Late Night DIY: Tube Talk – Tubes, Rectifiers, and Electricity

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Instead of building something in this “episode”, we’re going to talk about tubes again. Specifically, tube rectifiers. I’ve already done a 4-part series on tubes/amps so to can refer to those for more tube info.

A Tube Primer for Guitarists: Part One
A Tube Primer For Guitarists – Part Two
A Tube Primer for Guitarists, Part Three: Hybrid Amps
A Tube Primer for Guitarists: Part Four – Tube Distortion

A TubeAny of you out there who like the sound of tube amps should realize that some tube amps use solid state rectifiers. In fact, I think that there are more tube amps with solid state rectifiers than with tube rectifiers. How do they affect the sound of the amp… or do they at all?

First, let’s define what a rectifier is and what it does. Electronic devices all work internally on DC (direct current) voltage. That sounds a little confusing, right? It’s simple really. DC voltage means that the electricity flows from negative to positive… period. Ground is always negative and the power supply puts out positive voltage. The electricity that is available from the wall socket in your home is AC (alternating current) voltage. That simply means that the negative and positive poles swap places sixty times per second. See Drawing 1. (For more complete info on this stuff, search “electricity” on the web.)

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The power supply in your amp takes this AC voltage (120VAC) and, through the power transformer, bumps the value up to around 300-400 volts AC, the voltage tubes like to work at. For you knowledgeable people out there (yes, you nitpickers, too) there are all different types of tubes that work on all kinds of voltages, but I am keeping it simple for people that just don’t know how this stuff works. The purpose of the rectifier is to convert this AC voltage into DC voltage so everything works. Different voltages for different tubes in the circuits are created by a network of capacitors and resistors with output points at several places to provide whatever the circuits call for.

Drawing 2 shows a solid state rectifier section and the relationship of the other parts of the supply I discussed above. Check out Drawing 3 to see a tube rectifier section.
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If you compare Drawings 2 and 3, you’ll notice a similarity to the rectifiers. The diodes in 2 have the positive ends connected to the transformer just like the plates (positive end) in the 5Y3 in 3. So every time the AC voltage swings, one or the other diodes/plates conduct, resulting in a DC waveform that looks like drawing 4. After cleaning up by the voltage divider/filter section, the voltage looks pretty close to the DC in drawing 1.

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So, there you have the function of the rectifier in a nutshell. Those of you who avoid electricity like the plague are probably confused so I’ll sum up the purpose of the rectifier in one sentence: The rectifier converts wall socket electricity to a different form that your amp can use to amplify your guitar’s input signal. There you go.

So, What’s the Difference?

Now we get to the meat of the whole thing – what is the difference between solid state and tube rectifiers? Solid state components (transistors, diodes, integrated circuits) react in microseconds to changes at their inputs. They tend to have a very “tight” sound with very little lag time or “sag” to the output. Since they are basically electronic switches, either on or off, you get a very precise output with very little ramp up to full output and very little ramp down to off. A solid state rectifier section gives your amp a solid, right there now voltage, which is relatively unaffected by hard playing (like power chords). The amp will have a “tighter” sound, a little more “in-your-face” to it.

A tube rectifier section is more laid back, taking milliseconds to react and exhibiting a little sag now and then when your amp is running on eleven. Simply put, your note is softened by the slower reaction of the tube as the demand for voltage runs high on attack and then the note swells as the tube catches up and provides full voltage. The Champ’s 5Y3 probably has the most “sag” or lag time, but that is a big part of the Champ’s great sound when cranked up. Many of your favorite guitar recordings from the past (and the present) were/are recorded on this 5 watter.

Different rectifier tubes, the 5Y3, 5AR4 and 5U4, the most common types, have different characteristics when it comes to response time and their ability to carry a voltage load under demanding conditions. For instance, the 5Y3 doesn’t produce as much DC out as a 5AR4, given the same input voltage… nature of the beast. Does that mean you can swap in a different rectifier tube to get tighter or looser response from the amp? NO. Some amps allow this, but always check with the manufacturer if this practice isn’t spelled out in your manual or on the amp itself. You can really toast your amp if you do this in an amp that’s not set up for it. ($$$$$$) The end result here is the same reason tube guys like tube amps – the expressiveness or feel of the amp. However…

It all depends on how you play and what you’re after. Solid state designers are getting better and better at emulating a tube sound, with all the sag and flow, out of amps that are more reliable and cost less. They don’t need those big voltages either so they weigh less, too. Don’t believe all the hype that a tube amp has to have a tube rectifier. Some of the great tube amps all have solid state rectifiers; Fender blackface Twins, Soldano SLOs, Marshall 100w Plexis and many others. I don’t know anyone who would say a Marshall Plexi isn’t an in-your-face amp when cranked.

The Vox AC 30 and AC 15, Fender Bassman and tweed Deluxe, and many others, have tube rectifiers. Are they the sound for you? By anyone’s measure, a Vox AC30 can get right in-your-face, too. (Wish I still had mine.)

The entire discussion comes down to what works for you. The only judge you have to please is yourself. It all depends on what you want and how you play. The only way to find out is to try as many amps as you can when you’re amp shopping. Even renting amps to try with the band is a good way to go as the music store trial just don’t get it most of the time. I’ve said it before and I’ll say it again… there is no right or wrong, there is only the right thing for you.

More on Tubes

Guitar Amps: The Sound of Tubes

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Doug Knight

Our “Man on the Street” reporter, with his “What’s New in Music Stores?” series, resides in Coos Bay, OR. You can find him on Friday nights at The Small Events Center at OrCoast Music in Coos Bay.

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