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Thursday, April 18, 2013

Work smarter not harder

I just worked on an Ampeg B15 and realized at the end that I could have saved some time had I approached the problem differently.

The amp had a loud 60 hertz hum with nothing plugged in, so my first thought is the filter caps are the problem area. I pulled the phase inverter tube and the hum was still there, so it's being introduced early in the power supply.

The first filter cap on this amp is a discrete one and the other 3 in the power supply are in a mulit-cap can. Since the hum is generated early in the power supply, I subbed in a good cap for the discrete unit, and the hum is still there. So I proceeded to sub in 3 more caps for the multi-cap can, and…the hum is still there. The only remaining cap is the fixed bias supply filter cap, which is where the problem turned out to be.

I got to the problem, but realized that if I had paid attention to the frequency of the hum, I would have started with the bias filter cap and saved some time. The only way you get 60 hertz hum is from external sources, or somewhere before the rectifier tube. The rectifier tube is a full wave device which takes the incoming 60 hertz AC voltage and flips the bottom half of the wave up to produce 120 hertz pulsating DC (see the diagrams below). If the problem had been the filter caps the hum would have been up an octave, close to the open B string on a guitar. As is was, the hum was closer to the B at the 2nd fret of the A string on a guitar.
60 hertz AC
120 hertz pulsating DC

The fixed bias power supply uses a single diode for half wave rectification--it doesn't fold the bottom half up and produces 60 hertz pulsating DC (see diagram).
60 hertz pulsating DC
Chalk it up to experience.

Sunday, December 30, 2012

What's ahead?

I find it interesting that when I read some interviews with musicians, they talk about looking for their sound, getting a different sound, or searching for something new. Yet when you get to the description of the gear they used on their last recording, it's almost impossible not to find the word "old" or "vintage."  They're trying to think outside of the box while being planted firmly inside of it. On the other hand, some people are already so far outside that I have a hard time keeping up with what they're doing. Instead I find myself wondering how the box we're in is going to grow.

One technology area that's overdue for a good idea is loudspeakers. If you look at a speaker objectively, it's pretty crude--very inefficient, limited in the range of frequencies it can reproduce, delicate--it's made of paper!  Certainly there must be better ideas out there to move air. Piezos may be a first step, and I've read about thin speakers and vibrating membranes, but don't know of any commercial products.

Similarly, magnetic pickups are due for an upgrade. The signal they produce is tiny and prone to noise, plus the magnetic field can interact with the string if it's too close. What about optical sensors? There are several out there already that offer better sensing of the string's vibration and are immune to electrical interference from lamp dimmers and other devices.

Digital signal processing offers some exciting possibilities to make a guitar a better instrument. Here I'm referring to something like the Antares auto tune circuitry that does pitch correction on the notes you play so the guitar plays in tune when it's not, and intonates perfectly. It doesn't have to change the sound of the guitar, although it can (like making an electric sound like an acoustic). But think of the restrictions this removes:
  • the 25.5" scale length of a Fender is critical to its sound, but if you may prefer the 24.5" scale of a Gibson there was no way up until now that you could get the tone 
  • intonation doesn't need to be perfect on the guitar, so you don't need a Buzz Feiten tuning system or Plek setup
  • transposing a song won't affect the chord voicings
Digital modeling amps are a great idea on paper, but the reality of it hasn't panned out. There's something a little off in the sound and the feel. Maybe that's the Turing Test for amps when you can't tell whether you're plugged into a blackface Super Reverb or a digital model of it. Similarly I heard that playing a Variax guitar felt "wrong" for some of the sounds--like playing an electric guitar, but  hearing a 12 string acoustic is "wrong." (Maybe that point above about scale length won't hold true.)

So as the calendar clicks off one more year, I'm wondering what the future will bring.

