, a vintage Plexi head would be of the DIODE variety as opposed to the OP AMP variety?

[jtienhaara]

Sorry for the confusion -- a late 60s Plexi would have a silicon (op amp) rectifier.

Anyway I really was just nit-picking -- the important info came from object88.

Hope this helps Caldo! Cheers,

Johann

[wowandflutter]

Great thread!

Just one nit-pick though: diodes are not solid state.

Johann

Actually arent diodes solid state. They are just a positive and negitive (PN)
pieces of semi conductor material sandwitched together. The semiconductor material being that which makes it solid state. Just like NPN or PNP Transistors are solid state as well regardless if they are discreet or in chip form.

Unless when you refer to diode as they used to refer to them in tube form ie: diode, triode, pentode, etc.

[jtienhaara]

Hey you're right! I stand corrected. I always assumed silicon was the only semiconductor material. But germanium is, too.

How's that for muddying the water!

[nacho459]

I never understand why some Mesa Booty amps have multiple rectifiers, IE Dual Recto, Triple Recto. They sure look cool, but what's the point?

[puffpastry]

If I may add to the discussion...

The big difference between tube rectifiers and solid-state rectifiers is twofold

Actually, there's more differences between the two. Solid state rectification adds IR noise to the power, tube rectification does not. That IR noise shows up in the audio signal--it's kind of a low-level whine, sort of a mechanical noise for lack of a better description. This gives the audio a bit of a harsh edge that's not found with tube rectifiers. Also, a diodes ability to supply ample current under load is an issue that I've heard discussed, although disagreement exists over how and why this affects audio. In hi-fi applications, many designers use FReD or UFReD diodes to minimize these effects. I'm not sure why this isn't as prevalent in pro audio, especially considering how cheap these types of diodes have become over the last few years.

Tube rectifiers sag under load.

ALL rectifiers sag under load. The questions is--how does that sagging affect the audio signal? In a linear, regulated supply, error amplifiers make adjustments under load to keep current supply close to theoretical levels. But even if the SS regulator is doing this, it doesn't mean the SS rectifier is able to keep up with demand. Therein lies the debate over FReD diodes--some claim that, because they have a faster recovery time, that they are better able to keep up with current demands, thereby enabling the audio circuit to handle things like low frequencies and transients better.

[greatmagnet]

I could never understand why some Mesa Booty amps have multiple rectifiers, IE Dual Recto, Triple Recto. They sure look cool, but what's the point?

I thought the names "double", "triple" and so on referred to the number of footswitchable channels the amp has, and thusly each channel got it's own dedicated rectifier circuit...but I may be wrong there.

Mesa/Boogie Octuple Rectifier...for the serious metal player.

[nacho459]

I thought the names "double", "triple" and so on referred to the number of footswitchable channels the amp has, and thusly each channel got it's own dedicated rectifier circuit...but I may be wrong there.

Yeah, It actually has to do with the number of actual rectifier tubes. The Single is a 50 watt 2 power tube amp. The Dual is a 100 watt 4 power tube, and the Triple is a 150 watt 6 power tube amp. The original Dual and Triple, and the rack mounted Dual Recto had 2 channels with a single button foot switch. The newer ones are a three channel with a multi button MIDI foot switch.

[covert]

Okay, lets get some terminology straight here.

Diode goes back to the pre solid state days, in fact the first tubes produced were diodes (means two electrodes). This was the predecessor to the triode which could be made to amplify signal.

As far as solid state stuff goes, almost all of it is silicon based. Germanium was the stuff they "doped" the silicon with. There are other possibilities for doping material.

Op(erational) amps are a circuit topology, that could be built from tubes, from discreet transistors, or cast all on one chip, as they usually are today.

[Sayer]

"Yeah, It actually has to do with the number of actual rectifier tubes. The Single is a 50 watt 2 power tube amp. The Dual is a 100 watt 4 power tube, and the Triple is a 150 watt 6 power tube amp. The original Dual and Triple, and the rack mounted Dual Recto had 2 channels with a single button foot switch. The newer ones are a three channel with a multi button MIDI foot switch."

Yes the single or double does refer to the number of rectifier tubes and they are individualized for each foot switchable channel. But the terminology does not refer to the power tubes, which can be El 34s and 6l6s or even KT 66's In this "self biasing" (yeah right) amp.

(I have had to bias mine every time ive ever changed the tubes and mine is handmade)

[object88]

Excellent information here. Yeah, my use of "diode" isn't really the most acurate, as covert pointed out. But I think that these days, when people hear diode, they think of those little axial solid-state jobbers with the black band around one side. I, myself, do not know of a better term for them.

As for Mesa Boogies and their terminology, I thought that the "double" and "triple" is the count of rectifier tubes, but the reason for having multiple rectifiers is that a single rectifier can't put out enough volts to handle the larger tube count employed in those models.

Additionally, I'd like to point out that puffpastry is right on regarding hash noise from a diode, although my understanding is that it's possible (although certainly not trivial) to supress that noise.

I believe that all rectifiers sag under load, but my understanding is that diodes sag considerably less than tubes. There is a point where no rectifier will be able to handle the load. Perhaps with tubes, performance tapers off, while with diodes, it just hits a brick wall?