What's the deal with rectifiers?

[marnold]

I saw a Mesa-Boogie ad in Premier Guitar for dual and triple rectifier ampfs. M-B seems to be the only company that makes a big deal about that. What is a "rectifier" and what benefit is there to having more than one?

[Ch0jin]

A rectifier is an electrical device that converts alternating current (AC) to direct current (DC), a process known as rectification. Rectifiers have many uses including as components of power supplies and as detectors of radio signals. Rectifiers may be made of solid state diodes, vacuum tube diodes, mercury arc valves, and other components.

Thanks Wiki ;)

In layman's terms though, the socket on your wall supplies anywhere from 110 to 240 Volts AC (depending on your location). Your guitar amp, solid state or tubes, needs DC voltage to operate. Therefore the power supply of any guitar amplifier has the job of changing that 240V AC to whatever DC voltage your amp needs. (From 30-40V for a Solid State amp to several hundred volts for a tube amp). As you see from the Wiki definition, converting AC to DC is what rectifiers do, so you'll find one in every guitar amp ever made.



Most modern tube amps use a Solid State Rectifier (Silicon diodes) for a number of reasons, not least of which is cost and reliability. However you'll find tube rectifiers on vintage amps because silicon rectifier technology wasn't around back then. These days you'll find tube rectifiers in boutique tube amps because some people like the sound of an amp with a tube rectifier especially little amps like reproductions of the Fender Champ.

Now to answer the specific Mesa question..

I'm no boogie expert so I'm sure someone will correct me, but as I understand it, the dual recto's had (not surprisingly) two rectifiers, a Tube and Silicon and you could switch between them to get the sound you wanted. I believe the triple rec's include both tube and silicon rectifiers as well as a some kind of "bold/sponge" switch as well (but I have No Idea what that does....)

Overall though it's just a brand name. There are LOADS more tonal and usability changes between the dual recto and triple recto than type of power supply.

Oops one more thing...

People like tube recto's in small amps because of a phenomena known as "Sag". Basically tubes do a terrible job at rectification so as you wind up the volume and hit big bass notes, your power supply voltage will "sag" or drop. In a tube amp this (i think) equates to more compression which some people dig. Note that for this to occur you need to be -cranking- that amp. Totally reasonable in a 5W champ, aural suicide with a 150W tube boogie. Silicon is FAR better at it's job so you get no sag, potential for more volume given the same circuit and of course a different tone when you -crank- your amp.

[markb]

Some (very) early Marshalls had two rectifier tubes but this was due to a lack of the correct part, not a "feature". IME, a tube rectified amp will give more "sag" and "push-back" (honest!) than a silicon diode rectified PSU. This will depend on the PSU design as well. e.g. a Fender Twin has a very capable power supply while a Vibrolux's is slightly under spec. This limits the output of the amp but has a more marked effect on headroom. Take an old blackface or silverface Vibrolux and you'll notice almost no volume increase above about 5, just more compression and clipping. The Twin will stay cleaner further up the volume scale.

Note: I am not talking about reissues, reimagined designs or anything else that Fender has produced since the 70's here. Please don't confuse old designs with modern amps with the same names.

[tunghaichuan]

In addition to Chojin's excellent explanation:



Most modern tube amps use a Solid State Rectifier (Silicon diodes) for a number of reasons, not least of which is cost and reliability. However you'll find tube rectifiers on vintage amps because silicon rectifier technology wasn't around back then. These days you'll find tube rectifiers in boutique tube amps because some people like the sound of an amp with a tube rectifier especially little amps like reproductions of the Fender Champ.


Back in the 40s, 50s, and 60s when Leo Fender was building amps, solid state rectifiers were very expensive compared to tube rectifiers. If he were building amps today, he'd no doubt use solid state rectifiers.



I'm no boogie expert so I'm sure someone will correct me, but as I understand it, the dual recto's had (not surprisingly) two rectifiers, a Tube and Silicon and you could switch between them to get the sound you wanted. I believe the triple rec's include both tube and silicon rectifiers as well as a some kind of "bold/sponge" switch as well (but I have No Idea what that does....)

Overall though it's just a brand name. There are LOADS more tonal and usability changes between the dual recto and triple recto than type of power supply.