Thursday, December 20, 2012

Trainwreck amps demo

Just watched a great demo of 3 classic amps built by Ken Fischer, the 2 Trainwreck Express and 1 Liverpool. One Express were owned by George Lynch and the other was Ken's amp until he sold it to Billy Gibbons. The demo is great, using a Strat and Les Paul with different settings and playing. You can really hear the difference among the amps and guitars. Check it out:

Friday, December 14, 2012

The role of rectifiers

The job of a rectifier is to convert alternating current (AC) to pulsating direct current (DC). The filter capacitors that follow it in the circuit remove the pulsations, and provide a smooth DC power source. It seems pretty basic, yet how this is done has an effect on tone.

There are two types of rectifiers: tubes and solid state diodes. In the early vacuum tube days, solid state diodes weren't powerful enough or readily available, so rectifier tubes were used. In the 1960's, solid state technology had advanced enough where silicon diodes could be used. Sometimes several would be strung together to get the voltage rating needed, but now that's no longer the case.

One difference between the two devices is that a vacuum tube rectifier has an internal resistance (which is different depending on the type of rectifier tube used). Another is that tubes are sensitive to the amount of capacitance that is used to filter the pulsations out of the DC, and have a limit to the amount that can be used.

The way these two factors affect tone shows up as:
  • sag or compression in volume when playing loud, and
  • the amount of bass the amp can reproduce.

Both loud volume and/or bass frequencies pull more current through the power supply. Current through the resistance in a rectifier tube produces a voltage drop at the tube (Ohm's law), so the power supply voltage sags below what it is when the amp is idling. If the capacitors following the rectifier tube can't make this voltage up, the B+ voltage dips and the volume goes down slightly. When you're playing loud, or hit a loud note, this sounds like a little bit of compression. Or if there's a lot of bass, the tone changes because the bass is amplified less than the other frequencies.

Diodes don't do this. The voltage drop across a diode is constant and very small in comparison to a tube. They also don't care how much capacitance is at their output, so larger capacitors are often used. This makes an amp play "tighter." Metal players usually prefer diode rectification for its tightness, while many blues players like the looser tube rectification because the compression makes the amp sound like it is singing.

Diodes are cheaper and will last longer then rectifier tubes, but amps still use rectifier tubes for tonal purposes. Most of the time the rectifier type is built into the amp design, but some amps (Mesa) allow switching between tube and solid state rectification. There's also a mod that uses a diode to isolate the preamp and power amp capacitors. This allows the preamp to stay tight, since it stops the power amp's filter capacitors from drawing on the stored voltage in the preamp capacitors when the amp is working hard. It's also possible to build  or buy a rectifier tube socket with solid state diodes wired into it to change the rectification method. (You need to be careful here as this also raises the B+ voltage slightly and may stress aging filter capacitors.) And there are even some solid state rectifiers that have a resistance built into them so you get solid state longevity with tube-like tone.

Sunday, November 25, 2012

What to do with that extra channel

Almost all the Fender blackface and silverface amps have 2 channels, a Normal and a Vibrato (which may also have reverb). This is a historical feature. In the early days, one amp was used for the guitar, microphone, and possibly an accordian. Now with the one-amp-per-instrument model, having two similar channels is not the most flexible setup.

For one thing, the Normal channel requires a 7025/12AX7 tube, and if you're not going to use it, why buy one? You can not install the first preamp tube, but there's a subtle difference in sound. The B+ voltage goes up a hair (probably not sonically noticeable), but because the cathode resistor one the second triode in the tube is shared between the Normal and Vibrato channel, the Vibrato channel's tube is now set up differently. Instead of the cathode resistor being 1500 ohms, it drops to 820, which changes the bias and makes it distort a little easier. Try it and see if you hear a difference.

What about running the channels in parallel by plugging into #1 on the Vibrato channel and running a short patch cord from Vibrato #2 to Normal #1? On the non-reverb amps like the Band Master and Dual Showman, this works, and is an interesting sound. But on the reverb amps, the two channels are out of phase and there is some cancellation which doesn't sound good. Each time the audio signal passes through a tube stage, its phase is inverted. The tubes used in the reverb circuitry invert the phase of the signal when compared to the Normal channel signal, so this isn't a good idea for most uses. (Maybe you're one of the few that has a use for the phase cancellation sound.)

Running the preamps in parallel works really well for Marshall 4-input amps. A lot of players jumper the two channels of a SuperLead as a standard way of setting up the amp. This came about for another reason (see my June 20, 2012 post) but creates a nice thick tone.
There's a simple mod that lets you use the Normal channel for finer reverb control (on amps that have reverb). Run a shielded cable from the output of the reverb tank to the input on the Normal channel (you'll need and RCA to 1/4" cable). The Volume knob is now similar to the Reverb knob in function, but you have tone controls. Easy enough to try without a big commitment.

For more intrusive uses, you can cascade the channels, which was the idea Randall Smith had in the '70's which became Mesa Boogie. This is a bit of an art, as you can't get a good sound by just running one channel into the other (see the Marshall One-Wire mod on my website). Distortion sounds better if it's built up over several tube stages, so you'll need to attenuate the signal. Plus bass muddies the tone, so you'll also have to add a high-pass filter to get something usable.

Another thought is to re-voice the channel so it sounds different. One idea on my website is to turn it into a Tweed style preamp. Other people change the tone stack components, or add a Marshall style preamp (where the tone controls are after the 2nd triode stage). Something to give a different sound and make the amp more flexible. By using an A-B switch box you can add channel switching to these amps.

Consider it--it might be a fun way to play with your sound. But be cautious about modifying these amps in a way that can't be returned to stock (like by drilling holes). As they get older they're becoming more valuable, and sometimes it's better to mod your amp ownership rather than your amp.

Monday, November 5, 2012

If it's too loud, you're too old

Made by Dr. Lex
I read a really well-done article by Dr. Lex describing a trend of reduced dynamic range in music recordings for the sake of perceived loudness. The graph here is from his web site, and shows this trend starting about 1990, and getting increasingly more apparent over the next 20 years.

I started paying attention to music seriously in the '70s which was a relatively stable period for dynamic range--it was present in recordings. The vinyl albums I was listening to had a lower possible dynamic range than CDs have, but still it was used to effect--there were quiet parts and drastically different loud parts. The first album that I remember being extra loud was Alice Cooper's "Killer" in 1971. At the time I didn't understand why or how this could be, but it fit with the nature of the album and was cool. I wasn't alone, my friends noticed it too. But this may have been one of the first shots fired in what Dr. Lex calls the "volume wars."

I think the ear training that I received in the '70s colors the way I hear contemporary mixes--they're less interesting. Younger people are growing up with this reduced dynamic range which will be their ear training that they will take forward with them. It will be interesting to see where it goes.

It's ironic that modern media has the ability to reproduce a greater dynamic range than was available in the '70s, but it isn't being used due to the nature of how the data is delivered and consumed--MP3s, earbuds, etc.

In the meantime, I must be too old, because it's too loud. I miss dynamic range.

Saturday, October 20, 2012

What a difference a (tube) stage makes

Blackface Fender Deluxe amps sell for much less than blackface Deluxe Reverbs. An informal survey of completed Ebay sales shows about $1000 difference between the two.

This is due to more than the lack of reverb; the Deluxe Reverb has just a touch more gain to it, giving it a slightly different sound. There is one extra triode that's used to mix the reverb return with the dry signal, and even with the reverb knob set to 0, it's still in the signal path of the Vibrato channel. This, I think, is what endears it to a lot of guitar players.

When you add the reverb into the picture, the signal now goes through 3 additional stages, so the reverb adds additional coloration to the amp. I've read newsgroup posts where the Deluxe's fans feel they can get the same thing through a reverb pedal or other effect, but it doesn't duplicate the the signal path. There's reverb, but it's not achieved the same way.

The Deluxe is a great amp and has great sound. Although it's similar in name to the Deluxe Reverb, it's a different amp, and not as flexible. Its two channels are differentiated by one having tremolo, and the other not. A Deluxe Reverb's channels on the other hand, are almost like having two amps. The normal channel is a Deluxe with the tremolo turned off, and the vibrato channel adds a gain stage, reverb, and tremolo.