The Dual Rectifer actually has two 5U4 rectifiers in it and solid state rectifiers, IIRC. Adding an additional tube rectifier in parallel with an existing one does two things: it cuts the internal resistance of both tubes in half which causes less voltage drop. Since the tube rectifier has internal resistance there is some voltage drop in the power supply dependent on how much current the amp is drawing.

The other thing that having multiple rectifiers does is cause less sag than just one. Sag occurs in an amp when the current draw from the power amp circuit exceeds the rectifier's ability to supply current. Adding another one increases the current capability of the power supply.



People like tube recto's in small amps because of a phenomena known as "Sag". Basically tubes do a terrible job at rectification so as you wind up the volume and hit big bass notes, your power supply voltage will "sag" or drop. In a tube amp this (i think) equates to more compression which some people dig. Note that for this to occur you need to be -cranking- that amp. Totally reasonable in a 5W champ, aural suicide with a 150W tube boogie. Silicon is FAR better at it's job so you get no sag, potential for more volume given the same circuit and of course a different tone when you -crank- your amp.

Except that sag doesn't occur in a Class A circuit. So most Champs do not really benefit from a tube rectifier. Except that the Champ uses a Princeton transformer which would have too high voltages if SS rectifers were subbed in. So this can be a useful thing for a Class A circuit, to cut down on transformer voltages that would be normally too high to use in a 5W SE circuit.

tung

[marnold]

I see! (said the blind man to his deaf wife as he picked up his hammer and hand grenade) Thanks for the explanation.

I read something on the FDP forums where one amp tech I know said that they made a solid state rectifier that can mimic the sag effect of a tube. I don't remember the details but remember thinking it was quite clever. I'd imagine, though, that with the typical Mesa-Boogie usage sag would be an undesirable thing since you want all power, all the time.

[Ch0jin]

Hey Mark, what's "Push-back"?

Thanks Tung for cleaning up my brain dump ;)

It's probably not the place for it (or maybe it is) but why do class A amps not experience sag? I'm clearly very rusty on my amplifier theory, but I just assumed sag was purely a reaction to your output stage trying to draw more current than the power supply stage can provide, thus increasing the voltage drop across the internal resistance of the rectifier and hence lower B+ resulting in your waveform being "compressed" to fit between a slightly lower voltage range.

[Ch0jin]

Marnold,
Frankly, at the end of the day mate it's marketing. Specifically, product differentiation. The recto range have selectable power supplies and the competition (i.e Peavey and Marshall) don't, so of course thats the angle Mesa lean on for promotion.

" I'd imagine, though, that with the typical Mesa-Boogie usage sag would be an undesirable thing since you want all power, all the time."

Kinda.

Sag gives compression remember. I'm fairly sure your typical triple rec user is after gobs of sustain and compression for metal leads. Also, the triple rec is rated at something insane like 150W. Unless your playing to a stadium without a PA, I'm sure you've got quite enough volume on tap to alleviate any concerns about not getting the last few dB from your rig.

All that aside, I got to mess around on a Dual Recto years ago and they kill for hard rock/metal etc. I was playing bass in a band that had one guitarist with a dual recto and the other with a 5150, both through marshall cabs and whilst the 5150 had serious balls, the mesa was just brutal when you got stuck into it.

[tunghaichuan]

I see! (said the blind man to his deaf wife as he picked up his hammer and hand grenade) Thanks for the explanation.

I read something on the FDP forums where one amp tech I know said that they made a solid state rectifier that can mimic the sag effect of a tube. I don't remember the details but remember thinking it was quite clever. I'd imagine, though, that with the typical Mesa-Boogie usage sag would be an undesirable thing since you want all power, all the time.

It is actually pretty easy to simulate sag in an amp with solid state rectifiers. All you need to do is to put one 100-130 ohm, 5w resistor between the high voltage lead and the ss diode. When drawining minimal current from the power supply, there is minimal voltage drop across the resistor. When the amp is cranked and drawing full current, there is massive voltage drop across the resitor and the amp sags. Again, this only works in Class AB amps. In Class A amps the current draw on the power suppy is constant whether there is any signal or not.

One of the reasons that Boogie uses two rectifier tubes in their amps (or three in the case of the Triple Recto) is that two rectifiers will sag less than one. Three will give even less sag. Some people claim that tube rectos sound different thant SS diodes, but I don't have enough experience with that to say one way or the other.

Finally, one limitation of tube rectos is that there is a limit to how big the first filter cap can be. I believe 20-30uF is the maximum for a 5Y3. If the filter cap is too big it wears out the rectifier tube prematurely. With SS diodes there is no limit. I've used up to almost 400uF with no ill effects. This does make the amp very stiff sounding, and is very good for heavy metal players. Not so much for people that like vintage tones.

tung

[Ch0jin]

Gah I just read your post and worked out the answer to my question about class A amps :) *facepalm

I'll slink away humbly now....

Oh and your last point about inrush current is a good one I hadn't considered.

[tunghaichuan]

No worries, it was a few years and about 5 champ builds before I found out about tube rectifiers and Class A. :rotflmao:

tung


Gah I just read your post and worked out the answer to my question about class A amps :) *facepalm

I'll slink away humbly now....

[markb]

Hey Mark, what's "Push-back"?

Thanks Tung for cleaning up my brain dump ;)

It's probably not the place for it (or maybe it is) but why do class A amps not experience sag? I'm clearly very rusty on my amplifier theory, but I just assumed sag was purely a reaction to your output stage trying to draw more current than the power supply stage can provide, thus increasing the voltage drop across the internal resistance of the rectifier and hence lower B+ resulting in your waveform being "compressed" to fit between a slightly lower voltage range.

Good question! I've seen it a description of the tonal interaction after the sag. "Bloom" might be another way of putting it or it's just another bit of tube snob nonsense :) .

[Robert]

My Heatseeker has solid state rectifier with "sag" effect.

The Bad Cat I recently tried allowed for switching between solid state and tube rectifier. Didn't notice a huge difference, although I could tell there was a difference. Either would have been good for me on that particular Bad Cat amph.

[sunvalleylaw]

I don't really know anything about rectifiers other than I like the Rect (rectifier) model on my son's Peavey Vypyr. :rockon:

[Ch0jin]

Something else just occurred to me. Previously I made mention that rectifier sag is only apparent when your amp is cranked, which in the case of most tube amps means LOUD!

But I forgot about attenuators :)

All of a sudden I have renewed interest in owning an amp with a tube recto and a big fat attenuator. Could be some nice tones to be had.

[Ch0jin]

I don't really know anything about rectifiers other than I like the Rect (rectifier) model on my son's Peavey Vypyr. :rockon:

On my POD that's known as "Treadplate" or "Big Treadplate". Ya know, just in case anyone cares haha. I don't use it much though because the JCM800 patch I made sounds amazing ;)

[markb]

The Valvetronix ACxx models have put me off the idea of owning any more temperamental VOX amps. The AC15 is particularly wonderful, it sags and blooms just the way a good AC15 should (if you can find a good one that is). It's worth owing one just for that. I do miss the smell of my old AC30s though ;)

I don't think MESA have helped our understanding by trademarking the word "rectifier".

[Ch0jin]

Interesting you say Mesa Trademarked "Rectifier".

I don't know a thing about trademark law (or if thats even whats it's called) but as a former electronic engineer I find that ridiculous. (of them, not you). Thats the equivalent of trying to trademark "Resistor" or "Capacitor".

Then again, at least "rectifier" is a common term for the device in English. Every time I post here I try to remember to say "Tube" instead of "Valve". In Australian English it's always been "Valve", which honestly is a slightly better descriptor than "Tube". Seems like we name it after what it does (kinda) and the US name it after what it looks like ;)

Then again, the Epiphone was named the VJ not the TJ which made me smile :)

[hubberjub]

My Mesa Boogie Maverick is a Dual Rec. I just keep it on the tube at all times. It doesn't make a big difference. The solid state gives the amp more clean headroom.

[Tone2TheBone]

:munch: :)

[M29]

Tone what are you grinnin about:poke: Does this have somethin to do with Marshall:D

[markb]

http://www.mesaboogie.com/Trademarks/Trademarks.htm